Dissertations / Theses on the topic 'CFD study'

This master thesis project consisted of three parts that all were performed through CFD simulations with the purpose to develop Scania's methods in the subject of CFD. All parts included simulations on Scania's SC92T70 centrifugal compressor. Part one consisted of performing a mesh study for the purpose of reliability, to investigate the convergence of different parameters by refining the boundary layer. The method used is an inflation option called First layer thickness. Five different meshes were generated where the Richardson extrapolation method was used to examine the parameters between the mesh renements. From the result from the examined parameters, an approximate relative error could be calculated to be less than 0.52 %, and a numerical uncertainty of less than 0.35 %, between Mesh3 and Mesh4. In addition to that, Mesh3 had a simulation time of one hour less than for Mesh4. These results motivated the use of mesh3 to be refined enough for further work in this thesis project. This mesh ended at 37, 915, 257 number of elements. The second part consisted of performing steady state CFD simulations, to examine different parameters in order to find indications of the phenomena surge. Here, experimental data was used as reliance to perform CFD simulations on the compressor. Design points from experimental data was used, that ranged from low mass flow rates where surge arises, to high mass flow rates where another phenomena called choke occur. Except for the design points taken from experimental data, a few extra design points where included at low mass flow rates (in the region of surge). The goal was that the analysis of the different parameters would generate fluctuations on the result for the design points in surge region. Four different rotational speeds on the compressor were examined, 56k, 69k, 87k and 110k revolutions per minute. A total of 140 different parameters were examined, where 10 of these indicated on surge. All of these parameters that indicated on surge where found in regions of vicinity to the compressor wheel, which are the regions subjected to the phenomena.The parameters indicating on surge where mass flow, pressure coefficient, static pressure and temperature. Indications where found at the wheel inlet, ported shroud, and wheel outlet interfaces. The indications were only found for the two lower rotational speeds of the compressor wheel. To capture the behaviour on higher rotational speeds, more design points in the region of surge are needed, or transient simulations. Part three of the thesis project consisted of investigating the methodology of performing a Conjugate Heat Transfer model (CHT) with the CFD code CFX. This part has not been performed by Scania before, so a big part of the problem was to investigate if it actually was achievable. The goal was to use this model to calculate the heat transfer between fluid and solid parts, as well as between the solid parts and the ambient. One question Scania wanted to answer was if the CHT model could generate aerodynamic performance that corresponds to Scania's traditional adiabatic model, as well as to experimental data of the compressor. In this part, both solid and fluid domains were included in the geometryto calculate heat transport, in contrast to the traditional adiabatic model that only uses the fluid domains. Because of that, a big part of the work consisted of defining all interfaces connecting together surfaces between all domains. This is needed to model heat transport between the domains. In the set up part in CFX, the CHT model differed a lot from the traditional adiabatic model in that way that the outer walls was not set up as adiabatic anymore. In the CHT model, instead heat transfer is allowed between the outer walls of the fluids and the solids. From the result simulations, one could see that the CHT model was able to compute the heat transfer between fluids and solids. It also managed to export thermal data such as heat flux and wall heat transfer coefficient to be used for mechanical analysis, which is an important part in Scania's work. From the analysis of aerodynamic performance, a conclusion was drawn that the CHT model was able to compute efficiency and pressure ratio that followed the behaviour ofthe traditional adiabatic model as well as experimental data. However, for lowermass flows, the CHT model started to underpredict which could be explained by the geometrical differences between the CHT and adiabatic model. By analysis of temperature, one could see quantitative differences compared to the traditional adiabatic model. For other parameters (static and total pressure), there were no experimental data to be used for comparison. Because of that, an important part in future work of this CHT method development is to perform more experimental test for CFD data to be compared against. Another important part to compare the models is to have an identical geometry. Without an identical geometry, deviations in result will occur that depends on geometry.

Nowadays electrification is one of the leading fields of engineering as it is seen as one of the key factors that can reduce environmental impact of human activities by reducing theirs polluting emissions. Mobility is the sector in which electric driven systems are diffusing the most. The search for performance in one of its main component, the electric motor, is therefore of fundamental importance in terms of efficiency and reliability of every electric driven applications. The optimization of thermal aspects covers a primary role and highly affect power consumption and lifetime of components. With this intent the use of CFD, Computational Fluid Dynamics, allows to exploit most of heat transfer aspects which concurs on thermal behavior both for design phase and performance estimations. The work of this thesis investigates the cooling performances of the motor driving an electric vehicle made by Engines Engineering -EE- which is a company that projects motorbikes for thirds and is recently going to expand in the field of electric mobility. Beside the specific case studied, the methods can be extended to any component which require a thermal management as CFD tools are fundamental in a very wide spectrum of application in engineering.

