Rozprawy doktorskie na temat „Research Subject Categories – TECHNOLOGY – Engineering mechanics”

The occurrence of a severe natural disaster causes loss of life and destruction of properties. The overall criticality of the disaster depends on the nature of the disaster and the physical characteristics of the affected locations. In the aftermath of a natural disaster, multiple emergencies often evolve at different geographical locations with casualties and infrastructure damage. In post-disaster scenarios, responsible authorities should initiate relevant disaster management activities to mitigate the devastating effects of natural disaster. Resource allocation is an integral part of the post-disaster activities. In general, resource allocation deals with the issue of distributing necessary resources to multiple users depending on their demand and the availability of resources. It aims to achieve efficient and fair assignment of limited resources. The devastation caused by a natural disaster enforces the need for various critical resources in disaster-affected locations to reduce the impact of the disaster. When adequate resources are available, the problem of allocating resources becomes trivial, and all the crisis locations can be fully satisfied in terms of their resource requirements. However, if there is a scarcity of essential resources after the simultaneous occurrence of multiple emergencies at distinct geographical locations, providing resources to all those regions and fulfilling their demands simultaneously becomes challenging. In such situations, efficient decision-making is necessary to execute a fair and socially agreeable allocation of resources to the affected locations. One cannot rely on human-controlled decision-making since it can have a bias for, or prejudice against, some of the disaster locations. A fair and impartial approach to the allocation of resources can be implemented by designing an automated decision-making system. This thesis proposes a game-theoretic framework which can form the basis for such a system. In this thesis, we develop a multi-event emergency management system using a non-cooperative, single-stage, strategic form game model to facilitate the allocation of resources to the respective disaster locations. Each emergency event is assumed to occur at different locations simultaneously, and some amount of resources are demanded by each location to mitigate the impact of disasters. These locations are represented as players in the game, which are assumed to play in a self-interested manner with the other players to get an allocation of scarce resources available at the resource station. However, it should be noted that the disaster locations are not actively involved in playing a game. It is a centralized decision-making executed by the responsible disaster management authority, which implements the algorithm designed using the game-theoretic framework to decide reasonable allocations to the players. The authority assumes different allocations to be the possible strategies of the players and arrive at a fair solution. As a game utility, the authority imposes a non-monetary cost on each player for obtaining a certain amount of resource units. The objective of the proposed game is to derive socially acceptable strategies for an effective and fair allocation of resources to the respective players. In the thesis, it is established that the game model is unique in structure and always possesses pure strategy Nash equilibria (PSNE). Each PSNE consists of possible allocations to the players; hence, those can be implemented by the disaster management authority as potential allocation vectors. As the resources needed during disaster management can be both divisible and indivisible, we investigate the game for both types of resources. Mathematical analysis shows that the existence of PSNEs is independent of the nature of resources. The only difference it makes is that in the case of indivisible resources, the players have a discrete set of strategies, and divisible resources make their strategy sets continuous. It is also shown that the game-theoretic algorithm can be used for any number of players or disaster locations at various stages of resource allocations. The investigation is conducted using twoplayer, three-player and n-player game models. Different case studies are presented in the chapters of this thesis to validate the mathematical results developed in this work and to indicate how this proposed method can be helpful in practical disaster resource allocations. This work also includes the statistical analysis of the game-theoretic algorithm and the study of its computational complexity. This thesis also includes a study on the preparedness and damage assessment of a natural disaster using unmanned aerial vehicles (UAV). Preparedness is a pre-disaster activity which is essential to build resilience against natural disasters. Damage assessment is one of the post-disaster activities which estimates the loss of human lives, properties, and infrastructure. This phase is important to initiate the response and recovery work after a natural disaster. These activities become challenging and time-consuming when human effort is the only option. In our study, we focus on the possible applications of UAVs to make these activities speedy and effective.

Ribbons exhibit fascinating buckling-dominated behavior under mechanical loading because of a unique combination of geometric dimensions. The recent interest in examining engineering applications of ribbon-like structures underscores the need for dedicated structural mechanics models to predict their complex behavior. In this thesis, we deal with ribbons that have at unstressed con figurations. Due to their physical appearance, such ribbons are typically modeled either as rods with highly anisotropic cross-sections (width of the cross-section is much larger than the thickness) or narrow plates. We speci fically examine the predictive capabilities of the Geometrically exact two-director Cosserat rod and Geometrically exact one-director Cosserat plate models. We measure ribbon shapes in various bending-dominated experiments and compare them with predictions computed using detailed finite element simulations of these models. We nd the plate theory to be particularly useful under a broad range of loading conditions, mainly because it captures nontrivial (and nonlinear) curvature distributions realized in the material bers oriented along the ribbon's width. This feature, which is noticeably absent in rod models, contributes to their poor predictive capabilities. We then propose a phenomenological one-dimension ribbon model by dimensional reduction from the Cosserat plate theory. Speci fically, we impose kinematic assumptions on the displacement field's dependence along the width direction of a ribbon to permit non-trivial lateral surface curvatures observed in the Cosserat plate solutions corresponding to various experiments. We speci fically examine polynomial dependences for the displacement field on the coordinate along the width. In principle, we expect a quadratic dependence to suffice since it helps to reproduce non-zero curvatures along the width. However, we nd that the resulting restricted kinematics is prone to membrane locking. Presuming a cubic dependence helps circumvent the issue. Alternately, resorting to selective reduced integration techniques during numerical approximation using finite element methods helps alleviate the issue.

In recent times, usage of electronic devices inside cars while driving has increased due to the introduction of new technologies to keep drivers comfortable and entertained. Systems like satnav ease out the navigation by offering optimised traffic plans at times of complex traffic and road conditions. Though technologies like music, radio and phone facilitate onboard communication and entertainment, they have potential in distracting drivers. The interaction with such technologies while driving takes the attention of drivers away from driving. As distraction of drivers leads to car crashes and fatal accidents, the research community investigated detecting and reducing such distractions. Operating a secondary task while driving is one of the key reasons for driver distraction. It is challenging to detect the inattention blindness of drivers compared to detecting instances of eyes-off road. Recent studies have found that high perceptional load results in increased inattention blindness. This dissertation investigates methods to reduce driver distraction caused due to operating secondary tasks. It proposes new interactive technologies involving virtual touch and eye gaze tracker to undertake secondary tasks in both head down and head up displays. It also proposes a new machine learning model to estimate cognitive load from ocular parameters and validates with respect to EEG parameters from studies involving professional drivers operating real vehicles. Finally, grounded theory method of qualitative research is used to understand and explore concerns and issues of professional drivers, resolve them, and look for factors contributing to acceptance of the proposed interaction technologies. This dissertation discusses the potential of the proposed methods for applications beyond automotive context. As the proposed systems are tested in simulation and real driving environments, they are considered to be deployed by industries.

Experimental techniques to measure and visualize kinematics in structural and solid mechanics range from humble strain gage rosettes to sophisticated digital image correlation methods. This thesis develops a stereo visionbased optical measurement technique and evaluates its efficacy for measuring three-dimensional elastic deformations of slender structures. Our motivation to develop the technique stems from the need to quantify/digitize the kinematics of slender elastic structures undergoing large displacements and rotations within the small strain regime. As devices composed of highly flexible elements become ubiquitous in engineering applications, especially at small length scales, it is imperative to examine the mechanics underlying their functioning through a combination of modeling and experimental studies. The technique proposed here is a step towards addressing challenges in the context of the latter. We adopt elastic ribbons as prototypical examples in our study. Owing to the disparities in their dimensions (length ≫ width ≫ thickness), ribbons naturally contort into complex three-dimensional energy-minimizing configurations in response to simple loading scenarios. For this reason, elastic ribbons furnish excellent test cases to investigate the capabilities of the proposed technique. Besides, such measurements complement on-going efforts within the research group to understand the mechanics and envision novel applications of elastic ribbons. The proposed technique relies on familiar principles of stereo vision— a pair of calibrated digital cameras, an ansatz for pixel correspondences, and triangulation of corresponding pixel pairs to reconstruct points of interest in the scene. Hence, we will photograph a ribbon sample from multiple vantage points using a pair of digital cameras and reconstruct the locations of markers labeling its surface. The novel aspects of the technique include: the choice of fiducial markers to paint flexible surfaces, an algorithm to encode/decode 5-letter marker dictionaries that helps limit the number of distinct markers required to label surfaces, and an optimization-based algorithm to determine full-field approximations of deformations mappings by unifying independent Lagrangian marker and Eulerian shape measurements. The set of markers labeling ribbon surfaces serve multiple purposes. They define Lagrangian coordinates that can be tracked during deformation, establish pixel correspondences between cameras in a stereo arrangement, and aid in registering partial reconstructions to a common coordinate system. Throughout our study, we adopt the ArUco marker system that is commonly used for positioning and alignment of coordinate systems in virtual reality and robotics applications. This choice is mainly based on convenience; alternate marker systems (e.g., AprilTags, color-based markers, shape-based markers) along with reliable detection algorithm can be employed as well. Through a detailed set of measurements of shapes and displacements of straight and annular ribbons, we quantify the accuracy of the proposed technique at a desktop-scale. When available, we also compare the measurements with idealized finite element simulations available in the literature. We conclude the thesis by listing possible strategies to improve the technique.

In this work, radiation and transmission of sound through flexible perforated panels set in infinite rigid baffles are investigated. The treatment is largely analytical using Fourier transforms and contour integrations. Numerical calculations are only used occasionally. The work is largely divided into three parts: the first part involves radiation and transmission studies using the one-way coupled formulation, the second part investigates the same problems using the two-way coupled (or the fully-coupled) formulation and the third part involves derivations of closed form expressions for the modal coupling coefficient using contour integration. In the first part, the panel with perforations is placed in a baffle that is perforated or unperforated. Having an unperforated or a differently perforated baffle presents challenges. It causes a certain coupling of wavenumbers leading to an integral equation. In the literature so far, the baffle has been taken to be similarly perforated, thus, simplifying the situation. The perforations are arrays of circular holes and are mathematically modeled using a perforation ratio. An existing model for a circular hole that transmits sound is used and the collective array is modeled using a perforate impedance. Since, there is an escape of fluid through the perforations as the panel vibrates (radiating or transmitting sound) an averaged fluid particle velocity over the panel surface is derived using fluid continuity and momentum equations. This averaged fluid velocity is then used along with impedances to compute the pressures and sound powers. In addition, the presence of the holes shifts the resonance frequencies and modifies the modeshapes. This shift is accounted for using the Receptance method. The entire derivation is done in the wavenumber domain (spatial Fourier transform). And at the end, numerical calculations are done. For the radiation and transmission problems, the results are presented in terms of the radiation efficiency and the transmission loss, respectively. It is observed that the perforations reduce the in vacuo natural frequencies of the panel. For the radiation problem, analytical expressions for the radiated power and radiation efficiency are derived in an integral form and numerical results are obtained for different perforation parameters such as perforation ratio, hole diameter and number of holes. It is observed that a reduction in the perforate impedance leads to a decrease in the radiated power and also in the radiation efficiency. The effects of resistive and reactive hole impedances on the sound radiation are also discussed. For the transmission problem, it is found that the perforate impedance acts in parallel to the panel impedance and for a real-world scenario, where the perforate impedance is less than the panel impedance, a reduction in the transmission loss (TL) can be achieved with perforations on the panel. For small holes at lower frequencies the resistive impedance dominates over the reactive impedance. This results in a higher TL at lower frequencies for a micro-perforated panel as compared to that for a panel of same perforation ratio but with larger holes. In the second part, the same two problems of radiation and transmission of sound through perforated panels set in rigid baffles are studied using the two-way coupled or fully coupled formulation. In addition to the details presented for the one-way cases above, here two equations are derived where the average fluid particle velocity and the panel velocity depend on each other. Thus, a coupled problem needs to be solved. Due to the inclusion of the fluid loading, a modal coupling coefficient arises in the formulation. This coupling coefficient is indicative of the degree of coupling between the in vacuo panel modes caused by the acoustic fluid. In several of the earlier studies on unperforated panels, in the literature, largely the self modal coupling has been investigated. Only a few studies have presented studies on the cross modal coupling. These studies were restricted to the low frequencies. The formulation is reduced to a single coupled equation and the system of equations (including the modal coupling coefficient) are solved numerically. Again, the results are presented in terms of the radiation efficiency and the transmission loss. The natural frequencies are identified from the peaks in the mean panel quadratic velocity spectrum and compared with results from the literature. It is observed that the radiation efficiency decreases with the increase in the perforation ratio, irrespective of the surrounding acoustic medium. For a given perforation ratio, the water-loaded panel radiation efficiency is found to be less than that for a panel immersed in air. It is also observed that for a light fluid like air, a one-way coupled formulation is adequate. Further, a fully coupled model for the transmission problem is also developed. It is observed that the TL of a perforated panel acquires negative values at low frequencies. This apparent anomaly is resolved by taking into account the additional power component that flows from the baffle region onto the panel at low frequencies. In the last part of the thesis, approximate expressions in closed form are obtained for the modal coupling coefficient using the contour integration. Analytical expressions valid for any given fluid loading conditions are derived for the modal interactions between the corner modes, single and double edge modes and the acoustically fast modes. This is further used to evaluate the natural frequencies and the radiation efficiency of the perforated panel. The results agree very well with those obtained earlier in the thesis using the numerical integration. Also, plots of the resistive and reactive parts of the modal coupling coefficient are presented and discussed.

One of the major challenges in space technology is to achieve high resolution imaging of the intended area on demand to meet civilian and military applications. Spacecraft formation flying with coordinated control of smaller satellites offers improvement in image resolution using distribution of payloads across the formation group. Quick revisit and stereo coverage, as well as provision of modularity and redundancy, are other benefits achieved by formation flying. The thesis addresses spacecraft formation flying with coordinated control of smaller satellites in realizing this high resolution system without compromising agility. The work focuses on coordination and control realization which is achieved through a distributed control strategy that leads to consensus agreement among formation members. To begin with, the problem of formation control for a two spacecraft leader-follower (L-F) system is formulated and is subsequently extended to a fleet of multiple satellites. Synchronized relative position and attitude control forms a major role in achieving the stereo-imaging requirement. Navigation information is obtained by fusion of Inertial Navigation System (INS) and Carrier phase Differential GPS (CDGPS) measurements. Mono-propellant thrusters and micro-reaction wheels are used as control actuators for the position and attitude control, respectively. For a two satellite leader-follower formation, the control system has been designed using Proportional-Derivative (PD) control as well as Linear Quadratic Regulator (LQR) based optimal control techniques. While extending to more than two satellites, graph theory based consensus agreement concepts are applied to achieve desired formation goal. In this case, control system is designed using distributed formation control based on Linear Matrix Inequalities (LMI). The use of LMI-based control methods help in achieving global stability of formation groups while satisfying the consensus property among members. Detailed simulations are performed and analysis of the results assure that the designed system is capable of meeting the task of formation flying with micro-satellites. A brief analysis of the collision avoidance concept using eccentricity-inclination vector separation for passive orbits has been made. Null Space based Behavioral (NSB) approach, which determines the guidance trajectory based on priority among the tasks, namely, collision avoidance, plume avoidance and tracking error reduction, has also been presented. The distributed control system for a group of six satellite formation in Projected Circular Orbit (PCO) configuration has been demonstrated using LMI based controller together with consensus observer. The collision avoidance between the formation members has been demonstrated by assessing the inter-satellite distance between the formation members. The formation stability has also been proved through simulations and analysis. The proposed control design is expected to improve the revisit capabilities of the mission in addition to improving the spatial and spectral resolution of the remote sensing satellite systems by aperture distribution and baseline enhancement.