Race car aerodynamics has played a vital part to improve lap times over the years of motor racing. Having good road adhesion with slick tires and aerodynamic downforce will increase the vehicles maximum lateral force and thus higher cornering speeds can be achieved. The undertray and diffuser is the most efficient aerodynamic component on most racing vehicles and is capable of producing six times more downforce than its contribution to drag plus, if optimized correctly, able to significantly reduce the vehicle's drag coefficient. The intent of this project is to optimize a completely unique undertray design for the KTH Formula Student teams racing vehicle DeV17. The undertray is inspired by the Aston Martin Valkyrie venturi tunnel design and is optimized by iterative change in CAD design parameters for three different chassis designs. The results are obtained with CFD RANS simulations using the k-ω (SST) turbulence model with the Siemens Star-CCM+software. The optimum design gave 530 N and 90 N of downforce and drag respectively at a velocity of 80km/h. The venturi tunnel design is proven to give a 29% downforce improvement over a conventional flat plate design with stronger longitudainal vortices and lower, more widespread, minimum pressure distribution. The most important aspects that affect downforce in undertray design is concluded to be a diffuser outlet height, upsweep and vehicle ground clearance. No specific aerodynamic advantages in having a convergent tapering of the tunnel cross-section is observed, meaning the undertray can be represented as only consisting of an expanding diffuser. The tunnel design is considered to give promising track testing results and be a spark for further innovative ideas with aerodynamic design for both the automotive and racing industry.
Tävlingsbilars aerodynamik has spelat en viktig roll för att förbättra varvtiderna under åren inom motorsport. Att ha god väghäftning med 'slicks' däck och aerodynamisk nedåtkraft kommer att öka fordonets maximala sidokraft förmåga och därmed kan högre hastigheter i kurvor uppnås. Underredet och diffusern är den mest effektiva aerodynamiska komponenten på de flesta racerfordon och kan producera sex gånger mer nedåtkraft än dess bidrag till luftmotståndet och, om den optimeras korrekt, kan den avsevärt minska fordonets luftmotståndskoefficient. Syftet med detta projekt är att optimera en helt unik underredes design för KTH Formula Student lagets racingfordon DeV17. Underredet är inspirerat av Aston MArtin Valkyrie venturitunnel design och optimeras av iterativ förändring av CAD designparametrar för tre olika chassidesigner. Resultaten erhålls med CFD RANS-simuleringar med turbulensmodellen k-ω (SST) och programvaran Siemens Star-CCM+. Den optimala designen gav 530 N och 90 N nedåtkraft respektive luftmotstånd under en hastighet på 80 km/h. Venturitunnel designen har visat sig ge en förbättring på 29% nedåtkraft jämfört med en konventionell platt design, med starkare längsgående virvlar och lägre, mer utbredd, minimitryckfördelning. De viktigaste aspekterna som påverkar nedåtkraft i underredes designen dras som slutsats till att vara diffuser utloppets höjd, upphöjning vinkeln och fordonets markfrigång. Inga specifika aerodynamiska fördelar med att ha en konvergerande avsmalning av tunnelns tvärsnitt obververades, vilket innebär att underredet kan antas endast bestå av en expanderande diffuser. Tunneldesignen anser ge lovande bantestresultat och vara en gnista för ytterliga innovativa idéer inom aerodynamisk design för både bil- och racingindustrin.