The present study concerns experimental and numerical investigation of the combustion of low-calorific value syngas in an optically accessible reverse flow combustion chamber. Several modes of operation are investigated to identify the best strategy for stable operation with low emissions of NOx and CO. The first part of the study investigates the combustion dynamics in the chamber and establishes the range of parameters for stable operation using OH* chemiluminescence (5 kHz), noise (50 kHz), and exhaust emissions measurements (NOx and CO). The combustion dynamics have been investigated as a function of the global equivalence ratio (0.32 - 0.89), O2% in the co-flow (7.6 - 21%), and the oxidizer preheat temperature (~ 400 - 800 K). The variation of these parameters resulted in different operating conditions designated as: conventional (Φglobal = 0.8), ultra-lean (Φglobal = 0.32), transition (Φglobal = 0.47, O2 = 14.3% in oxidizer), and MILD (Φglobal = 0.89, O2 = 7.6% in oxidizer) combustion modes. For all cases, autoignition was observed to be the mode of flame stabilization that indicated the role of H2 in reducing the ignition delay. The conventional mode displayed the highest sound pressure level (SPL) and fluctuations in the reaction zone (OH*). The most stable operation was obtained for the MILD case where the SPL decreased by 6 dB caused by a suppression of the high-frequency (> 800 Hz) longitudinal modes. In the second part of the study, OH concentration and temperature are measured using Planar Laser-induced Fluorescence (PLIF) and Rayleigh thermometry to provide a detailed understanding of the reaction zone structure. The OH radical, which is a marker of the reaction zone, shows maximum intensity for the conventional case and lowest intensity for the MILD case. The instantaneous images show a complex reaction zone with thin structures near the inlet and progressive distribution of OH at the bottom. The temperature measurements reveal a uniform thermal field throughout except very close to the centreline. Such a distribution can provide superior heat transfer characteristics in furnaces. The maximum temperature is measured for the conventional case (~ 1700 K), while the temperature is similar for the ultra-lean, transition, and MILD cases (~ 1300 K) supporting the observations of low NOx emissions. In the third part of the study, we evaluate the performance of the combustor by measuring NOx and CO emissions. The NOx emission is less than 1-ppm for all the cases, while the CO emission is highest for the MILD case (461-ppm) and lowest for the conventional case (32-ppm). In the last two parts of the study, the experimentally generated data is used to validate models that are subsequently used to numerically simulate scaled-up designs of the combustor with power ranging from 3.3 kW to 25 kW. The influence of four different scaling criteria on the performance of the combustor is evaluated. These are constant velocity (CV), constant residence time (CRT), constant volume-to-jet momentum ratio (CM), and constant volume-to-jet kinetic energy ratio (CK). The CV criterion performs the best in terms of pressure drop and CO emissions. Overall, the current investigation establishes that the combustion of low calorific value syngas can be performed in a reverse flow configuration with low emissions and potential for scaling to industrial sizes.

The interaction between premixed flames and turbulence is an inherently non-linear phenomenon. Understanding such interactions has profound practical implications towards the development of better combustion devices and turbulent combustion models. To this end, three statistically planar, freely propagating, turbulent premixed lean H2-air flames with varying turbulence intensities are generated using Direct Numerical Simulations (DNS). A newly developed backward tracking technique is applied to identify the source locations of iso-scalar surfaces of the turbulent premixed flames. In this technique, flame particles embedded on the iso-scalar surface are tracked backwards in time. Using the available flame particle trajectories, finite-sized Lagrangian triangles are created and tracked forward in time to investigate changes in their shape and size. These changes approximate corresponding modifications of the underlying flame surface. Based on the inferences obtained, a phenomenological model proposed for the evolution of geometric structures in non-reacting flows is modified and validated for the present cases. The evolution of probability density function (pdf) of Lagrangian triangle area is then studied to understand the conditional stretch rate of the triangles, as they disperse out to generate the complete flame surface. An optimization problem is posed to obtain the conditional stretch rate, and it is found the stretch rate is dependent on the instantaneous triangle size. Based on the outcomes of the above-mentioned exercises, the expressions for turbulent flame speed and hence the burning rate of the flame are found to be implicitly dependent upon the statistics of the leading portions of the flame surface, but at an earlier time. This signifies the importance of these surface generating locations that have been identified as the “leading points”, a concept used in turbulent combustion modelling. In summary, Lagrangian methods have been utilized in this work to investigate the generation mechanism of turbulent premixed flames in their statistically stationary state

The mechanical behaviour of deformable bodies in a particulate environment has been an area of increasing interest across a wide spectrum of systems and scale. A composite ensemble of deformable structures and discrete particles involves coupling of component responses, large displacements of structures, and multiple dynamic interactions that lead to inherent contact nonlinearity. To describe these structures and their interactions with particles, we apply a particle simulation approach based on the discrete element method (DEM). There exist alternative frameworks too such as continuum modelling with techniques like finite element method (FEM), or a combination of continuum and discrete modelling, or lumped modelling with mass-spring systems. Owing to the convenience and robustness provided by a single approach, this thesis aims to develop a single framework with the discrete modelling approach. The mechanical behaviour of particulate models of slender elastic continua is first validated with their analytical or FEM counterparts, and then particle-structure interactions are considered. We develop elastically deformable particulate models of straight beams and shallow arches. We evaluate the geometrically nonlinear response of particulate beams under a variety of static, dynamic, and impact loading scenarios. We also model particulate arches and assess their ability to exhibit two force-free equilibrium states, namely bistability. To illustrate the utility of these particulate representations, we first consider a case study of an undulating beam in a particle medium. The dynamic beam-particle interactions propel the beam within the medium, resembling the self-propulsion of reptiles in granular environments. In another case study, we take up a relatively sparse environment of mobile particles and oscillating cantilever beams. The interplay between particles and beams is shown to drive particles for capture. We also demonstrate particle-arch interactions in bistable mechanisms that result in particle gripping and trapping. We draw insights from factors that regulate the governing dynamics of such coupled phenomena. Next, we model particulate thin films that undergo deflections in linear and geometrically nonlinear regimes and describe both plate-like and membrane-like behaviours. A notable instance of this particulate perspective of thin films occurs in the context of microscopic biological material such as cells and their organelle. We present in this context a discrete particulate description for the nucleus of a biological cell. A three-dimensional model that incorporates the nuclear envelope and chromatin-containing nucleoplasm is developed and subjected to micropipette aspiration. Our work on particulate systems is implemented within Altair EDEM, a commercial DEM software. While most available DEM packages, including EDEM, provide a ready-to-use interface for the modelling and analysis of granular and bulk materials, they lack similar modules for particulate structures. The preprocessing stage thus involves substantive customization to build algorithms for particle generation, contact physics, external couplings, and parameter definition appropriate to our studies. By customizing this process, we utilize the graphical interface capabilities of EDEM to simulate, readjust, and visualize the analysis of structures under applied forces and particle interaction. Taken together, the studies in this thesis facilitate a comprehensive investigation of the particulate approach’s efficacy to model a variety of deformable structures, capture geometric nonlinearity in their response, and simulate the interaction dynamics of coupled particle-structure systems.

The main theme of the work carried out and reported in this thesis is to understand the processes involved in a co-current downdraft biomass gasification reactor and to analyse the data obtained with an attempt to provide a consistent and rational scientific reasoning of the behaviour and processes studied. The work carried out largely comprises of experiments involving parametric and investigative studies on an experimental setup built specifically for this purpose. The background for this study is based on the IISc design for the biomass gasifier where the technology developed at CGPL, IISc has been well recognized for its performance in the field with many industrial applications, meeting electrical and thermal energy requirements. However, even though the processes have been well defined, based on the literature it was found that adequate insight into the processes and the parametric dependence of the process are not brought out adequately and required further study on this. The present work is an effort in reducing this gap. The theme of the study is set out in two streams, one in establishing basic process parameters that influence the behaviour of the reactor and the other one, to establish the characteristics of the reactor in its functional aspects. The first part of the study involves use of only primary air to the reactor and to carry out the parametric studies. This part of the study has enabled understanding the basic phenomena that controls the processes in the reactor. The second part of the experiments involves introducing secondary air to the reactor along with the primary air and to extend the study that gives an insight in to the change in the behaviour of the reactor with dual mode air injection and to evaluate the performance of the reactor in this configuration. The structure of the thesis is set out with five chapters enabling the presentations to be made, categorising the theme of work. Chapter 1 provides an introductory background of biomass usage as an effective renewable fuel and its relevance to the current energy scenario. The principle involved in biomass gasification, a process that lets to have a clean combustion which has been known to be a challenge with biomass. Chapter 2 provides description of experimental setup, measurement options and experimental procedure for carrying out test. Chapter 3 provides the report of the experimental work carried out on the reactor with primary air, to have a comprehensive study of propagation rates and gas compositions at varied conditions of air flux, differently for biomass moisture content and biomass species. The chapter presents the experimental data with analysis, modelling and interpretations with consistent and scientific reasoning on all the observations. Chapter 4 provides the results and details of experiments carried out with dual mode air inductions, with both primary and secondary air to the reactor and gives an account of systematic parametric study carried out that provides an insight into the process behaviour and performance of the reactor, over the spectrum of study. The parameters, particularly the flux ranges selected are much beyond the normally reported so as to get an overview on the functional limits of the reactor. Chapter 5 gives out a consolidated review of the work and provides a comprehensive overview on the data obtained during the experiments along with the concerned analysis carried out and also narrates the limitations in the studies carried out. Chapter 6, sums up the outcome of the work providing a highlight of the contributory points. Each of the chapters have been provided with a summarising note at their tail end that provided the reader an overview of what is addressed and presented in the chapters concerned. The experimental test-runs were quite time and resource demanding, lasting for 3 to 8 hours for each run and including the resetting of setup for the next run, the test cycles are seen to take it to 2-3 days. In view of this, priority was set for the study parameters in addressing and understanding the processes and to characterise their influence on the processes in the reactor. With the time constraints and based on the priority set out, some of the studies that include the measurement of tar and particulates in the gas produced were taken off from the theme of this work. However, in conjunction with fair amount of studies carried out on this parameter by earlier researchers, though not tagged to the entire range of parameters studied here, it makes the present study of practical relevance and offer a higher quality of design guidelines. The recognizable outcome of the study from the work carried out for this thesis includes reliable data from carefully designed and carried out experiments, well resolved behaviour of flame propagation in the biomass bed, the distinct differences in the behaviour of the reactor under single and dual mode of air inductions, gas compositions and performance evaluation at all the parametric variants. A detailed overview of the outcome of the thesis work and contributory points are provided in the concluding note in the chapter 6

This is a detailed study on the aerodynamics of low Reynolds number airfoils. E387 airfoil that comes under the category of trailing-edge stall is used extensively in this study. The reduction in lift due to low Reynolds number effects can be modeled by the so-called viscous decambering of airfoils. In particular, we show this viscous decambering effect can be subsumed in the zero-lift angle term. It is further demonstrated that leading-edge camber angle plays an important role in explaining the stall characteristics of different airfoils. An attempt has been made to heuristically illustrate the different stall characteristics from thin-airfoil theory, and also to understand the basic design philosophy of low Reynolds number airfoils. Laminar Separation Bubble bursting is a deleterious phenomenon (in turbine blades, airfoils etc.) resulting in a loss of lift and increase of drag at relatively low Reynolds numbers. Bursting of a laminar separation bubble is characterised by a massive loss of lift in an airfoil; the surface pressure distribution departs significantly from the inviscid pressure distribution. There are some criteria in vogue in the literature to characterise onset of the bursting phenomenon. A relatively simple one-parameter criterion that was proposed in the past (Diwan, Chetan, and Ramesh 2006), has been found to be quite successful in characterising bursting and is well cited in the literature, including in unsteady flow contexts. This criterion is revisited and reassessed in the light of our present laminar separation bubble measurements over an Eppler 387 airfoil. Building on these foundations, the pursuit of a more robust bursting criterion that could also be predictive in nature, led to a new simple criterion. According to this, bursting is signalled by the ratio of the freestream velocity at reattachment to that at separation reaching a critical value of 0.86. New engineering correlations for length and height of laminar separation bubbles are also proposed. These correlations, along with the new bursting criterion, should be extremely useful in the design of low Reynolds number aerodynamic configurations. One of the central foci of the thesis is to study the phenomenon of bursting of laminar separation bubbles. A pertinent question to ask, on the physics of bursting, is whether the onset of bursting is coincident with absolutely instability? In this study, it is unequivocally shown that whether the bubble is short, long or transitional it is convectively unstable. The absolute instability characteristics are only seen momentarily in the unforced flow conditions, which change the state of the flow. Low frequency activity is prevalent in all the bubbles, but for the short bubble the amplitude of this activity is small compared to transitional and long bubbles. Impulse response shows absolute instability behavior in the low-frequency part of the flow-field for all the bubbles, reminiscent of some global mode oscillator. At (and post) bursting, the Reynolds shear stress 􀀀u0v0 is found to be negative in the initial region (close to the maximum time-averaged vorticity of the flow) of the transitional and long bubbles. Similar observations were reported by O¨ zkol,Wark, and Fabris (2007), Simoni, Ubaldi, and Zunino (2012), and Lengani et al. (2017). The effect of 􀀀u0v0 < 0 is that one of the turbulent kinetic energy production term, namely 􀀀u0v0 ¶U ¶y , becomes negative. This term is an important contributor to the overall rate of production of turbulent kinetic energy, and if it is negative, development of turbulence is compromised and the reattachment of the separated shear layer is delayed. This provides an explanation of the delayed reattachment for the long and transitional bubble post bursting. This is attributed to the increased curvature of the separated shear layer, and results in the runaway effect. Reduction of drag by passive flow control techniques, like roughness induced flow control, is investigated in this work. Roughness is found to be most effective when it is placed in the vicinity of the locus of inflection points of the base velocity profile. Analysis of receptivity of this flow by the solution of the adjoint Orr-Sommerfeld equation also supports the experimental observation. A more clear flow physics by consideration of the energetics is obtained from the inhomogeneous Orr-Sommerfeld equation. From this, the roughness seems to be effective where the inflection point is close to it and the shear-strain rate of the base flow at the wall is relatively high. This knowledge of passive control study is applied on the next study, where the relative impact of LSBs is investigated. The conventional wisdom says that LSBs deteriorate the performance of an aerodynamic system. Here, we systematically study the LSB in the Reynolds number range of 40,000 to 200,000. We find that for lower Reynolds number of 40,000 to 60,000, suppressing LSBs by tripping the boundary layer is beneficial to the overall performance of the airfoil. On the other hand, for Reynolds number of 100,000 and above, suppression of LSBs deteriorates the performance further. So, it seems that LSBs at these Reynolds numbers of 100,000 and 200,000 act as efficient switches of the oncoming laminar flow to transition to a turbulent flow, and it is not ideal in trying to suppress them by boundary layer trips. Whereas, for lower Reynolds number of 40,000 and 60,000, the trip is recommended as it is able to bring down the drag, and increase the lift on the airfoil to a considerable extent. The reduction in lift at low Reynolds numbers is attributed to the modification of circulation by the boundary layer vorticity. A non-zero pressure difference across the trailing-edge of an airfoil can be directly related to the integrated flux of counterclockwise vorticity emanating (diffusing) out of the wall. The loss in lift DCl is successfully correlated to this DCP across the trailing edge, and this seems to be true for long bubbles, and even fully separated post-stall flow situations.

This thesis is an attempt to understand the characteristics and responses of plasmon phonon coupled dynamics in nanocrystalline structures. An energy based Lagrangian formulation is developed with fundamental field quantities like displacement, charge, electric and magnetic field as variables. Governing differential equations of motion are derived from variational calculus. A new computational framework is implemented to solve for system degrees of freedom, which involve energy and force exchanges between atomic and continuum representation of the system. The developed simulation framework is employed to investigate the phonon and plasmon characteristics for nanocrystalline copper thin films and carbon nanotubes under decoupled scheme. Several fundamental properties are estimated which are found in agreement with experimental and first principle calculations. The simulation framework is applied to analyse the coupling behaviour in a nanocrystalline copper, free standing structurally engineered graphene, pure carbon nanotubes, macromolecular carbon nanotube composite structure, engineered graphene on a silicon substrate and zinc oxide on a zinc substrate. Intense coupled modes are identified using these material models to demonstrate the usefulness of the developed framework and simulation results. The one-dimensional ultrathin copper nanowire has shown the existence of optical phonon modes in copper. Three different plasmon extinction peaks are observed for nanowire. The coupling between the plasmons and optical phonons is reflected in the form of splitting, screening and intensity rise in the phonon density of states. Device configuration with nanowire mounted on a substrate shows a strong excitation of surface plasmon and the phonon oscillations. The charge localization near the defect induces a secondary plasmon excitation. The repetitive electromagnetic irradiation heats up the graphene lattice and achieved a complete screening optical phonons. Uncontrolled heating initiated a lattice failure in graphene with hole defect. A good control on transport of thermal energy is achieved with the help of graphene boundary engineering. The coupled analysis for a single walled carbon nanotube is estimated under controlled temperature at different stages of mechanical elongation. A step wise decay in the plasmon energy is observed with increased in the lattice strain. The local plasmon induced electric fields modulate the phonon intensity and mode switching in strained nanotubes. Macro-molecule involved nanotube binding and the coupled characterstics are analyzed. The combination of zinc oxide nanowire with zinc substrate shows a decrease in the plasmon energy due to the chemical bonding between them. The experimental equivalent growth process is modeled using atomic interaction potentials. The nucleation of zinc oxide from the zinc substrate was confirmed with the bond legth, bond angle and charge computations. The thermodynamic stability of graphene on silicon substrate is investigated through the variation of bond energy and bond density with graphene orientation. A global minimum is found in middle of armchair to zigzag configurations. The silicon-graphene shows two extinction peaks ans strong difference in the plasmon induced coupled phonon mode of vibration. The developed theoretical model and its numerical implementation are useful to get a higher level understanding of dynamical behaviors of various nanoscale devices.

Carbon nanotubes (CNTs) have been extensively researched for diverse applications in recent years. In the present work, the use of low frequency non-uniform alternating electric fields to manipulate the alignment behavior of CNTs in an epoxy matrix has been explored for use in structural applications. CNT alignment was accomplished based on the dielectrophoresis (DEP) principle. The alignment methodology was developed and CNT alignment effectiveness was assessed through in-situ current measurements and polarized Raman spectroscopy data in addition to optical microscopy. Electrical and mechanical characterization studies were then performed on the nanocomposites containing aligned CNTs and the improvement in properties with respect to the control samples was evaluated. Further, the alignment methodology was extended to the case of hierarchical composites with geometric discontinuities. The effect of CNT orientation on the open hole tensile behavior of uni-directional glass fiber-epoxy hierarchical composites containing CNTs was evaluated as a typical case of structural loading. Different electrode configurations were employed to control CNT orientation locally around the hole with respect to the loading direction. The results indicate that altering CNT orientation locally around the hole influences the overall response of the hierarchical composite. The thesis then discusses multiphysics simulations performed to model CNT behavior in epoxy resin considering time varying non-uniform electric fields and matrix viscosity. Considering a nanocomposite plate with a rectangular filleted notch subjected to tensile loading as a case study, numerical models were developed to enable electrode configuration design to control CNT orientation around the notch to mitigate stress concentration effects. Experimental studies were then performed with inputs from simulation studies facilitating the development of electrode set-up. The results indicate a significant enhancement in notched strength of the nanocomposite plates. Numerical and experimental studies on the development of nanocomposites containing varying concentration of aligned CNTs are also presented.

This thesis performs an LES study of the flow through a swirling nozzle. The nozzle has one single stream with swirl being generated by vanes mounted on an axially aligned centrebody. The end face of the centrebody coincides with the nozzle exit. The nominal Reynolds number based on the jet diameter and bulk flow velocity is 82,000 and the swirl number is 0.8. Two simulations are performed at this condition for a nominal end face diameter, Dc = 10mm (LES85A) and Dc = 5mm (LES85B). An additional test case for an Re ∼ 50, 000(LES50) was performed to validate the LES method against experimental flow field measurements for this nozzle at this condition. The LES results agree closely with PIV measurements. Coherent PVC oscillations are observed in the LES85B case. A linear stability analysis around the time averaged flow reveals the presence of a marginally stable helical mode that shows strong radial flow oscillations at the centreline, as may be expected. The mode is stable in the LES85A case. Structural sensitivity analysis reveals that the complex eigenfrequency of the mode is sensitive to changes in the linearised governing equations (structural perturbation) due to changes in the time averaged flow in the region corresponding to the merger of the wake behind the centrebody and the leading edge of the vortex breakdown bubble. Comparing mean fields between LES85A and LES85B shows that the reduction in Dc results in a large change in mean radial velocity alone at a z/D ∼ 0.1 , where the bubble and the centerbody wake meet. This can be conceptualised as the resultant of a localised radial force proportional to the oscillating velocities, being applied at the wake-bubble merger region. The relevant components of the structural sensitivity tensor terms that characterise the response of the eigenvalue to such a force suggests an increase in the growth rate and frequency. This is consistent with the marginally higher growth rate and frequency of the unstable linear mode and the presence of PVC oscillations in the LES for the LES85B case. Therefore, we conclude that the larger value of Dc in the LES85A case results in a nominally longer recirculation zone in the wake of the centerbody which then merges with the vortex breakdown bubble. This stabilises the PVC eigenmode and thereby, suppresses PVC oscillations.

Over the past decade, solar powered supercritical carbon dioxide (s-CO2) based Brayton cycle has been identified as a promising candidate due to its potentially high cycle efficiency (50%, for turbine inlet temperatures of ~ 1000 K). Materialization of this cycle requires development of solar receivers capable of heating s-CO2 by over 200 K, to a receiver outlet temperature of about 1000 K. Due to the extreme outlet conditions (~1000 K, 20 MPa), tubular solar receivers which typically employ panels consisting of metallic circular tubes for transfer of the incident concentrated solar radiation to the fluid flowing within the tubes are considered to be a suitable option for direct heating of s-CO2. In the design of receivers/heat exchangers for s-CO2 Brayton cycle, equipment wall temperatures above 1000 K are anticipated. While CO2 is considered to be transparent to the solar radiation spectrum, it has considerable absorption component in the longer wavelength range. This absorption effect may be present for s-CO2 also, yet its participating nature in radiation heat transfer has been traditionally ignored for flow through tubes. In this study, a numerical analysis using existing analytical data for s-CO2 absorption spectrum has been performed to study the fundamental aspects of a developing laminar flow of s-CO2 for a constant heat flux boundary condition. It is observed that while the velocity profiles remain largely unaffected, augmentation of overall heat transfer coefficient and Nusselt number due to presence of radiation heat transfer in addition to convection and conduction has a significant effect on the temperature distribution on the tube wall and its vicinity. It is found that for accurate design and estimation of heat transfer performance of s-CO2 equipment, the participating nature of s-CO2 can be critical for laminar and low Reynolds number turbulent flows. In general, the effect of absorption can be increasingly significant for lower values of Reynolds number and larger values of tube internal emissivity, tube diameter, tube length and the incident heat flux. In addition to absorption of radiation, emission by s-CO2 may also be significant and has been ignored in the literature in spite of the high temperatures involved. To investigate this aspect, a novel experimental method for measurement of radiation emitted by s-CO2 at high pressure and high temperature is presented in this thesis. Due to high pressure conditions, use of conventional spectroscopic methods to measure radiative properties of s-CO2 is challenging. In the present method, supercritical conditions are created in a shock tube by using carbon dioxide (CO2) as the driven gas, and a platinum thin film sensor is used to measure the radiation heat flux emitted by s-CO2. The total emissivity for s-CO2 is estimated and the value compares favourably with that predicted theoretically using a standard method available in literature. It is estimated that the total emissivity value in supercritical conditions is nearly 0.2 for the conditions studied, implying that s-CO2 acts as a participating medium for radiation heat transfer. The outcome of this study has a significant impact on the design and analysis of heat transfer equipment where laminar or low Reynolds number turbulent flows are encountered. For accurate and realistic design of a tubular solar receiver, a novel methodology for coupled optical-thermal-fluid analysis is presented in this work and the proposed methodology is utilized for developing a prototype of s-CO2 receiver consisting of panels constituted by tubes. First, a preliminary analysis is presented, detailing the methodology for coupled analysis. The effect of staggering the tubes to increase the effective absorptance and reduce the reflective losses is explored. A receiver consisting of a single large panel is analysed to establish the methodology and to estimate the tube wall temperature and efficiency for a typical incident flux distribution on the receiver tubes in conjunction with flow of s-CO2 through the tubes. Subsequently, detailed optical-thermal-fluid analysis and design of a s-CO2 tubular receiver with flat panels is performed. Different flow arrangements with and without recirculation, aim point strategies and power levels of operation are studied for comprehensive evaluation of the receiver performance under different conditions. It is found that the receiver designed is able to provide the required temperature rise while the pressure drop and peak receiver temperatures are within allowable limits. As found in a recent study at Sandia National Laboratories, arrangement of the receiver panels in the form of blades can result in an increase in the overall receiver efficiency by up to 5 % compared to the flat receiver arrangement, due to better optics. This bladed receiver arrangement is adopted in the final stage of this work for modelling, testing and design validation of the s-CO2 receiver using compressed air as the heat transfer fluid. Details of the bladed receiver configuration, coupled modelling, prototyping and testing are presented in this thesis. For high temperature on-sun testing on a solar tower, despite the limited availability of pressurized air, a unique test strategy is employed. The receiver is successfully demonstrated to safely heat air up to a temperature of 700 K, with receiver wall temperatures approaching 1000 K. To account for the thermal mass associated with the transient nature of the tests and heating of the non-irradiated part of the receiver structure during the on-sun tests, a modified receiver efficiency calculation is proposed in this work. The agreement between the measured and simulated values of receiver efficiency, temperature and heat flux distributions is found to be satisfactory.

Sedimentation – settling of particles in a fluid- is observed in nature like rain droplets and dust particles in the atmosphere, and in a variety of industrial processes, like to clarify liquid as well as separate particles of different size and density. The simplest system is the sedimentation of mono-disperse particles in a vast stationary fluid. The main parameters are Particle Reynolds Number (〖Re〗_p based on terminal velocity), ratio of particle density to fluid density (ρ_p/ρ_f ), particle volume fraction (φ), and container dimensions for experimental and numerical methods. Two main questions arise: what is the mean settling velocity (V_g), and nature and values of fluctuation in particle velocity (V^/), and how do they compare with the terminal velocity (V_t) of an isolated particle in an infinite fluid. At low particle Reynolds number, V_t is given by the Stokes law. Experiments have been typically performed in a tank containing the fluid with particles initially well mixed and tracking the motion of the particles or performing PIV to obtain mean settling velocity (V_g), fluctuating particle velocities (V^/) etc. The main focus of these studies has been to correlate different parameters like mean settling velocity, velocity fluctuation, correlation length with volume fraction, and dimension of the container. Though this apparently simple problem has been studied theoretically, experimentally, and numerically over many decades, there are several unanswered questions. For example, the experimental results for velocity fluctuations do not agree with the theoretical predictions. The origin of scalings for velocity fluctuations are unclear. In our study, we try to address some of these issues using a new type of experiment. In our experiment, particles are fed at a constant rate at the top and allowed to settle in a long vertical tube containing quiescent fluid, closed at the bottom. The constant particle feed rate ensures mean steady particle settling in contrast to the standard experiments done previously where the settling process is transient. Also, the long vertical extent of the tube ensures Axial Homogeneity. We have done two types of experiments: water droplets (10 μm, 〖Re〗_p~〖10〗^(-3)) falling in the air, and spherical glass beads (110 μm, 〖Re〗_p~1 ) settling in water. The estimated volume fractions for the former is 〖10〗^(-7) and for the latter, it is〖 10〗^(-3). For the droplet-air system, the tube dimension is 5×5 〖cm〗^2 and for the particle-water system, three tube dimensions (4×4 〖cm〗^2 , 5×5 〖cm〗^2, 7×7 〖cm〗^2 ) have been used. Experiments have been done with different mass flux values. We have used high-speed imaging illuminated by a sheet of laser light to visualize the particle motion fields and Particle Image Velocimetry (PIV) to get the mean and fluctuating particle velocities and the spatial and temporal correlations. We have observed a variety of sedimentation-induced convective motions, including regions of particle patches moving upwards. The conditions at the tube end significantly alter the convective patterns and the fluctuating velocities. Convective motions, though hypothesized to exist, have not been observed in earlier experiments. We present results for the mean and fluctuating velocities and spatial and temporal correlations of the velocity fields for the range of mass fluxes and different tube dimensions. Besides the existence of convective motion, the main findings are: the mean settling velocity varies between 0.80-1.1 V_t. The fluctuating velocities are in the range 0.30-0.80 V_t and strongly depend on mass flux. Correlation lengths scale with tube width. We present these results in a non-dimensional form which suggest different scaling laws.

Environmental issues caused by the non-biodegradability of synthetic thermoplastics have increased interest in more environmentally friendly alternatives that should be derived from renewable resources. This work focuses on producing green composites and biofilms by synthesizing nanocellulose (NC) from readily available natural and renewable sources like cellulose. The term NC refers to cellulose that has been reduced to the nanoscale. NC is used to describe a variety of cellulose nanostructures, including Micro Crystal Cellulose (MCC), Cellulose Micro Fibrils (CMF), Cellulose nanocrystals (CNC), and Cellulose nanofibers (CNF). Because of its versatility, low toxicity, biodegradability, and carbon neutrality, nanocellulose has attracted considerable attention for generating new materials in several industrial, technological, and biological applications. Nanocellulose has a sizable global market and demand because of these numerous applications. One of the challenges for commercial and industrial production of nanocellulose is finding a source of cellulose that is economically viable, abundant, and sustainable. Bacteria, plants (including trees, shrubs, and herbs), algae, and animals (Tunicates) are the primary sources of nanocellulose. In this study, cellulose nanofibers are synthesized from the bamboo pulp through TEMPO oxidation, high- pressure homogenization, and ultrasonication treatment. Characterization of CNF was done using Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Fourier-Transform Infrared Spectroscopy (FTIR), X-ray powder Diffraction (XRD), and Dynamic Mechanical Analysis (DMA). After characterization, the synthesized NC was evaluated by turning it into films and using it as reinforcement to prepare composites. Bamboo particles are reinforced with CNF suspension to prepare composites using the hot press (HP) and oven-dried (OD) methods. The mechanical properties of these fabricated composites are investigated, such as modulus of elasticity (MOE), modulus of rupture (MOR), and interfacial strength. The properties of the NC and composites are compared for 1st , and 3rd homogenization passes. The mechanical characteristics of the film have been investigated using uniaxial tensile and nanoindentation experiments. The morphological characteristics examined using SEM and TEM showed that the cellulose in the bamboo pulp is reduced to the nanoscale, confirming the formation of CNF. The average diameter of nanofibres of bamboo calculated from TEM images was 8 to 10 nm, respectively. The CNF analyzed using FTIR spectroscopy exhibited the presence of functional groups and their vibrational modes. Further, crystal size (CS)andcrystallinity index (CI) of CNF synthesized from different homogenization passes were calculated usingthe XRD technique. Properties like viscosity, storage modulus, and loss modulus were determined from the rheological characterization, which confirmed the non-Newtonian behavior of pure NC suspension. In addition to morphology analysis, mechanical properties of NC films computed from uniaxial tensile test and nanoindentation showed a 58 MPa tensile strength and a 0.2228 GPa hardness, respectively. Also, dynamic mechanical properties like storage modulus, loss modulus, and tanδ were determined to examine the viscoelastic behavior of the film as a function of temperature. However, from the results, no phase transformation was noticed until 75°C. Furthermore, composites reinforced with 1 % weight consistency of CNF showed an increase in interfacial strength, MOE, and MOR as a function of density. The MOE increased 9 times for HP samples compared to the OD samples. MOR nearly increased by a factor of 4, and the energy-absorbing capacity also increased to 182% in the case of HP samples. From these results, it can be inferred that hot-pressing was an effective manufacturing method than oven drying as it removes maximum moisture content and enhances adhesion between bamboo powder and CNF. Therefore, it can be concluded from the results that nanocellulose derived from bamboo has highly entangled fibers, which can be transformed into a film and used in packaging and biomedical applications. Also, bamboo CNF acts as a binding agent to prepare composites. Hence, the concept of using matrix and reinforcement derived from the same natural sources can be used to make green composites. Consequently, there is a great deal of potential for the TEMPO-oxidized bamboo celluloses to develop into ground-breaking nanotechnology that will connect the domains of biomass and forest refinement with cutting-edge high-tech research.

The topic of this research in this thesis is creativity assessment of solutions developed in the engineering design process as a means of aiding designers to focus their creative effort during the process. Both engineering design process and creative process are planned, and occur, together in practice. Engineering design is a complex process involving both problem finding and problem solving, broadly divided into four stages: task clarification, conceptual design, embodiment design and detailed design. Literature on design models, theories and approaches is studied to understand the nature of design models developed in literature and to find the various characteristics of the design process, such as design stages, inputs and outputs of the stages, etc., leading to a detailed understanding of the current models of the design process, and summarised using an extended, integrated model of designing. In particular, a detailed characterization of the design stages is carried out using the extended-integrated model of designing. The method of mapping is used to extract the underlying common constructs in the steps within task clarification, conceptual and embodiment design stages of the design process, so that these can be used in the evaluation of creativity consistently across the design stages. The results of this characterization are discussed and summarized with a focus on creativity assessment methods. It is found from the characterization that requirements and solutions are important constructs of designing. Further, it is found that pairing of requirement and solution is necessary for evaluation of creativity during the design process. It is also found that level of abstraction is important, as requirement-solution pairs belong to various levels of abstraction. The concept of creativity, and major indicators of creativity, are reviewed through a study of literature. Current methods for assessing creativity are also reviewed, and their shortcomings identified. It is found that, novelty and usefulness are the two umbrella indicators of the creativity. However, it is also found that it is not possible to use usefulness during the design process. Therefore, requirement satisfaction is proposed as a proxy indicator for evaluation of usefulness. Using the above, a creativity assessment method is proposed to address the limitations found in the literature. The creativity assessment method is described in detail and an example is used to explain how the method can be applied during the design process. In this example, the application of the method in guiding the effort during the design process is also explained. Further, as to how the method should be able to overcome the limitations of earlier methods is discussed. Evaluation of the method is carried out for its correctness, and partially for its usability and impact. Correctness is evaluated by comparing the assessment using the method with the benchmark – the collective assessment by a group of experienced designers as judges; the comparison showed that the assessment using the creativity assessment method closely matched those of the benchmark. It was also found that assessment of degree of requirement satisfaction (one part of the above creativity assessment method) during the design process led to an improvement in creativity of the designs produced.