In this master thesis the wake of a hydrofoil have been investigated using PIV. The main goal of this work have been to investigate how vortex generators can create mixing and smoothing of the velocity deficit in hydrofoil wakes. This study is motivated by the rotor stator interactions in Francis turbines with the idea that smoother wakes from the stator can reduce the forces on the rotor and hence increase the life span of Francis turbines. A literature survey of foil theory and wake flows have been carried out. This survey motivated the use of a normalization of the velocity in the wake. Experimental work was carried out at the water tunnel facility at Saint Anthony Falls Laboratory at the University of Minnesota. Tests were performed on a NACA0015 hydrofoil with four different vortex generator configurations, for a range of different angles of attack and velocities. Lift and drag forces on the hydrofoil was measured using a force balance. Because the drag measurement had poor accuracy, it could not be used to compare the different vortex generator configurations in terms of drag. As a result the drag was investigated using the velocity deficit in the wakes. The quality of this analysis have been discussed with the use of CFD. CFD is also used to gain insight into how pressure and velocity is distributed in the water tunnel. The PIV images from the tests have been processed into vector fields with the commercial PIV software DaVis7. For analyzing the PIV data further, different post-processing schemes in DaVis7 was investigated together with programs developed in Matlab. In order to compare the wakes resulting from the use of different vortex generators with measurable quantities, the use of a standard wake profile has been investigated. The standard wake profile is symmetrical and could hence only describe wake measurements done at an angle of attack close to $0^{circ}$. Furthermore it turned out that most vortex generators resulted in a wake that could not be described with the standard wake profile. The vortex generator configurations that gave the best smoothing of the hydrofoil wake for the investigated operation points turned out to be a $1unit{mm}$ V-shaped vortex generator. This vortex generator also caused less drag than than the other vortex generators tested. However, the use of vortex generators resulted in increased drag compared to the plain hydrofoil for the analyzed operating points. The velocity deficit in the wake is shown to get so well smoothed out for some tested cases that it is considered worth while to continue the investigation on vortex generators capability to increase the lifespan of Francis turbines.

The purpose of this thesis is to design and study an aircraft which implements the Co-Flow Jet (CFJ) airfoil concept, as well as to study the CAARC standard highrise building. The design concept is verified mainly by the use of a Computational Fluid Dynamics (CFD) package. A thorough methodology for geometry and mesh generation is developed, and subsequently applied to the two cases. The first case studied is that of the CFJ Airplane (CFJA). It consists of a threedimensional, highly blended, ying wing geometry implementing the Co-Flow Jet airfoil concept. Though a thorough comparison to a baseline geometry, it is shown that usage of the CFJ airfoil cross-section greatly improves aircraft performance by increasing lift, reducing drag, and providing a source of thrust over the operational range of angles of attack. A steady state CFD simulation is used for this case, as the air ow around an airfoil cross-section is inherently steady for attached ows. CFD results are used to support the Engineless Aircraft" concept, where the CFJ airfoil is used as the sole form of propulsion. The second case studied consists of a rectangular high-rise building undergoing a wind condition with Mach number of 0:1 and a Reynolds number of 160000. Due to the non-streamlined geometry of the building cross-section, aerodynamic instabilities due to uid separation are present, and therefore an unsteady CFD analysis is necessary to fully resolve all of the ow phenomena. Preliminary steady state results are presented, and a plan is laid down for the future study of this highly complex case. Results are presented for a variety of angles of attack in the case of the CFJA, and for the main ow direction in the case of the CAARC building. Results are compared with baseline geometry in the case of the CFJ Airplane. The CFJ Airplane case is simulated using a 3rd order steady state scheme, which is sufficient to achieve valid results for the ow regime. The CAARC building, which has inherent ow separation, requires the use of high order schemes.

A conceptual study of performance enhancing devices for an airfoil is performed using Computational Fluid Dynamics. Three simple, passive devices are examined to explore alternate methods for stall control and lift-to-drag improvement. The motivation behind this research is to study effective techniques to improve performance with fewer drawbacks than previously existing methods. An evaluation scheme is presented to compute airfoil lift, drag and pitching moment for a range of angles-of-attack up to stall. NACA 641-212 single-element and slatted airfoil CFD results are compared with experimental data to validate the computational model. Evaluations on the first conceptual design (Stall vane) show elimination of the separation at 15 degrees of angle-of-attack where the flow reversal normally starts at 86% - chord. A total drag increase of 22% is detected because of the sharp leading-edge of the device, but the main element drag has a reduction of 43%. The maximum lift coefficient does not show a significant change on the same model. The second device (Cylinder) has a negative effect, initiating flow separation and causing a significant decrease in lift-to-drag ratio at a given lift coefficient. The third device (Dimples) demonstrates the potential of lift-to-drag ratio improvement at the higher angle-of-attack. Further investigation is required to verify the results since the improvement is small.
Thesis (M.S.)--Wichita State University, College of Engineering, Dept. of Aerospace Engineering.