A particular form of winged seed (samara) dispersal technique adopted by nature uses autorotative (unpowered rotation of the wing generating thrust force against gravity) descent; for eg. in Maple and Mahagony trees. This technique provides the lowest descent velocities among various seed dispersal techniques found in nature ensuring the safety of the delicate seeds. Bio-mimicked solutions to important engineering problems in aerospace as well as in disaster management - air dropping of life-saving packages during floods can be inspired from the samara. The samara is a complex structure having a bluff root containing the seed attached to a three dimensional wing. The dynamics of the samara from the instant of release is entirely unsteady, involving an initial transition phase where the samara tumbles until it achieves autorotation leading to a steady descent velocity. The distribution of mass and aerodynamic forces in this single structure ensures its stability during descent. Studies to comprehensively understand the physics of the samaras are limited. Recently, Leading Edge Vortex (LEV) has been found to be responsible for the high thrust forces achieved during autorotation. The dependence of LEV on the morphology of the seed needs to be understood to design optimal devices for engineering applications. The principal aim of this study is to understand the effect of morphology on the aerodynamics of the samara with a particular focus on the characteristics of the LEV. The flow field around the autorotating samara is experimentally obtained using Particle Image Velocimetry (PIV) in a specially designed vertical wind tunnel. However, since each natural samara is intricate, optimal, and unique, it has limited utility for parametric studies. Therefore, 3D printed models are developed that closely mimic the functions of the natural samara. A new design methodology has been developed to generate autorotating samara models. Drop tests of the natural samara and the 3D printed models show that the dynamics of the models and the samara are similar. Three 3D-printed samara models with different spanwise distributions of chord and mass are considered. For the first time, a complete characterization of the spanwise distribution of LEV has been carried out on the samara models. We show that in the neighborhood of the maximum chord location multiple LEVs are present, which leads to significantly higher local lift forces compared to other cross-sections near the root and the tip. The elaborate spanwise survey also shows that the locations near the wing tip behave similar to a bluff body, while sections near the root undergo a reversal of flow topology. A new non-dimensional parameter has been defined using Buckingham 𝜋 analysis that encompasses the dominant parameters involved in the study. This enabled us to understand the inter-relationship between observed flow physics, morphology and performance parameters.

Shock wave unsteadiness is an important phenomenon in high-speed aerodynamics. This phenomenon is observed in many locations of high-speed vehicles, such as supersonic inlets, ramjet engines, transonic airfoils, high-speed fans, and compressors. In supersonic inlets and ramjet engines, unsteady shock motions can lead to large undesirable local fluctuations in properties such as pressure and heat transfer rate, besides overall thrust fluctuations. Shock unsteadiness in transonic airfoils can induce structural vibrations known as buffeting, while in gas turbine fans/compressors, shock oscillations can lead to blade vibrations known as flutter. Motivated by the above problems, the purpose of the present experimental study is to understand the response of a normal shock subjected to downstream pressure perturbations. Although several studies pertaining to shock dynamics due to downstream pressure perturbations have been reported in the literature, only a few of them have concentrated on the effect of downstream perturbation to the normal shock behavior in a constant area duct, with detailed flow field measurements that can help to understand the flow physics that drive these shock oscillations. With this in mind, in the present work, a detailed experimental study of shock dynamics due to downstream pressure perturbations within a constant area duct have been done in two different configurations. The first configuration is one in which the downstream pressure perturbations are generated in the far field region by rotating a triangular cross-sectional shaft, this being an idealization of inlet shock dynamics in ramjets caused by downstream combustion chamber pressure fluctuations. The second configuration is one in which the downstream pressure perturbations are generated in the near field region by heaving an airfoil, and this may be considered as an idealization of the unstarted cascade flutter of high-speed compressors. In both cases, the normal shock is induced and stabilized at low Mach numbers (M∞ ∼ 1.3) within a supersonic/transonic tunnel, and the shock dynamics in response to the downstream pressure perturbations are visualized using high-speed shadowgraphy. In addition to the high-speed shadowgraphy, high-speed wall pressure measurements have been carried out in the first configuration, and in the second configuration, unsteady force measurements have been carried out to understand the influence of shock oscillations on airfoil flutter. In the far field pressure perturbation case, pressure perturbations are generated by rotating a triangular cross-sectional shaft which is 580 mm downstream from the normal shock. The normal shock is induced and stabilized in the constant area section of a supersonic wind tunnel which is operated at M∞ = 1.4. The main parameter varied in this case is the perturbation frequency ( f ), which is varied from low frequencies to 60 Hz in steps of 10 Hz. High-speed shadowgraphy visualizations indicate that the shock oscillates in response to the exciter perturbation frequency, with a phase difference between exciter motion and the shock displacement. The shock shows large streamwise motions (up to 60 mm), with distinct differences in the shock structure and velocity during its upstream and downstream motions. It is also observed that the amplitude of shock motion decreases with increase in perturbation frequency, while the shock velocity is almost independent of the perturbation frequency. The results from-high speed pressure measurements indicate that the downstream pressure fluctuations are nearly 3-5% of the mean static pressure at the exciter region. In the near field pressure perturbation case, pressure perturbations are generated by heaving an airfoil (at frequency f ) with its leading edge being 0.1 chord length downstream from the normal shock. The normal shock is induced and stabilized in the constant area section of a transonic wind tunnel which is operated at M∞ = 1.3. The parameter varied in this case is the reduced frequency (k = π f c/U), which is varied from low values up to 0.264. Flutter characteristics of the airfoil are deduced in terms of the energy transfer to the heaving airfoil from the measured unsteady loads, and it indicates that there are two excitation regions, one corresponding to lower reduced frequency and other corresponding to higher reduced frequency, which is similar to the case of unstarted cascade flutter observed by Jutur (2018). High-speed shadowgraphy visualizations have been carried out at different airfoil heave frequencies, and the results indicate that the shock oscillates in response to the airfoil heave motions, with the phase between the shock motion and the airfoil motion being dependent on the reduced frequency. The correlation between the shock motion and airfoil position indicate a negative correlation value at k = 0.049, and for all cases with k ≥ 0.117, it is positively correlated. In summary, measurements from both configurations indicate that the shock oscillates in response to the exciter perturbation frequency, with a phase difference between shock motion and exciter motion. This phase difference observed between the shock displacement and the exciter for variation in perturbation frequency in the first configuration may be attributed to shock wave boundary layer interactions, while in the second configuration it is the phase of the unsteady shock motions with respect to the airfoil motion that is important in deciding the flutter characteristics of the downstream airfoil. Further, in both the configurations, the amplitude of shock motion is found to be decreasing with increase in perturbation frequency

Manipulation of an array of sessile droplets organized in an ordered structure turns out to be of immense consequence in a wide variety of applications ranging from photonics, near field imaging and inkjet printing on one hand to bio-molecular analysis and DNA sequencing on the other. While evaporation of a single isolated sessile droplet has been well studied, the collective evaporative dynamics of an ordered array of droplets on a solid substrate remains elusive. Physically, the closed region between the centre and side droplets in the ordered array reduces the mobility of the diffusing vapour, resulting in its accumulation along with enhanced local concentration and a consequent increment in the lifetime of the center droplet. Here, we present a theoretical model to account for evaporation lifetime scaling in closely placed ordered linear droplet arrays. In addition, the present theory predicts the limiting cases of droplet interaction; namely, critical droplet separation for which interfacial interaction cease to exist and minimum possible droplet separation (droplets on the verge of coalescence) for which droplet system achieves maximum lifetime scaling. Further experimental evidences demonstrate the applicability of the present scaling theory to extended dimensions of the droplet array, generalizing our physical conjecture. It is also worth noting that the theoretical timescale is applicable across a wide variety of drop-substrate combinations and initial droplet volumes. We also highlight that the scaling law proposed here can be extended seamlessly to other forms of confinement like an evaporating droplet inside a mini channel as encountered in countless applications ranging from biomedical engineering to surface patterning. Having established the framework of collective dynamics of droplet evaporation, we turn our attention to the case of self-agglomeration deposits, which are observed in evaporating sessile linear array of droplets. When a spilled drop of coffee dries on a solid surface, it leaves a dense, ring-like deposit along the perimeter, i.e., forming “coffee ring” on the surface. Ring-like stains are not particular to coffee and are commonly seen in the droplets containing dispersed solutes. Many of the industrial application requires uniform deposition like in inkjet printing, genotyping and complex assembly. In this section, we have presented the mixing of colloidal particles suppresses the coffee ring stains and forms uniform deposition. One of the promising areas where the self-aggregation of colloids is highly useful is photonic crystals. Photonic crystals have emerged as a potentially powerful platform that were previously impossible. Photonic crystals have emerged as a powerful tool to achieve light manipulation. Central concept for the photonic behavior is the formation of a photonic ‘band gap’ - a range of frequencies for which light is forbidden to exist within the bulk of the photonic crystal. The presence of a band gap depends on a particular periodic structure within the crystal. Self-agglomeration of colloidal particles can also form periodic arrays, which may serve as a template of photonic crystal. To find out the arrangement of the colloidal particles, packing fraction of colloidal particles have been calculated from centre to the edge of the agglomerate. Optical reflectance shows the variation in the reflectivity along the diametrical line which have been correlated to the peak reflectance of light beam incident on the colloidal crystal. We show that by controlling the evaporative behavior of the droplet in a linear array, it is possible to effect changes in the photonic behavior of the final precipitate.

Wall-bounded turbulent flows that possess finite curvature of the mean flow streamlines are known to be starkly different from their plane counterparts. One of the primary distinguishing features of such flows is the existence of a distinct mechanism of instability (centrifugal instability) that is altogether absent in planar flows. This fact is seen to have deep consequences with regards to flow dynamics and coherent structures and begs further inquiry into the underlying physics. With this as the primary motivation, we have chosen a wide-gap Taylor-Couette (TC) configuration with rotating inner cylinder and a fixed outer cylinder as a model problem to isolate and study the effects of flow curvature. Direct numerical simulation (DNS) of the incompressible Navier-Stokes equations in cylindrical polar co-ordinates has been employed for obtaining high-resolution spatio-temporal data of the flow field. The radius ratio and the aspect ratio chosen for the simulations are $0.1$ and $5.5$ respectively. Accurate representation of the flow field in such a large domain demands significant resolution in space and time which directly translates into large memory requirements and runtime of the DNS code. As an attempt to alleviate these issues, the first part of the study deals with the implementation of an efficient multi-core algorithm using an influence matrix based domain decomposition technique. Using this efficient code, we proceed to study the centrifugal instability that occurs around an impulsively rotated cylinder in otherwise quiescent fluid. We find that the critical wavelength and the critical boundary layer thickness (i.e. $\lambda_{c}$ and $\delta_{c}$) at the onset of the instability scale with Reynolds number as $Re^{-2/3}_{a}$. A critical Taylor number defined as $Ta = Re^{2}_{a}(g/a)^{3}$ ($g$ is the gap-width and $a$ is the radius of the inner cylinder) with $g$ being replaced using either $\lambda_{c}$ or $\delta_{c}$ is found to achieve a constant value at large Reynolds numbers. This result is analogous to the one obtained by Taylor \cite{Taylor1} wherein the critical Taylor number for small-gap approximation is $\sim1708$, independent of the Reynolds number. The investigation proceeds to a systematic study of the statistically stationary flow in the annulus of the wide-gap TC configuration described above. Both axisymmetric and fully three-dimensional simulations are performed to obtain the mean and turbulent statistics for $5<(Ta/Ta_{c})<5\times10^{4}$ ($Ta_{c}$ is the critical Taylor number for the onset of the first instability). A striking feature of the flow is the presence of significant asymmetry in the flow dynamics at the two walls as evidenced by both the global torque fluctuations and the spatio-temporally averaged velocity profiles. It is found that the mean azimuthal velocity profiles near the inner and outer walls plotted in wall co-ordinates (i.e. $\bar{u}_{\theta}/u_{\tau} = F((r-a)u_{\tau}/\nu))$ significantly deviate from the classical log-law profile. Guided by the fact that for curved flows a finite production of energy exists for the radial r.m.s velocity component, we seek an alternate velocity scale (to friction velocity) which is in many respects similar to the Deardorff scale \cite{Deardorff} commonly used in free convection. Starting from Karmans' law of the wall and assuming the profiles far from the wall to be independent of fluid viscosity, we obtain the result that $r\bar{u}_{\theta}\sim(r-a)^{-\frac{1}{3}}$ in the inertial regime. A similar result was obtained by Claussen \cite{Claussen} whose argument was based on Monin-Obhukov similarity theory as applied to circular Couette flow. Surprisingly, the profiles of mean angular momentum near the inner wall for both the axisymmetric and fully 3D simulations display this scaling, while the profiles near the outer wall do not; a fact that further elucidates the asymmetry of the flow dynamics at the two walls. The mean temperature profile of convective turbulence is found to have a similar variation in the inertial regime (i.e. $\overline{T}\sim z^{-1/3}$)\cite{Priestley}, a fact that hints at similarity between buoyancy driven and curved wall-bounded turbulent flow. Further, we explore the scaling of the time averaged torque with Reynolds number and find that the torque non-dimensionalized by the laminar torque (a ratio analogous to the Nusselt number of free convection) scales with Taylor number as $(Ta/Ta_{c})^{0.21}$. The exponent obtained here is significantly smaller than $0.33$, the exponent predicted by marginal stability theory (MST) of Marcus \cite{Marcus1} which although is applicable only for small gap-widths. We propose that the torque scaling obtained is directly related to the boundary layer dynamics, with the boundary layer thickness determined by centrifugal instability. Next, we discuss some salient flow features both with regards to the time averaged and the instantaneous flow. Time averaged flow fields display large scale structures with wavelengths that are typically larger than the wavelength of the Taylor vortices of the initial instability. The wavelengths of these large scale structures are seen to increase with Taylor number, a result that lends some support to the experimental findings of \citet{Burkhalter}. Instantaneous flow fields are investigated to identify some plausible coherent structures in the flow that may be responsible for the torque transport. $\lambda_2$ criterion as proposed by \citet{Jeong} is used for this purpose and its contours at a particular value show the presence of stream-wise oriented vortices that resemble those predicted by G\"{o}rtler \cite{Gortler} to be the dominant structures in a centrifugally unstable boundary layer.
DST

The injector faceplate of a liquid propellant rocket engine is comprised of numerous single-element atomizers to inject propellants into the engine thrust chamber. The sprays from these single-element atomizers interact and mix, and then develop a combined spray. The present study investigates the characteristics of combined spray from a multi-injector assembly discharging three identical hollow cone swirl sprays arranged in an equilateral triangular configuration. The experiments are carried out in a spray test facility using water as the experimental liquid for different values of pressure drop (ΔPl) across the atomizers. The images of combined spray, captured using the technique of backlighted shadowgraphy, are used to deduce quantitative details of spray interaction behavior, and laser-based optical diagnostic systems (Phase Doppler Interferometry, and Spraytec) are used to record droplet characteristics of the combined spray. A mechanical patternator is used to describe the evolution of liquid mass distribution of the combined spray at different values of axial distance (Z) from the atomizer exit. The interaction process between the individual sprays influences spray width and liquid sheet breakup characteristics of the combined spray, particularly for sprays with low ΔPl. The interaction zones of the combined spray are marked by three lobes of high liquid mass flux, which develop asymmetry in the spray cross section perpendicular to the spray axis. It is showed quantitatively that the level of asymmetry in the combined spray decreases with increase in Z. The analysis of droplets characteristics of the combined spray reveals the presence of droplet coalescence for sprays with low ΔPl and droplet shattering for sprays with high ΔPl, which highlights droplets collision effects caused by the interaction and mixing of individual sprays in multi-injector thrust chamber.

A methodology has been developed for the computation of phononic band structure of cellular periodic structures, structures with spatially repetitive patterns, using Frequency domain Spectral Finite Element Method along with Bloch Theorem. Band structures depict the nature of wave propagation in periodic structures and are characterized by the presence of banded frequency zones where wave may or may not propagate. These zones are known as 'Pass Bands' and 'Stop Bands' respectively. Bloch theorem is conventionally used for such band structure computation in order to reduce computation of an infinite structure to that of a single unit cell- a unit of the structure containing the repeating pattern, and to enforce interrelationship between equivalent points in the unit cell. Frequency domain Spectral Finite Element Method uses exact generic solutions as shape functions, and can be very efficient for high frequency wave propagation problems. Wittrick-Williams method has been used to solve the resulting eigenvalue problem. The impact of periodic defects in the wave propagation has been studied through the use of supercell, a unit cell comprising multiples of the primitive unit cells, the smallest unit cells. Toward that end, the band structure of a supercell has been compared with that of the primitive cell and the differences are analyzed. A methodology has also been developed for the band structure computation of 3-D cellular structures with tapered elements using a Frequency domain Ritz Method. Wave propagation in various types of pentamodes has been taken as examples. A pentamode is a type of mechanical metamaterials possessing semi-fluidic properties. The auxetic cellular materials, structures having negative Poisson's ratio, is an important class of cellular structures. A novel design has been proposed that seamlessly combines conventional and auxetic honeycomb cores to develop cellular structures with a significantly larger bandgap than the pristine counterparts. Subsequently, a methodology has been developed to efficiently account for the effect of joints in the cellular structures using Spectral Superelement Method, the Finite Element Method that combines Frequency domain Spectral Finite Element with conventional Finite Element Method. The impact of various types of inclusions and features have also been investigated using this framework. Lastly, the methodologies developed so far have been used to present an atomistic-continuum modeling of graphene, the 2-D allotrope of carbon, by comparing the phononic band structure of the proposed model against the standard Density Functional Theory based band structure, as found in literature. The atomistic-continuum model parameters, thus arrived at, may be used for an approximate computation of very large systems where sophisticated methods could be prohibitive.

Ribbons are slender structures characterized by three disparate geometric dimensions: length >> width >> thickness. Such a dimensional disparity enables ribbons to bend, buckle, twist and crease response to simple loading conditions. Their nonlinear deformation behavior, once considered a hindrance, is now routinely exploited in engineering applications related to stretchable electronics and flexible robotics. Such applications demand a systematic understanding of the mechanics of elastic ribbons using experiments, modeling, and simulations. This thesis is a step in this direction. Experiments using annulus-shaped ribbons and Moebius strips serve as our point of departure. The critical challenge in these experiments lies in measuring complex three-dimensional deformations observed. Routinely used techniques turn out to be inadequate, either due to the compliance of ribbon structures (e.g., contact probes, strain gauges) or due to the large displacements and rotations involved (DIC). We leverage novel computer vision techniques developed in the lab to faithfully digitize shapes and sample deformation maps of ribbons in the experiments. These measurements lead us to the main contributions of this thesis--- a detailed examination of the predictive capabilities of commonly used modeling approaches and the formulation of a dedicated one-dimensional ribbon model. The physical appearance of ribbons motivates modeling them either as thin elastic plates or as elastic rods having a slender cross-section. Widespread adoption of the von Karman plate theory and the Kirchhoff rod model exemplifies this dichotomy. Somewhat surprisingly, comparing finite element simulations of these models with experimental measurements reveals both approaches to be deficient, even in simpler scenarios than ones where they are routinely used. These studies show that it is essential to permit large displacements and rotations in ribbon models and that compliance in the direction of the width, though small, plays an important role. Indeed, the experiments with annular ribbons and Moebius strips are designed to highlight these deformation features. We propose adopting the small-strain Cosserat plate theory instead. The model's generality, along with a robust finite element implementation that addresses issues of numerical locking by adopting high order elements and approximating large rotations using exponential maps, translates to excellent agreement with experimental measurements. The model faithfully reproduces measured shapes, displacement fields and curvature distributions, as well as bifurcations and energy localization phenomena observed in experiments. We then propose a dedicated reduced-order one-dimensional ribbon model by systematically incorporating kinematic assumptions in the plate theory. The model is Sadowsky-type theory that requires one additional field to describe lateral curvatures along the width of the ribbon. We examine the model's predictions through challenging examples, including one involving twist-induced snap-through. The model promises to be a valuable tool to develop insights into the mechanics of ribbons, besides being a compelling alternative to the Sadowsky and Wunderlich ribbon models routinely used in the literature.

Fiber reinforcement plays an important role in the structure and function of biological materials. Soft connective tissues in human body like artery, heart tissues, skin etc. exhibit anisotropic material responses due to the orientation of fibers along specific directions. At a cellular level, stress fibers in the cytoskeleton play an important role in maintaining cellular shape and influence cell adhesion, migration, and contractility. Cells respond to changes in their mechanical milieu and drive biochemical processes which induce growth and remodeling of the underlying material properties. This results in non-uniform changes to the structural form and function over time. Continuum mechanics-based approaches to address biological growth and remodeling demonstrate an intimate relationship between the cellular level mechanobiology and the underlying tissue properties. How does orientation of fibers affect mechanical response of tissues? How do mechanosensing processes influence cellular growth and remodeling under stretch? I have combined experimental techniques and analytical models, to quantify structure-property correlations in cells and biomimetic materials under stretch. In the first study, we investigated the role of fiber orientations in the mechanics of bioinspired fiber reinforced elastomers (FRE) fabricated to mimic tissue architectures. We fabricated FRE materials in transversely isotropic layouts and characterized the nonlinear stress-strain relationships using uniaxial and equibiaxial experiments. We used these data within a continuum mechanical framework to propose a novel constitutive model for incompressible FRE materials with embedded extensible fibers. The model shows that the interaction between the fiber and matrix along with individual contributions from the matrix and fibers were crucial in capturing the stress-strain responses in the FRE composites. The deviatoric stress components show inversion at fiber orientation angles near the magic angle (54.7°) in the FRE composites. These results are useful in soft robotic applications and in the biomechanics of fiber reinforced tissues. Secondly, we apply these formulations at a cellular level to quantify the role of stress fiber elongation and realignment to changes in cellular morphomechanics under uniaxial cyclic stretch. Cyclic uniaxial stretching results in cellular reorientation orthogonal to the applied stretch direction via a strain avoidance reaction. We show that uniaxial cyclic stretch induces stress fiber lengthening, realignment and increase in cortical actin in fibroblasts stretched over varied amplitudes and durations. Higher amounts of actin and realignment of stress fibers were accompanied with an increase in the effective elastic modulus of cells. We modelled stress fiber growth and reorientation dynamics using a nonlinear, orthotropic, fiber-reinforced continuum representation of the cell. The model predictions match the observed increased cellular stiffness under uniaxial cyclic stretch. As a final study, we have designed and fabricated a microscope mountable cell-stretching device and used it to quantify stretch-induced changes in cellular contractility by measuring the changes cellular traction forces. Our results show a significant 2 increase in cellular traction forces when subjected to prolonged duration of cyclic stretch. Together, these studies demonstrate the importance of uniaxial stretching in mechanotransduction processes which are essential in understanding growth processes and in disease models of fibrosis.

Burgeoning dependence on fossil fuels for transport and industrial sectors has been posing challenges such as depletion of fossil fuel reserves, enhanced greenhouse gas (GHG) footprint, and imminent changes in the climate, etc. Fossil fuels are regarded as unsustainable and questionable from economic, ecology and environmental point of view. Therefore, the pursuit for an environmentally benign and sustainable source of energy started with the advent of oil crisis in 1970, major focus of energy policies shifted to biofuel production from renewable resources. Biofuels minimizes the fossil fuel burning and CO2 production, as it is produced from biomass such as plants or organic waste, thereby decreasing the dependence on oil. The review on evolution of biofuel production from first to second-generation feedstock revealed that the process requires higher concentration of chemical usage due to the presence of lignin. In addition, this feedstock requires arable land and freshwater source for their large-scale cultivation. Recent studies on biofuels indicate that algal biomass; particularly from marine macroalgae (or seaweeds) have the potential to supplement oil fuel Macroalgae with higher concentration of varied carbohydrates (constituting as a source of energy) are emerging as a potential and renewable feedstock towards sustainable biofuel production due to their higher growth rate and availability. Macroalgal biomass cultivation with biorefinery approaches not only aid in empowering rural women with the better economic prospects, but also aid in addressing strategic energy security of the nation. Ceasing of cultivation of salt tolerant paddy and recurring losses with the prawn cultivation (due to viral diseases); have forced abandoning of gazni lands, which can now be used for large-scale cultivation of macroalgae. Macroalgae biomass with higher composition of carbohydrates is relatively higher in gazni, ponds compared to natural rocky shores. In addition, harvesting of algal feedstock in gazni ponds is economical compared to sparsely distributed macroalgae in rocky stretches of the region. Biorefinery approach of macroalgae and macroalgal cultivation for this purpose in Aghanashini estuary could be a good income-generating proposition for the fishermen as well as aid in empowering coastal women

This thesis deals with ultrasonic guided wave based non-destructive inspection and health monitoring of aircraft structural components using a non-contact sensing scheme based on laser Doppler scanning imaging technique. Ultrasonic guided waves are elastic waves which travel relatively long distance and are guided by the geometry of thin structures with minimal loss in amplitude. Experiments are conducted using piezoelectric actuator and a 3D scanning Laser Doppler Vibrometer (LDV). The piezoelectric actuators bonded on the structure excite ultrasonic guided waves. The laser beam from the LDV senses the surface vibration based on optical interference of the laser beam. The optimal frequency range is selected based on the wave dispersion characteristics, wave mode strength, thickness of the test specimen and size of the defects that need to be determined. Analytical models are developed to determine these relations and signal calibration. For defect detection, schemes for both baseline-free and baseline-dependent algorithms are developed. The baseline-dependent algorithms use amplitude, phase and Time-of-Flight (TOF) information to detect defects. Experiments are conducted on large structural panel with inaccessible defects. The measurements are obtained from sparse sensing points which are in form of either compact or a sparse distributed array on the accessible side of the panel. Triangulation technique along with TOF information is used to localize the defects. Sparse measurements are used to reconstruct virtual spatio-temporal wavefield using a pixel-based image reconstruction technique. The schemes allow fast inspection of large and complex structural parts without structural disassembly. In a baseline-free detection process, the schemes that are developed include a spatio-temporal wavefield imaging scheme, a spectral power flow scheme, a local wavelength and mode conversion strength-based scheme. The spatio-temporal ultrasonic wavefield reconstructed using scanning LDV is further used to understand the wave scattering behavior in the presence of defects. Further, the spectral power flow-based scheme is developed that can estimate the location and size parameters of defect simultaneously. This scheme uses an invariance property of the guided wavefield to detect defects. A spatial power flow map is further derived from this scheme to eliminate the background wavefield and highlight the defect. These schemes are tested on a metallic structural panel having sub-wavelength defects such as notch and pitting corrosion. In a composite structural component, a delamination is first identified using the spatio-temporal ultrasonic wavefield imaging scheme. Further, the delamination parameters are estimated using local wavelength-based scheme involving dynamics of sub-laminate wise wave splitting in the delamination region. The technique is tested on a laminated composite and a co-cured co-bonded composite T-joint. For fast inspection of structural components, a sparse sensing scheme with a circular array of sensing points is chosen. Using this scheme, along with the estimated mode conversion strength due to wave scattering, the problem of classifying different defects are explored. Toward the end of the thesis, a reliability analysis is carried out to establish the newly developed inspection technique in terms of sensitivity and probability of detection. Environmental and measurement uncertainties are introduced by adding noise in the excitation signal. Probabilistic detection curves for varying defect parameters are obtained from the experimental data. Future potential of the developed technique in the aircraft maintenance, repair and overhaul industry is highlighted.

Pit toilets satisfy the sanitation requirement in regions with no access to piped sewerage facilities. Black water discharged from pit toilets is a major source of groundwater pollution. The primary focus of the thesis is to understand the fate of ammonium-N in blackwater generated in pour flush pit toilets, in blackwater contaminated soils in vicinity of leach pits that are subject to moisture evaporation and lastly develop a methodology to reduce nitrate contamination of aquifers by blackwater released from leach pits of pour flush toilets. In the first part of the study, the characteristics of black water from a pour flush pit toilet located in Mulbagal town, Karnataka, India, for nitrogenous constituents and other physico-chemical parameters are examined. The impact of moisture evaporation on fate of ammonium-N reactions in blackwater contaminated soils is next investigated in this study. Methodology for in-situ removal of soluble N fraction from blackwater in leach pit of pit toilet is developed. Guided by results from laboratory experiments, design for modified twin pit toilet is proposed that reduces the contamination load on groundwater

In this thesis, we develop different modelling frameworks to capture the processes of opinion formation and disease spread. The common theme binding them is a nearest-neighbour-based interaction rule; while the closeness of opinions fosters inter-personal interactions, physical proximity facilitates disease spread. Since the nearest-neighbour rules with dynamic local interactions have been shown to enable consensus about the heading of self-propelled particles in a non-equilibrium system, we adapt the Vicsek model. In the first part of the thesis, we discuss two modelling frameworks that are rendered suitable for the study of opinion formation and influence diffusion. We work on the presumption that the agents' opinions are analogous to directions in the opinion space. We assume that the agents with vectorial opinions related to a common subject form groups. We characterise these groups by the distribution of the initial opinions of the agents, and accordingly, they are either Conservative or Liberal. This modelling aspect exclusive to our work is intended to capture the impact of opinion bias of the group on the eventual behaviour. We also account for the heterogeneity among agents and broadly classify them into rigid and flexible. Although this classification does not seem unique, their characterisation differs from the conventional ones. It is based upon the inclination of agents to update one's opinion and susceptibility to the influence of peers with contrasting opinions. Since the interactions among agents of a group on virtual social platforms are oblivious to the physical distances separating them, we assume them to be arranged on a time-varying and directed influence network. In the first model, the agents are placed on directed influence networks based on opinions, individual tolerance and familiarity. In contrast, the network is generated using Watt-Strogatz's model in the second. This arrangement of agents is unlike the uniform random distribution of particles inside the square box. Additionally, not all interactions are equal; some are more important than others and is quantified using inter-personal weights. The two processes, opinion formation and evolution of the network, in tandem, give rise to several behavioural patterns. We evaluate trends in the behaviour of groups upon varying several model-specific parameters through extensive simulations. In the second part of this thesis, we discuss two other modelling frameworks proposed to capture trends in disease spread due to human mobility. While the opinion models borrow the directional attributes of particles from the original formulation of the Vicsek model, the motion of the self-propelled particles is of interest in the context of the spread of infectious diseases. However, the rules governing the movement of particles cannot describe the human movement straight away, thereby necessitating suitable modifications. We propose an agent-based framework equipped with a population mixing algorithm and stochastic disease transmission and evolutionary dynamics. The population mixing algorithm incorporates the simple rules governing the movement of particles in the Vicsek model, together with collision avoidance and goal following to mimic human motion. This algorithm generates human motion patterns ranging from short-distance and long-distance movement to activity-driven mobility. On the other hand, the disease models characterise the health condition of agents using three crucial traits of the disease, (1) infection status, (2) severity and (3) awareness, endowed with age-dependent probabilities for transmission, progress and recovery of the disease. The representative population, motion model and stochastic age-specific disease transmission dynamics are used to evaluate different scenarios. The scenarios are combinations of different motion patterns of the agents; we have chosen them to reflect restricted human mobility during phases of the COVID-19 outbreak from the past.

The modelling of spray-wall interaction encountered in engine combustors relies heavily on studies of single drop impact on heated solid surfaces. In the present thesis, the impact of a single fuel drop on a hot, smooth stainless-steel surface is experimentally examined using high speed visualization. Four different fuels of significantly varying physical properties (heptane, decane, Jet A-1 and diesel) are considered. The condition of impacting fuel drop, characterized in terms of Weber number (We), is varied in the range 27-903. The temperature of the solid surface (Ts) is varied in the range 25-410 oC encompassing all heat transfer regimes from convection to film boiling. The analysis of fuel drop impact on an unheated surface (Ts = 25 oC) reveals that the lamella spreads sluggishly even beyond the end-point of the inertia driven primary spreading phase. This new phase of fuel impact dynamics is termed in the present study as ‘post-spreading’. The spreading rate of the fuel drops in the post-spreading phase is dependent on We and is much lower than that dictated by Tanner’s Law for spontaneous drop spreading. For the impact of fuel drops on hot stainless-steel surfaces, regime maps with We on the X-axis and Ts on the Y-axis, highlighting various heat transfer regimes and morphological outcomes are constructed. Quantitative trends on the variation of maximum spread factor (max) for the fuel drops on the hot surface are presented. With the help of existing theoretical models for predicting max, it is concluded that viscosity of the fuel plays a major role in the determination of temperature dependency of βmax. For drop impact in film evaporation regime, an empirical model for max involving an explicit surface temperature term in the form of normalized value T* is formulated from the experimental data and is found to agree well with similar data from literature. Further details of fuel drop impact dynamics on the hot surface in other heat transfer regimes including contact boiling and Leidenfrost regimes are presented.