This thesis presents the research on direct-fuel-injection (DFI) performance analyses of the two-stroke engine using computational fluid dynamics (CFD). The aims of this research are: (1) to generate a finite volume mesh that can be used to simulate the moving of the piston and opening and closing the ports of the cylinder, (2) to achieve an numerical flow pattern of the scavenging process and (3) to study the DFI process using the mesh and the flow pattern obtained from the first two parts. The three parts in the analyses, therefore, are the engine geometry modelling, scavenging process modelling and DFI modelling. CFD software STAR-CD was used to write the programme and perform the analysis. The geometry model used a moving mesh mechanism with variable openings to simulate the piston motion and port area changes. The scavenging model was constructed to calculate 3D, compressible, turbulent, transient flow with heat transfer and changing volume. Results of the calculation provided a large number of data, including flow patterns, pressure and temperature distributions and fresh-charge concentrations. The DFI process was simulated as a gasliquid two-phase flow. Fuel droplets dispersed in the continuous gas phase were calculated using the Lagrangian model. Four DFI cases that differ in position and number of injectors were simulated. Fuel droplet distribution, including droplet size, velocity, temperature and position, and fuel-vapour concentration were obtained. The result of the geometry modelling shows that the finite volume programme performed well for this particular task. Because of the limitation of the computer hardware used, this programme was restricted to be used for the simulation of the engine process before the ignition, i. e., cold running condition, therefore, combustion process was not included. The findings from the analyses, with very limited resources, would help improve the engine design. The results of all four cases indicate that, even only for the period of engine process before the ignition, the DFI two-stroke engine can significantly reduce hydrocarbon emissions compared with the conventional carburettor engine.

In the present work the aerodynamic forces on trucks driving in so-called platoon are investigated in a numerical fashion. Driving in platoon, or convoy, refers to in an orderly manner driving in a line, one truck after the other, taking advantage of the unrecovered _ow behind each truck. The phenomenon is called slipstreaming or drafting. The Compu- tational Fluid Dynamics (CFD) software STAR-CCM+ is used to calculate the flow field around a platoon consisting of two and three trucks at different distances, ranging from 5 to 70 m. Two numerical approaches are used, one is the Reynolds Averaged Navier Stokes based (RANS) two-equation turbulence model k 􀀀 " realizable model with a two-layer treatment. The second one is the Menter's Shear Stress Transport (SST) k 􀀀 ! Detached Eddy Simulation (DES) model. The first one is time independent, so-called steady-state, where platoons consisting of two and three trucks are used in the simulations. However, the nature of the flow field around vehicles is inherently time-dependent, which makes it difficult to receive a steady-state solution and thus, the reliability of the result is neg- atively affected. The second model is time dependent and much more computationally expensive, where only a platoon consisting of two trucks is simulated. Addition to this, simulations with an isolated truck will be conducted in order to make a relative study for both turbulence models. Since numerous of errors are introduced when approaching the problem numerically, it is important to have a reference case to compare with, set under the same conditions. Also, comparisons with other studies are done. A mesh independent study is conducted with the function of investigating how the mesh density influences the result, together with a mesh quality study, both helpful when assessing the credibility of the results. For the RANS approach, it is shown that for the 2-truck platoon, drag reductions are the greatest at the closest distance, 5 m, with 26:9 and 28:1 % reductions in drag for the leading and trailing vehicle, respectively, compared to the isolated case. There is an increase in drag for both vehicles with increasing distance, however, the trend turns around at 10 m for the trailing vehicle, where it also reaches its maximum, 5:5 % larger drag than that of the reference case's. Then a reduction is seen for all distances greater than 11 m. For the leading truck, the drag coeffcient CD is equal to the reference case's around 18 m, with an overshoot of 2 􀀀 3 % afterward, which may be a result of numerical errors. The same trend is seen for the 3-truck platoon, with largest reductions at the closest distance 5 m, with the reductions 31:5 %, 48:5 % and 33:2 % for truck one, two and three, respectively. At 10 m, there is also an abrupt increase in drag for the trailing trucks, however, the drag never reaches over the drag of an isolated truck. An overshoot is also seen for the first vehicle in the 3-truck platoon and it stops benefiting from platoon driving around 22 m. It was found that at 10, thick low-velocity boundary layers were formed on the leading trucks, which may be one of the reasons for the increase in drag. For the time-dependent approach, the drag behavior is similar to the RANS cases for the leading vehicle, but no overshoot is seen, instead the drag is always smaller than the reference case's. The maximum reduction is also found at 5 m, with the value 31:7 %. A completely different trend is found for the second vehicle, where the drag decreases with increasing distance, where there is a minimum reduction at 5 m (4:0 %) and a maximum reduction at the largest investigated distance 50 m (24:3 %). This kind of trend is also seen for the RANS-based simulation in the interval 10 􀀀 50 m, but the reductions are not as large. After 12 m, the trailing truck benefits the most. It was found that the vortices and the time dependence of the flow field are important features. The RANS-based model produced poor results in region of strong swirl and therefore it is not a suitable model for the flows of this type. Also, based on the good agreement with PowerFlow VLES (Very Large Eddy Simulation) simulations with the DES ones even further puts great distrust on the RANS simulations. The k 􀀀 " realizable model with a two-layer treatment has also shown deficiencies in predicting the downstream effects (over predicts) and the size and intensity of recirculation areas (for instance, the wake) as shown in the work of P.L. Davis, A.T. Rinehimer and M. Uddin, 20th Annual Conference of the CFD Society of Canada.