Shock boundary layer interactions (SBLIs) rank among the most fundamental problems in high-speed aerodynamics and are a primary concern in hypersonic flows. Shocks at hypersonic speeds are typically strong enough to separate the boundary layer, which can lead to dramatic changes in the entire flowfield structure with the formation of strong vortices and complex shock patterns characteristic of the SBLI phenomenon. Their presence in the systems in which they occur results in large-scale unsteadiness and high aerodynamic heating loads which curtail the performance of these systems and in many cases, pose insurmountable problems which limit the practical realisation of such systems. Following a wide-ranging literature survey which lays bare the dearth of experimental data and computational assessments in hypersonic SBLIs in high Reynolds number flows, an experimental campaign was undertaken on a configuration consisting of an impinging oblique shock interacting with the boundary layer on a flat plate. The effects of the impinging shock strength, the shock impingement locations on the flat plate and the flow Reynolds number were investigated for a range of configurations and flow conditions. The HST2 shock tunnel facility at LHSR (IISc) was used in the straight through and reflected modes of operation, and a Ludwieg tunnel constructed from the HST2 facility enabled the investigation of Reynolds number effects. The experiments were performed over a vast Reynolds number range between 1.13 million/m to 44.4 million/m, and it is this parameter whose effect is of primary focus in the present study. Of the 49 experimental runs performed, 4 were at a nominal Mach number of 6 (straight through mode of shock tunnel) while 45 runs were at a nominal Mach number of 8 (reflected mode in shock tunnel and all Ludwieg tunnel runs). The flow enthalpies for the present study fall in the low enthalpy range (varying from 0.3 MJ/kg for the Ludwieg tunnel runs to 1.67 MJ/kg for the reflected mode of shock tunnel), thus allowing the analyses to proceed with the assumption of negligible variations in the molecular composition and intrinsic characteristics of the flowing gas. The flow diagnostics consists of primarily Schlieren visualizations, along with augmented pressure distribution data obtained from PCB pressure sensors at a few locations on the centre line of the flat plate. The experiments revealed a nonlinear scaling of the separation bubble with the shock impingement location, and a clear influence of the flow Reynolds number on the extent of the interaction, with the separation bubble size decreasing dramatically under a range of flow conditions as the Reynolds number is increased. A correlation law was formulated to account for the effect of the flow Reynolds number on the phenomenon for the Ludwieg tunnel cases with finite separation. The correlation law proposed a factor of 𝑅𝑒𝐿19 to be included in the scaled separation term, and this law provides an accurate quantitative relationship between the separation length, point of shock impingement, flow Mach number, flow Reynolds number, and the imposed pressure ratio for the test conditions used in the present study. Following the experimental study, three dimensional steady (time-invariant) simulations of the full test section with the test model were performed for a few configurations with the aim of assessing the fidelity and accuracy of RANS simulations for the particular case of leading edge separation. The computational results have shown good agreement with the experimental data procured, and the three-dimensional nature of the interaction over the flat plate is captured. The thesis concludes with a summary of the results obtained, and with suggestions for future work.

For economical air transportation, aero engines with lower specific fuel consumption, lower emissions and lower noise levels are required. To this end, the use of turbofan engine with high bypass ratio is an option. With the increase in bypass ratio of a turbofan engine, the flow path of the inter turbine duct (ITD) which connects the high pressure turbine and low pressure turbine, becomes more aggressive in terms of slope and curvature distribution. Further, ITD offer the potential advantage of reducing the flow coefficient (with the increased area ratio of the duct) in the following stages, leading to increased efficiency. Together with the higher duct wall slopes and increased area ratio, ITD becomes highly prone for flow separation. In general, ITD can be represented by an annular curved diffuser and during its conceptual design, classical Sovran and Klomp (SK) framework is employed. In SK ducts framework, ducts are much less aggressive in terms of wall angles (< 20 deg), with straight wall ducts, tested in incompressible flow regimes. On the other hand, the ITD flows are known to be compressible involving curved walls at high angles (> 30 deg). Therefore, it is immoderate to use SK’s performance charts for conceptual design of the modern high slope ducts. Hence, it is required to establish the performance charts with compressible flows, curved walls and high wall angles. Having certain guidelines to design the ITD with better performance while avoiding the flow separation would be helpful during the conceptual design. Attempts are made in this work to address these aspects. Influence of curvature distribution and area-ratio distribution on the pressure fields within the curved annular diffuser are discussed through heuristic arguments. Further, these arguments were demonstrated through Computational Fluid Dynamics (CFD) simulations. The approach presented here, deals with the sensitivity of the duct performance parameters to duct wall modifications. In that sense, this work is not about a description of an automated optimization process, but rather about the physical principles that can guide such an optimization. From the CFD results and discussions, detailed guidelines to control the adverse pressure gradients (APG) on duct walls are tabulated. A geometry generation methodology which enables the design of curved annular diffusers based on the evolved guidelines, is discussed. An aggressive diffuser design space is identified with ducts of maximum slope of 50 degree and maximum divergence angle between the outer and inner walls of 10 degree for axial-length to inlet height ratio ranging from 1.25 to 2.5. Part of the identified design space for which the flow separation can be eliminated based on the evolved guidelines is demarcated. The need for flow control, possibly passive, is established for more aggressive designs. The use of splitter blade as a passive flow control mechanism in the design of separation free aggressive annular diffuser is explored through CFD Simulations. The fundamental working principle of a splitter blade in case of a two-dimensional rectangular diffuser is argued. Using these arguments, the effects of a splitter blade and its configuration in an annular diffuser are discussed. Guidelines to choose the splitter blade configuration to control the APGs on the duct walls are provided. One or more splitter blades are employed in the duct to eliminate flow separation for all the ducts in the aggressive design space considered. Requirement of number of splitter blades in the aggressive design space is demarcated. Performance charts for the ducts in this aggressive design space are established and can be used during conceptual design of an aggressive annular diffuser. The methods demonstrated in this work to eliminate flow separation on the duct walls were invoked in an attempt to design an open literature ITD with the reduced length. For this, ITD of Pratt & Whitney’s Energy Efficient Engine design (which is an open source geometry) was considered. Through the CFD simulations, an attempt was made to reduce the length of the duct up to 50% without having any flow separation on the duct walls. The performance parameters and the exit flow quality for the shorter ducts are presented, which could be helpful for taking trade-off decisions (trade-off between length reduction and performance drop).

A unified numerical framework for progressive damage analysis in polymer matrix composite (PMC) laminates has been developed by explicitly accounting for damage progression at the micro-level. A three-dimensional repeating unit cell (3D-RUC) at the micro-scale considering plasticity-based matrix damage, maximum stress based fiber failure, and a traction separation based interface failure has been developed. Images of the micro-structure of unidirectional long fiber reinforced plastic (UD-FRP) laminates have been used to develop the random micro-structure for the 3D-RUC. Coupling of macro- and micro- levels has been accomplished using non-linear homogenization principles. User material models for up-scaling have been developed to capture the damage progression in a UD-FRP laminate off-axis to the tensile loading direction. In addition to incorporating constituent damage models in the 3D-RUC framework, the effects of spatial randomness in the micro-structure have been studied by modeling randomly distributed fibers (spatially) embedded in the matrix. Randomness in strength properties has also been incorporated using weakest link based Weibull statistics. Using varying strength properties along the fiber length enables capturing of multiple fragmentation of the fiber under tensile loading. Predictions from the current analysis have been compared with the response of a composite UD-FRP plate with a central hole under tensile loading. The numerical prediction was found to match well with the experimental data

For many years, fluid flows have been modeled, starting from basic potential flow equations to full Navier-Stokes equations. The complexity of the flow increases as viscous effects, boundary layer, and flow separation are included in the fluid flow problem. However, at the preliminary design level, a simple technique to solve fluid flow problems becomes necessary for the quick assessment of 2-D aerodynamic concepts. Conventional panel methods have been popular in solving potential flow problems due to their ease of implementation for simple geometries such as circular bodies, airfoils, and 3-D applications such as wings. Nevertheless, the method becomes computationally expensive when a large number of boundary elements are required or for time-dependent problems. Moreover, due to established techniques such as panel methods, other methods go unexplored, or a smaller extent of literature is available. The present research aims to develop a Finite Element Method (FEM) for potential flows over a range of bluff bodies like cylinders to streamlined profiles such as airfoils. In contrast to conventional panel methods, Laplace’s equation describing the potential flow was solved here for the velocity potential function using the Galerkin method. A brief discussion on edge singularities in potential flows has also been presented using a half-cylinder case study. A novel method for implementing Kutta condition over airfoils to have a lifting flow has been investigated. Compared with other techniques such as finite difference and volume methods, the present methodology has proven to be computationally faster for airfoils with both finite angle and cusped trailing edges. The results have demonstrated excellent accuracy compared to analytical and panel methods. The present novel Kutta condition method has been extended to quasi-unsteady flows to show its ease of adaptability for various steady and time-dependent conditions. The process of vortex shedding in the wake of an airfoil and building up of forces was studied. A case study of a sudden step change in the angle of attack was considered for quasi-unsteady flow over an airfoil, and the results were in good accordance with the panel methods. Lastly, the application of the present FEM program was presented for a case of converting 2-D airfoil section data into 3-D wing data. 3-D wings with elliptic, rectangular, and trapezoidal planforms with tapering, sweep angle, and twist were considered. Mathematical formulas were derived from lifting-line theory, and an integration approach was used to calculate the aerodynamic coefficients. The results obtained are in good agreement with the experiments. Predicting data for 3-D wings from 2-D section airfoil using the present FEM program appears to be a very viable and cost-efficient method. Finally, the 2-D longitudinal profile of a sedan-type vehicle was considered to check the program's capability for evaluating geometries other than an airfoil. The present potential flow results for a sedan car and a modified sedan car were compared with the viscous model in ANSYS Fluent. It was observed that streamlining the sectional profile of the sedan would predict results closer to the real viscous flow due to minor flow separation.

An advection velocity correction (AVC) scheme for interface tracking using the level-set method is presented in this thesis. The key idea is to apply a correction to the interface advection velocity at points adjacent to the zero level-set, so as to enforce the preservation of the signed distance function property at these points. As such, the AVC scheme eliminates the need for explicit sub-cell x approaches, as reinitialization at points adjacent to the zero level-set is not needed. This ap- proach of correcting the advection velocity eld near the interface and computing the signed distance function (SDF) to a high order of accuracy near the interface, rather than applying an explicit sub-cell x during the reinitialization step repre- sents the key novel aspect of the AVC scheme. In this thesis results from using the AVC scheme along with advection and reinitialization schemes using upwind finite differencing on uniform meshes are presented. These results are determined for four canonical test problems: slotted disk rotation, deforming sphere, interacting circles and vortex in a box. These results are compared with corresponding results determined using a recently proposed explicit sub-cell x based reinitialization scheme (CR2). These comparisons show that the AVC scheme yields significantly improved conservation of enclosed volume/area within the interface. Note that, the present AVC scheme achieves this by only modifying velocity field values at mesh points. Therefore, the AVC algorithm can in principle be used within the framework of nearly any numerical scheme used to compute interface evolution us- ing the level-set method, even on non-uniform and unstructured meshes, in order to achieve improvements in solution quality.

The sense of touch, along with the other senses, decides the perception of world around us. We touch, grip, and wear various products in our day-to-day life, be it the constant interaction we have with our mobile phones, the interaction with various machinery at our workplace or the clothes we wear all the time. It is necessary to quantify these interactions between skin and the product surfaces to be able to design the products for comfortable and desirable experience. Coefficient of friction (CoF) can be a good indicator of frictional characteristics and touch-feel perception of a surface. Friction is the resistance to the relative motion of bodies in contact, when one of the interacting bodies is human finger-pad the friction is termed as tactile friction. The present work is focused on designing the force sensors for measurement of tactile friction. Strain-gauge based force sensor is studied for its ability to measure CoF and characterize the surface. The observations have been discussed and limitations of strain-gauge based sensors for tactile applications have been highlighted. With focus on better dynamic performance, sensitivity, low cost and simplified manufacturing, Capacitive sensing principle has been selected for force measurement. Design of various mechanical components and selection of electronics used to develop the Parallel-plate Capacitive Normal Force sensor is presented. Experiments done on the sensor for characterization of sensor and measurement of force has been discussed. Based on this experience a 2D force sensor for simultaneous measurement of normal and tangential forces has been designed.

Shape memory alloys (SMAs) have the ability to recover large strains upon undergoing a thermomechanical cycle. In addition to being used as structural elements, they can be patterned as thin films which can be employed in MEMS devices where high activation force needs to be generated. SMAs typically exist in two phases, viz., a high-temperature austenite (A) and a low-temperature martensite (M) phase. This work is focused on the phenomenon of superelasticity, which occurs due to stress induced martensite phase transformation (SIMT) from the parent A-phase. While instrumented indentation is a useful tool to determine the mechanical behavior of small volumes of SMAs, it is a significant challenge to develop correlations between test data and material properties. The mechanics of indentation, which is complicated owing to an inhomogeneous stress field and ongoing SIMT underneath the indenter, is not well understood, especially in SMA thin films. Hence, in this work, analytical and numerical investigations of spherical indentation of SMA bulk and thin film specimens are performed. Since previous experiments have shown that SIMT can be pressure sensitive, a special constitutive model that can represent it is used. The studies are conducted at temperatures below the austenite start temperature, A_s, and above the austenite finish temperature, A_f, while varying the pressure sensitivity parameter γ_1 in the SMA model. An expanding cavity model (ECM) is developed for analyzing indentation of pressure sensitive SMAs. To this end, an analytical solution is derived for the stresses, displacements and martensite volume fraction in an internally pressurized SMA hollow sphere, initially in the A-phase. The above analytical solution, which is validated against finite element (FE) analysis results, is then applied to develop the ECM. The evolution of transformation zone size and mean contact pressure, predicted by the ECM, are found to agree with complementary FE simulations over a range of indentation strain from 0.01 to 0.04. Also, the indentation stress-strain curve predicted by ECM taking into account pressure sensitivity of SIMT compares quite well with available experimental data for a Ni-Ti SMA. In order to understand the spherical indentation response of thin films of SMAs, FE simulations are conducted on a superelastic film of thickness t_f bonded to an elastic silicon substrate. The results obtained are compared with those pertaining to a bulk SMA specimen. At small indentation depths, the response of the thin film sample matches closely with the bulk specimen. However, as indentation depth increases, the transformation zone in the former is constrained by the interface to expand only in the radial direction. This results in a stiffer response of the coated sample at higher indentation depths, as reflected in load-displacement curves and Oliver-Pharr modulus. In contrast, the mean contact pressure for both specimens continues to agree well. The roles of pressure sensitivity, temperature, and normalized indenter radius R/t_f are also assessed. It is found that R/t_f<0.5 should be used to minimize substrate influence and extract key properties of the SMA thin film.

We consider the problem of designing trajectory tracking feedback control laws for La- grangian mechanical systems in a Riemannian geometric framework. Classical nonlinear control techniques that rely on Euclidean parameterizations of nonlinear confguration manifolds, severely restrict the region of operation of the system due to singularities of local coordinate charts. The primary focus of our study is to develop a generic, con- structive and intrinsic (coordinate independent) procedure for global control design such that the closed loop operational envelop is signifcantly enhanced. An important class of systems where the proposed control design techniques have been applied are underactu- ated unmanned aerial vehicles (UAV). Such systems are physically capable of executing aggressive and global (unrestricted) maneuvers as a result of enhanced mechanical de- sign and actuation technology. However, developing autonomous controllers such that the closed loop system can execute such maneuvers is indeed a formidable problem. Part 1: Global control on Riemannian manifolds (chapter 3 and 4): In the frst part of the thesis, we consider simple Lagrangian mechanical control sys- tems evolving on compact Riemannian manifolds, whose coordinate independent Euler- Lagrange equations of motion are established through the Levi-Civita connection corre- sponding to the kinetic-energy metric tensor. When the system is fully actuated), using the Riemannian connection structure, we develop a generic and constructive trajectory tracking feedback control law based on integrator back-stepping where the confguration error is the gradient of the squared geodesic distance between the confguration of the system and the reference trajectory, and the velocity error is the di erence between the velocity of the system and the parallel translation of the velocity of the reference trajec- tory, along a minimal geodesic connecting the confguration of the system and the point on the reference trajectory. The control law is appended with a feed-forward term which is the covariant derivative of the distance-gradient and the parallel-translation term. We demonstrate that this control law achieves asymptotically stable tracking when the confguration of the system is within injectivity radius of the point on the reference tra- jectory. The primary reason is that the control law does not encounter the cut locus, where it is no longer well defned, and around which it is no longer smooth. We then use our study of the compact Riemannian cut locus (which is the primary topological obstruction in global control design) in chapter 2 where we establish certain structural and dynamical properties, and thereby show that the control gains can be chosen large enough such that the confguration of the system does not intersect the cut locus of the point on the reference trajectory for all positive time, provided it starts away from the cut locus (arbitrarily close to it) initially. We thereby extend the region of stability of the above control law to an arbitrarily large domain of the tangent bundle. We then append the control law with a dynamic feedback in order to achieve globally exponentially stable tracking. We now restrict our attention to compact Lie groups which are naturally equipped with a bi-invariant metric structure, which enables us in constructing an elegant and computationally simple version of the generic control law on Riemannian manifolds. Exploiting the isometry of the group action, we convert the global tracking problem to a local tracking problem within the injectivity radius, and a global stabilization problem. Unlike the generic Riemannian case, we show that the components of the control law can be easily computed using only the Lie group structure (i.e. the group actions, exponential and logarithm map). We fnally study the problem of under-actuated differentially constrained mechanical systems where the velocity is constrained to a regular distribution. The equations of motion are established through a metric-compatible connection called the 'constrained connection', which is not necessarily torsion free if the differential constraint is non- integrable (i.e. non-holonomic). We extend the control design techniques previously established, to achieve global output tracking of such systems. Part 2: Application to under-actuated aerial vehicles on SE(3) (chapter 5 and 6): In the second part of the thesis, we apply the geometric control design techniques to two under-actuated aerial vehicles; multi-rotors and thrust vectored vertical take-o and landing (VTOL) aircraft, whose confguration evolves on the Lie group SE(3). In our study of multi-rotors, we frst design a globally-exponentially stable controller for tracking the position and relative heading angle of a quadrotor, which is considered as a rigid body subjected to a force along the body z axis and three torques about the body axes. The equations of motion can be written as a cascade of two subsystems, one which is a fully actuated rotational subsystem on SO(3), the output of which is the input to a translational subsystem on R3. We use the previously described control design on R3 using the bi-invariant metric (from the cannonical Ad-invariant inner product on the Lie algebra) and cascade this control with a saturated thrust feedback control on R3 in order to achieve global asymptotically stable tracking at an exponential rate. This design is then augmented with a fault tolerant strategy, which ensures that the controller continues to track the position of the center of mass of the quadrotor in spite of a rotor failure. This is achieved by relinquishing control of the heading angle, and designing a reduced-attitude control law which tracks the orientation of the thrust axis on S2. We use the global output tracking control design discussed in part 1 to achieve this. The reduced attitude control law is then cascaded with the saturated thrust feedback control on R3 as in the previous case. We then study the bi-spinner problem which is a rigid body with only two fxed co-axial rotors. Such a vehicle is severely under-actuated and therefore global tracking control is indeed a formidable problem. We propose a multi-scale geometric controller under the assumption that the angular velocity of the bispinner about the thrust-axis is signifcantly higher than the other two angular velocity components. We design a control law which globally tracks position trajectories with only two functioning rotors. In our study of thrust vectored VTOL UAVs, we consider an axis-symmetric aerial vehicle subjected to a terminally applied vectored thrust and torque about the axis of symmetry. Typical examples of such vehicles are thrust-propelled rockets, submersible torpedos, tail-sitter drones etc. The diffculty in control design for such a problem is that we can no longer write the equations of motion as a cascade system as we did in the case of multi rotors. The reason is that the control inputs which produce torques for the rotations in SO(3) also generate forces which result in translations in R3. Further, this coupling results in an unstable inverse input-output system (non-minimumphase), which renders the control design problem formidable. In order to resolve this difficulty, we frst impose a non-integrable differential constraint on the system, such that the constrained system admits a differentially at output i.e. The Huygens center-of-oscillation. We reformulate the translational dynamics with respect to this point to convert the tracking control problem into one which involves a cascade system as in the previous case, and apply the reduced attitude and saturated thrust feedback law to achieve global tracking of the center of mass, when the differential constraint is satisfed. This control law is augmented with another component which ensures that the differential constraint is asymptotically stabilized which ensures global asymptotic tracking for all initial conditions in the tangent bundle. Another important factor that the control design adresses is the constraint on the thrust of the vehicle to be strictly bounded above zero. We provide simulation results which illustrate the effectiveness, robustness and global tracking performance of the proposed controllers.

Collective cell migration is characteristic in many physiological processes such as wound closure, morphogenesis, and cancer tumour metastasis. Experiments on cell monolayers demonstrated the formation of finger-like patterns that are guided by leader cells at their tips which help advance the edge during migrations. Recent studies also showed that the substrate stiffness was a critical factor in altering cell migrations on different substrates which were quantified based on areal expansion of the monolayer and peripheral speed of the cells in the edge. These differences correlated with the presence of differential actomyosin cable tension in the monolayer edge, the creation of differential numbers of leader cells in the monolayer, focal adhesion areas and cell expansion on the various substrates. These studies provided valuable insights on collective cell migrations but did not discuss the mechanistic reasons underlying these differences. In silico studies have showed the importance of intercellular forces, cell expansions, proliferations, and contour forces in the monolayer in modelling the monolayer expansion over time. The goals of my study are to better understand the influence of various factors in the monolayer cell migrations on substrate of different stiffness. We developed a particle - particle interaction model to simulate the circular expansion of cell monolayers that were based on published experiments. The model allows us to vary the acto-myosin contractility on the edges, border forces and cell expansions qualitatively as shown in the experiments on different substrate stiffness. We parametric varied these terms to quantify their individual roles in the collective behaviours of cells on two dimensional substrates of 9.4 kPa and 33 kPa which is similar to that of tissues. Our results show that the peripheral velocities and areal expansion of simulation were in the range of experimentally reported values as also the percentage of leader cells that have not been shown earlier. We also show that the percentage of leaders depend on the difference in the magnitude of border migration force and the bulk actomyosin contractility force. We used these parameters to explore the cellular migrations on substrate with stiffness of 21 kPa which demonstrated a higher percentage of leader cells as compared to migrations on 9.4 and 33 kPa substrates. The actomyosin contractility force and the border forces were similar to those in the case of the stiffer gel that had a lower number of leader cells. We show that the individual cell expansions greatly reduced the overall monolayer expansion in the model and also created consistently higher percentage of leader cells which were similar to that reported in the experiments. Together, these results demonstrate that the leader cell formation in monolayer expansions not only depend on actomyosin contractility force and border force, which act on the contour, but also depend on the expansion of individual cells in the monolayer

Modern high speed aircrafts feature highly swept and low aspect ratio wings, mainly to increase the critical Mach number and reduce the wave drag. A special case of swept wings is wings with triangular planform commonly known as delta wings. The delta wings provide advantageous aerodynamic characteristics such as small lift curve slope, non-linear lift and a higher stalling angle due to it’s ability to form two counter rotating leading edge vortices. These vortices are strong and are a source of high energy which enables higher suction on the lee-ward side of wing and results into an increase in lift. However, as the angle of attack is increased beyond certain value, these vortices are affected by changes in the flow behaviour, which causes them to become unstable and breakdown into an incoherent form. This angle of attack is termed as critical angle of attack, beyond which the organised vortical structures are disintegrated leading to loss in lift and wing stall, a phenomenon called as vortex breakdown (VB). The vortex breakdown is detrimental to the aerodynamic characteristics of the wing and can cause instability of the aircraft. Due to this adverse effect, it is important to understand the behaviour of such flows. The behaviour of the flow over slender delta wings under transonic conditions is highly complex. With the occurrence of a number of shocks in the flow, vortex breakdown is abrupt and the overall behaviour is quite different to that for subsonic flow. In fact, the critical angle of attack is significantly lower in transonic speed range compared to low speeds due to interaction of vortex and cross flow/normal shock. The lower critical angle of attack in transonic flow regimes puts serious limitation on overall flight envelope for high speed aircraft. To consider this, the flow over a AGARD-B configuration having a delta wing of 60◦ sweep angle with sharp leading edge, is investigated experimentally in the transonic Mach number range of 0.8 to 0.95 around the critical angle of attack in the range of 10◦ to 15◦. The experimental results are complimented with numerical simulations performed at Mach number of 0.85 at selected angles of attack. Also, a detailed experimental investigation is carried out on a pneumatic control to delay the vortex breakdown. This thesis presents an experimental study of vortical flows and vortex breakdown over an AGARD-B configuration in transonic Mach number regime. The effect of vortex breakdown on overall aerodynamic characteristics and the variation of critical angle of attack with free stream Mach number is determined. The experimental aerodynamic data of AGARD-B is validated with the literature. Few test cases at free stream Mach number of 0.85, are analysed with numerical steady state simulations performed using High Resolution Flow Solver for Unstructured meshes (HiFUN). The numerical simulation results are validated with the present experimental data. A brief literature review of the control strategies adapted for delaying the vortex breakdown is presented. The literature suggests that ‘along the core blowing’ or ‘spanwise blowing’ method for control are found to be effective especially in subsonic Mach number regime. However, evaluation of these methods in transonic Mach number regime is found to be limited in literature. Also, the basic knowledge of a jet interacting with the transonic cross flow and it’s effect on the oncoming transonic flow is found to be limited in literature hence an experimental study of the sonic and supersonic jet interaction with transonic cross flow is carried out on a flat plate. An empirical correlation is suggested to predict the stream wise pressure variation ahead and behind the round jet injection on a flat plate, for a range of transonic Mach numbers, jet exit conditions and momentum ratios. A judicious selection of jet exit condition is made based on these experimental results and is implemented on AGARD-B configuration. The numerical simulations are utilised to evaluate the effectiveness of ‘along the core blowing’ on the vortex breakdown for AGARD-B configuration for few test conditions. Numerical study reveal a number of spanwise shocks present on the leeward side of wing for a baseline case and with the control case. Implementation of the control jet injection seems to modify the shock shape and reduces the severity of the vortex breakdown and improves the suction on leeward side of the wing. The jet used for the ‘along the core blowing’ requires very low levels of blowing (≈ 0.026%) and significantly enhances the lift to drag ratio by about 17 % near critical angle of attack. A detailed experiments are carried out to understand the effect of ‘spanwise’ and ‘along the core blowing’, control jet exit conditions, the control jet injection location etc. using force and moment measurement, pressure sensitive paint technique and unsteady pressure measurements. Analysis of pressure sensitive paint images, unsteady pressure fluctuations is used to identify the vortex breakdown phenomenon and effect of jet injection and is compared with baseline case. The span wise control jet injection from very close to the apex and tangentially along the leeward surface of the wing is found to be enhancing the lift to drag ratio by 4 to 9% depending upon the free stream Mach number. Also, the critical angle of attack is found to be pushed by approximately 2◦ due to the ‘spanwise blowing’ as compared to the baseline case. The primary and secondary vortex are found to be energized due to the span wise control jet injection. It is felt that the oncoming vortex entrains the fluid from the control jet and eventually enhances the axial velocity of vortex core which helps in pushing the vortex breakdown location of primary vortex by about 40% and that of the secondary vortex by 22%. The overall or cumulative coefficient of fluctuating pressure CPrms measured in the rear portion of the wing shows that the control jet injection reduces the overall coefficient of fluctuating pressure significantly by 20% compared with the baseline case indicating a reduction in the buffet loads. It is also observed that the jet exit condition and thus jet momentum coefficient (J) or coefficient of blowing (Cμ) plays an important role in pushing the vortex breakdown (VB) without disturbing the oncoming vortex significantly, especially since the control jet is injected very close to the apex of the wing. The experience gained from these experiments reveal that, for an effective postponement of vortex breakdown, a sonic jet should be blown either ‘Spanwise (SWB)’ or ‘along the core (ACB)’ from very close to the apex of the delta wing and a moderately low momentum coefficient (J) should be chosen for the injection such that the oncoming flow is minimally disturbed.

High-efficiency power cycles for concentrating solar power (CSP) technology such as air or supercritical carbon-dioxide (s-CO2) based Brayton cycle require high-pressure high-temperature conditions at the gas turbine inlet. This requires heating of the heat transfer fluid (HTF) to those conditions (~3-35 bar, 900-1600 K for air and ~200 bar, 1000 K for s-CO2) using a solar thermal receiver. Thus, the design and analysis of the pressurized receiver system form an important part towards the development of such solarized power plants. A coupled optical and thermal model is developed for analyzing a cavity-based pressurized receiver, with dynamic variation of solar radiation input. The optical part involves the focal region flux characterization of a fixed-focus Scheffler reflector that provides a spatially resolved heat flux to the receiver cavity surface. This is achieved using a combination of on-sun experiments and ray-tracing simulation. On the other hand, the transience in heat input to the receiver is captured by curve-fitting the measured DNI variation with time corresponding to the experimental testing period. This spatially and temporally varying heat flux is coupled to the thermal analysis of the receiver to predict the flow field and the enthalpy gain by the heat transfer fluid (HTF) along with the thermal losses from the receiver cavity. This numerical model is subsequently validated with on-sun experimental testing of a hybrid tubular and cavity receiver using a 32 m2 Scheffler dish for heat input and compressed air at 20 bar as the HTF. The numerical and experimental results are found to be in good agreement under comparable conditions, thus proving the effectiveness of the coupled optical and thermal model. To account for the transient nature of the receiver heating during the on-sun experiments, the receiver efficiency definition is modified to include the thermal inertia of the receiver material. It is observed that natural convection is the dominant heat loss mechanism that significantly reduces the overall thermal efficiency of the receiver. In the context of cavity receivers, the rate of heat transfer to the pressurized HTF is limited by the forced convection mode. For the enhancement of heat transfer in such systems, a passive method using metallic wire meshes in the HTF flow path is explored. Firstly, a pore-scale analysis is performed on the inline stacked wire mesh geometry for determining the hydraulic and thermal characteristics of the medium. The heat transfer taking place between the wire struts and the airstream at the local level is captured by thermal analysis on a representative elementary volume (REV) defined for this mesh geometry. This yields an interstitial Nusselt number for capturing the local heat transfer between the two phases. Subsequently, a homogeneous equivalent porous medium is defined using the properties obtained from the pore-scale analysis. For modelling the heat transfer within the two phases, the local thermal non-equilibrium (LTNE) model is implemented using the obtained Nusselt number correlation. The numerical model is subsequently validated with laboratory-scale experiments performed on a channel stacked with wire mesh layers and thermal load provided using an electric heater. The model prediction shows good agreement with the experimental results. However, the LTNE effect is not that pronounced under the present thermal conditions. Among the storage options available for such applications, sensible heat storage using ceramic material with honeycomb structure having gas flow passages has been used for the present study, because of its cost advantage and stability at high temperatures and pressures. To enhance the performance of such systems, the effect of block arrangement is analysed with respect to the rate of charging and discharging. Towards this end, two configurations are explored for the same storage material volume; a bigger cross-sectional area system with smaller HTF flow length and a smaller cross-sectional area system with a longer flow length. These systems are thermally cycled between 443 K and 300 K using compressed air at 10 bar pressure. Analytical and numerical models are developed that are validated using laboratory experiments, and the results are in good agreement with both the modelling approaches. This study reveals that the block arrangement that allows for higher flow velocity through the honeycomb channels of the ceramic block charges and discharges the system at ~1.5 times faster than the other configuration with slower air stream velocity under identical thermal conditions.

The current focus of research in Mg alloys is to develop wrought alloys suitable for the automotive industry. The challenges to be overcome are low strength, poor ductility, and high cost associated with wrought Mg‒based alloys. With this objective, wrought Mg‒7Sn‒3Zn (TZ73) alloy was developed in the present work, which does not contain expensive rare‒earth (RE) elements and has much higher strength than the most commonly used commercial AZ31 alloy, with a reasonable ductility. The alloy was prepared by squeeze casting, followed by homogenization at 300°C for 24 h to dissolve the low‒temperature eutectic and achieve uniform composition in the as‒cast microstructure. The processing map was generated by conducting hot compression tests in the temperature range 200 ‒ 450°C and the strain rate range 10‒3 – 101 s‒1. Based on the results from the processing map, the homogenized TZ73 alloy was rolled at 350°C. It was also subsequently annealed at 215°C. A detailed investigation has been carried out at each stage of the thermo‒mechanical processes. The hot‒rolled sheet exhibited a high strength (0.2% PS: 315 MPa, UTS: 362 MPa) with reasonable elongation‒to‒failure (9%). Strengthening is contributed to the alloy by the combined effect of grain refinement, solid solution strengthening by Zn and Sn atoms, fine Mg2Sn particles, and crystallographic texture. After annealing, the strength was reduced but % elongation‒to‒failure was increased, the alloy exhibiting tensile properties ‒ 0.2% PS: 218 MPa, UTS: 311, and elongation‒to‒failure: 18%. The properties of the Mg‒based alloys are highly dependent on the crystallographic texture. A strong basal texture develops on rolling, which is detrimental to formability. The crystallographic texture depends on the strain path during rolling. Therefore, cross rolling (CR) and reverse rolling (RR) were also carried out at 350°C. The RR process with refined microstructure and weaker texture exhibited better tensile properties (0.2% PS: 325 MPa, UTS: 386, and elongation‒to‒failure (9%) with minimum anisotropy) than CR. The basal texture is also weakened by introducing high shear stresses by having different circumferential velocities of the upper and lower rollers, known as asymmetric rolling (ASR). ASR was also employed for this alloy and the ASR sample exhibited the same strength levels as in symmetric rolling but higher elongation‒to‒failure. The present work shows that the wrought TZ73 alloy can be a promising candidate for the automotive industry.

Matrix cracking is the first and most dominant mode of damage in polymer matrix composite (PMC) laminates resulting in significant stiffness degradation under static and fatigue loading. Matrix crack evolution and its effect on stiffness degradation in cross-ply laminates under static loading have been studied extensively in the past three decades. Various analytical framework based on energy and strength-based approaches have been used to predict the matrix cracking in composite laminates. However, there have been limited studies on multi-directional (MD) symmetric laminates. In the present study, in the first part of the work, an analytical framework for matrix crack evolution for an MD symmetric laminate under static loading has been proposed using oblique coordinate-based shear-lag analysis coupled with probabilistic strength approach and Weibull distribution. The statistical parameters have been estimated from an experimentally observed matrix crack evolution data termed as master laminate. A methodology has been proposed to account the in-situ transverse strength variation due to varying thickness and constraint due to neighbouring plies. The ply-by-ply crack density evolution has been simulated. The models have also been verified under bi-axial loading conditions. The approach developed for static analysis has been extended to estimate the stresses in a cracked laminate under fatigue loading. Smith Watson Topper (SWT) parameter has been used to model the number of cycles to initiate the first matrix crack, along with log-normal probability distribution to handle the scatter in the crack initiation life. The matrix crack growth rate has been modelled using Paris law based on mixed mode effective stress intensity factor. Employing the crack initiation curve and strength degradation based on Palmgren-Miner damage rule, further formation of new crack initiation has been simulated. The crack density evolution has been simulated for cross-ply and MD-laminates under various constant amplitude in-plane fatigue stress levels. The crack density evolution and associated stiffness degradation predictions under both static and fatigue loading have been compared with the existing experimental values. Good correlation was found to exist between the experimental data and the simulation predictions. In the present work, it has been shown that, the statistical strength-based approaches can be used successfully, to predict the matrix crack evolution under static and fatigue loading in an MD laminates. The methodology proposed based on the semi-analytical approach, can be easily used by the designer, as a first stage design tool to compare different laminate stacking sequence/material system to choose better matrix crack tolerant laminate.

The lattice Boltzmann method (LBM) has emerged as a highly efficient model for the simulation of incompressible flows in the last few decades. Its extension to compressible flows is hindered by the fact that the equilibrium distributions rep- resent truncated Taylor series expansions in Mach number, limiting the strategy to low Mach number flows. Numerous efforts have been undertaken to extend this method to compressible flows recently. Some of these approaches have resulted in complicated or expensive equilibrium distribution functions. A few approaches are limited to subsonic flows while a few others have too many tuning parameters without clear guidelines to x their values. In this context, it is worth exploring newer avenues to develop an efficient lattice Boltzmann method for compressible fluid flows. In this thesis, we utilize a novel interpretation of the discrete velocity Boltzmann relaxation systems to develop new lattice Boltzmann methods for compressible flows. In these new lattice Boltzmann relaxation schemes (LBRS), the equilib- rium distributions are free from the low-Mach number expansions. In fact, the equilibrium distributions are simple algebraic combinations of the conserved vari- ables and the fluxes. This novel LB method is tested for the 1-D and 2-D Euler equations using a D1Q3 and a D2Q9 model respectively. Various bench-mark test cases have been considered to demonstrate the robustness of the new scheme. This strategy can be easily extended to other hyperbolic systems of conservation laws representing the shallow-water flows and the ideal magnetohydrodynamics. In this work, the extension of LBRS to the shallow-water equations in both one and two-dimensions and to the 1-D ideal MHD equations is demonstrated through bench-mark test problems. Finally, extension of this new method to parabolic equations is demonstrated by applying it to the viscous Burgers equation. The formulation of LBRS for the viscous case is based on the interpretation of the diffusion term in the resulting relaxation system as a physical diffusion term. Two novel approaches to extend the new scheme to viscous Burgers equation are presented.

The current research work addresses two interesting systems related to rotating beams. The first dynamical system under consideration is an articulated uniform beam rotating in vacuum under vertical hub excitation along the axis of rotation. The study considers the ensuing parametric instabilities in the considered system. Parametric excitation manifests in the form of time-varying parameters of a dynamical system. Interestingly, more often than not, these time-varying parameters are time periodic in nature and possess inherent frequency/frequencies of oscillation. In an externally forced system, response grows increasingly large when the frequency of the external forcing is close to the system natural frequency/frequencies. In contrast, a parametrically excited system may show large responses under the effect of parametric resonance even when the excitation frequency does not coincide with the system natural frequency/frequencies. In the current study, the effect of parametric excitation on a simple articulated uniform rotating beam in vacuum is considered. The autonomous form (in the absence of base excitation) of the system possesses a non-trivial equilibrium point which is a function of rotation speed and the acceleration due to gravity. In the absence of damping, the considered equilibrium point is neutrally stable and happens to be stabilize in the presence of positive structural damping. However, on application of the hub excitation, the equilibrium point is annihilated, but the system still oscillates in the near neighborhood of the equilibrium point (corresponding to the autonomous system). The ensuing oscillations can be rendered unstable owing to parametric resonance through tuning of the excitation. The linearization of the system close to the equilibrium point leads to the celebrated Mathieu equation with external forcing. The stability surface dividing the stability-instability region of the resulting Mathieu equation is derived by invoking the Floquet theory. The nonlinear stability boundaries are constructed using Poincaré maps, and are compared with the linear stability boundaries. In order to validate the theoretical study, an experimental model is designed. Although, the primary resonance is captured sufficiently well, the higher and lower order parametric resonances are nuanced due to the presence of damping. However, the experimental results do in fact indicate the existence of these parametric resonances on the parameter plane. The second part of the study is concerned with the existence and bifurcations of nonlinear normal modes (NNMs) of a flexible beam rotating in vacuum with both transverse and longitudinal deflection. The nonlinear strain-displacement relation is considered while deriving the equations of motion which are coupled through quadratic and cubic nonlinearities. The discretized form of nonlinear coupled equations of motion is derived considering coupling between the first mode of transverse and longitudinal oscillations resulting in nonlinearly coupled oscillators. The effect of slenderness ratio, rotation speed and the system energy is considered on the evolution of the ensuing NNMs and sub-harmonics and the cascade of bifurcations experienced by these modes are explored.

Heavy fuel oil (HFO) is extensively used in industrial burners and marine engines. HFO droplet combustion gives rise to carbon particulates generated by pyrolysis, which are called cenospheres. There is a current hypothesis that ‘one HFO droplet generates one cenosphere’, though there is no strong experimental evidence in support of this hypothesis. The present research deals with experimental study of HFO droplet combustion and investigates effect of HFO spray characteristics on carbon particulate emission in a spray combustion environment. A new research burner is designed to study combustion characteristics of a sparse HFO spray in a controlled high temperature environment (800 K – 1300 K). A nebulizer system generates a sparse spray of HFO droplets which enables fundamental studies on droplet combustion. Initially, spray evaporation studies are performed with standard liquids such as n-decane and n-dodecane. The droplet evaporation rate constant Kevap data derived from experiments are found to be in good agreement with those from literature. The research burner is then utilized to study HFO spray combustion and particulate formation. A novel Laser-induced fluorescence (LIF) based optical technique is developed to optically image HFO droplets in the high temperature spray flame environment. Based on this data, fuel injection parameters are optimized to achieve spray characteristics with Sauter Mean Diameters (SMD) ranging from 24-μm to 53-μm. Soot measurements are carried out in the HFO spray flames using the Laser Induced Incandescence (LII) technique to identify soot formation and oxidation zones. Soot is observed to be produced at lower flame heights and subsequently oxidized along the spray flame height. It is observed that a reduction in SMD from 53-μm to 24-μm leads to a 58% reduction in soot 5 formation. To investigate the impact of HFO spray characteristics and environment temperature on droplet pyrolysis, the cenosphere sizes are measured using an aerodynamic particle sizer. With change in SMD from 53-μm to 24-μm, a drastic reduction (~90%) in cenosphere emission density (particles/cm3) is observed. It is also observed that higher temperatures ranging from 1073 K to 1223 K are favorable for cenosphere reduction. The morphological study of cenospheres indicates that these are nascent coke particles generated at the end of the droplet combustion phase. The results seem to indicate that the ‘one droplet generates one cenosphere’ theory is not applicable for smaller droplets. The data is further analyzed to establish the existence of a critical diameter of HFO, which undergoes complete combustion in the droplet combustion phase without generating a solid coke particle. In other words, if a HFO spray consists of droplets whose diameter is below this critical value, the particulate emissions can be nearly zero. From the experimental data, the critical droplet diameter is found to be in range of 18-μm to 23-μm for the medium grade of HFO used. Keywords: Heavy fuel oil (HFO), Spray, Droplets, Combustion, Particulate emission, Soot, Cenospheres, Laser Induced Incandescence, Laser Induced Fluorescence

Fire hazards pose an increasingly potent threat to modern societies. Early identification and mitigation of fire hazards are crucial to avoid the loss of human lives and property. Recent research suggests that finely-atomized water spray consisting of droplets with a Sauter Mean Diameter (SMD) of less than 100 µm is a superior fire-suppressant compared to traditional water sprinklers (SMD  0.1 – 1 mm). The addition of chemical inhibitors further improves the effectiveness of water mist as cooling, dilution, and chemical modes of fire suppression are combined. In the present thesis, the effectiveness of chemically active fire suppression agents for methane and LPG flames has been assessed through experiments and numerical modelling. The agents (K/Na-based compounds) are introduced in a counterflow diffusion flame of methane/LPG in the form of aqueous solutions, and their impact on the flame extinction is measured. Initial experiments conducted with pure water mist show a 45% reduction in the extinction strain rate (ESR) at a Y_(H_2 O)=1.5% in a methane flame. It is observed that for both LPG and methane flames, the addition of alkali compounds further improves the inhibition effectiveness of water mist. Four potassium compounds and six sodium compounds have been tested in the present thesis. All potassium compounds show superior effectiveness compared to those of sodium. Among the tested additives, potassium bicarbonate (KHCO3) and potassium acetate (CH3COOK) are established as the most efficient in both methane and LPG flames. In methane flames, the addition of KHCO3 at 2% solute concentration reduces the ESR by 20% as compared to that obtained using a pure water mist. Combinations of multiple compounds are also tested to identify possible synergistic/antagonistic interactions between the agents. Additive interaction is found in the KHCO3 -NaHCO3 mixture, whereas the mixture of KHCO3 – CH3COOK shows antagonistic interaction. To understand these observations, experimental results are compared with one-dimensional simulations using detailed chemical kinetic models. Numerical predictions of the ESR under the influence of water mist are in good agreement with measured data. However, the influence of alkali solutions is only captured qualitatively in the simulations. Numerically, the analysis is extended to other classes of fire suppression agents as well. The ranking among four agents (Fe, K, P, Br-based) is obtained in a methane flame. Additionally, the work closely examines the effect of flame residence time on the inhibitor performance. The importance of regeneration coefficients and radical pool composition in determining the inhibitor effectiveness is established. The detailed chemical mechanism describing the phosphorus-based flame inhibition contains 44 species and 213 reactions, thus making the simulations computationally expensive. The thesis presents two chemical mechanisms, i.e., a skeletal (4 species, 7 reactions) and a global (3 species, 3 reactions) mechanism, which lead to an 82% reduction in computational time with respect to the detailed mechanism. Overall, the thesis has led to an improved understanding of fire suppression under the influence of chemically active flame inhibitors, specifically identifying the most effective potassium-based chemical inhibitors for methane and LPG flames.

The present study deals with the numerical analysis of the effect of the swirl in the self-preservation region of the turbulent round jet. However, a large number of literature exists for the analysis of near-exit regions—very few deals with the self-preservation region of the jets far downstream. The present study attempts to provide insights into the effect of swirl on the turbulent mixing and jet spread rate by examining the self-similar solution in the far-field region of the jet. The study is divided into two main portions: a comparison of the turbulent swirling and non- swirling jets and the comparison between the turbulent jets having low to moderate values of swirls. A standard computation for a non-swirling jet is used to validate the flow solver. Simulations are carried out at a Reynolds number of 2,400 for the top-hat velocity profile at the inlet. All flow characteristics are computed in detail and compare the results with existing DNS data. Velocity profiles at different streamwise locations collapse on a single curve and closely match the available data. The jet decay and spread rates also align with the standard computed data. Large eddy simulation has been performed for non-swirl (S = 0), weak swirls (S = 0.3, 0.5) and moderate swirl (S = 0.7) at a Reynolds number of 11,000. In both the non-swirling and swirling cases, special care is taken to ensure that the computational domain is large enough to study the jet’s behaviour in a self-similar region. The research presents the effects of the swirl on a turbulent flow and compares the simulation results with available experimental data. Comparing the swirling and non-swirling cases indicates a changed turbulence structure to the effect that the swirling jet spreads and mixes faster than the non-swirling. With increasing degrees of swirl, the angle of spread of the jets is increased, and correspondingly, the decay of the maximum values of velocity components along the lengths of the jets is faster. Flow entrainment shows that the entrainment increases with swirls. The numerical simulations showed that the flow quickly achieved a self-similarity for the mean axial velocity. In contrast, the radial and azimuthal mean velocities reached a self-similar state after a longer period of jet development. Results of the decay of velocity and jet spread rate in the self-similar region of the swirling jet without vortex breakdown were found to vary linearly with the streamwise direction of the jet irrespective of the magnitude of swirl number, which is in line with the findings from experiments of Rose (1962), Chigier & Chervinsky (1967) & Pratte & Keffer (1972). In contrast, Craya & Darrigol (1967) has theoretically shown that axial velocity decay varies as three halves along the length of the jet. Additionally, mass flux shows higher mixing in swirling jets compared with non-swirling. The integrated axial fluxes of linear and angular momentums were conserved along the jet’s axis in the self-preserving region.

In the present work, an Implicit Gradient Reconstruction (IGR) method is proposed in the context of Finite Volume Methodology (FVM). There are three computationally intensive steps involved in a typical finite volume framework for a spatially second-order accurate upwind scheme. These are solution reconstruction, solution limiting and flux finding. Solution re- construction involves determination of gradients while solution limiting requires comparison of double precision numbers. Further, computation of gradient and solution limiting is cell based procedure while flux finding is edge/face based procedure. The proposed IGR procedure, which is edge/face based procedure, integrates all these steps obviating explicit reconstruction and limiting steps resulting in a considerable reduction in computational effort and associated memory footprint. In the modified CIR (MCIR) scheme of linear convection equation, a parameter φ is in- troduced to control dissipation. The IGR procedure is derived from the MCIR scheme. The relation between the φ parameter and solution reconstruction in a finite volume procedure is systematically established. The methodology is extended to multidimensions, where the use of φ implicitly represents a reconstruction step. Hence, this procedure is referred to as Implicit Gradient Reconstruction. In addition, it is brought out that the use of φ also serves the purpose of solution limiting. The spatial accuracy of this procedure is demonstrated by computing the 2-D circular convection problem. The methodology, when extended to the Euler equations of Gas dynamics, results in the reconstruction of the characteristic variables. Consequently, three steps in the computation of explicit residual, namely, solution reconstruction, limiting and flux computation, are seamlessly merged into a single step. Owing to its significantly smaller memory footprint, the procedure is particularly relevant to large scale parallel computing. This procedure can be effortlessly incorporated into any of the existing finite volume solvers where inviscid flux formulation is based on characteristic decomposition. The capability of IGR procedure is established through several test cases involving inviscid and viscous flow computations in one and two dimensions. The results obtained from IGR procedure are compared with reconstruction based solvers and wind tunnel data wherever available. The IGR procedure produces results comparable to the classical reconstruction based procedure. The aforementioned IGR procedure is applicable to any flux formulation involving charac- teristic decomposition. An Edge Based Reconstruction Limiting (EBRL) is proposed for flux formulation not involving characteristic decomposition. This procedure is very simple and does not require classical diamond path reconstruction. The results obtained from EBRL based Roe and AUSM Plus flux formulation on transonic viscous flow over RAE 2822 airfoil are very good and compare well with wind tunnel experiments.