This document is a report summering the master thesis work dealing with the Computational Fluid Dynamic (CFD) study of the flow around a high-speed train. The model is a scaled 1:50 generic train with two cars, one inter-car gap and simplified bogies. A platform is set on the side of the train since one of the aim of the study is to look at the consequences of the phenomena in the wake on people or objects standing on the platform. The slipstream is one of this phenomena, it is due to the fact that the viscous air is dragged when the train is passing. If too strong, it can move or destabilize people or objects on the platform. In addition of the slipstream study, a velocity profile study, a drag and lift coefficients analyze as well as a Q-factor study and a frequency study have been realized. Some results of these different studies are compared with the ones obtained on the same model with a Delayed Detached Eddy Simulation (DDES). Since the flow is turbulent, for those different studies, the flow has been simulated with a Reynolds Averaged Navier-Stokes equation model (RANS) which is the k-ω SST model for the turbulence. The study of the slipstream allowed to calculate the Technical Specification for Interoperability (TSI) which must not be higher that the European Union requirement set at 15.5 m/s, the result obtained is 8.1 m/s which is then lower than the limit. The velocity profile shows similarities with the DDES results even though it is less detailed. The same conclusion is done for the Q-plot where is clearly visible the two counter-rotating vortices in the wake. Finally, a Fast Fourier Transform algorithm has been applied to instantaneous velocity results in the wake of the train in order to get the frequency of the aerodynamic phenomena in that wake. The main frequency is 25 Hz and corresponds to a Strouhal number of 0.1, quite closed to the results obtained with DDES which is 0.085. The results of the RANS and DDES are reasonably similar and by regarding at the large difference between the cell numbers (respectively 8 500 000 and 20 000 000) it can be conclude that in some ways the RANS model can be preferred at the DDES to save time for the computation but it does not contain the small scales resolved by the DDES.

In Diesel engines, the internal flow characteristics in the fuel injection nozzles, such as the turbulence level and distribution, the cavitation pattern and the velocity profile affect significantly the air-fuel mixture in the spray and subsequently the combustion process. Since the possibility to observe experimentally and measure the flow inside real size Diesel injectors is very limited, Computational Fluid Dynamics (CFD) calculations are generally used to obtain the relevant information. The work presented within this thesis is focused on the study of cavitation in real size automotive injectors by using a commercial CFD code. It is divided in three major phases, each corresponding to a different complementary objective. The first objective of the current work is to assess the ability of the cavitation model included in the CFD code to predict cavitating flow conditions. For this, the model is validated for an injector-like study case defined in the literature, and for which experimental data is available in different operating conditions, before and after the start of cavitation. Preliminary studies are performed to analyze the effects on the solution obtained of various numerical parameters of the cavitation model itself and of the solver, and to determine the adequate setup of the model. It may be concluded that overall the cavitation model is able to predict the onset and development of cavitation accurately. Indeed, there is satisfactory agreement between the experimental data of injection rate and choked flow conditions and the corresponding numerical solution.This study serves as the basis for the physical and numerical understanding of the problem. Next, using the model configuration obtained from the previous study, unsteady flow calculations are performed for real-size single and multi-hole sac type Diesel injectors, each one with two types of nozzles, tapered and cylindrical. The objective is to validate the model with real automotive cases and to ununderstand in what way some physical factors, such as geometry, operating conditions and needle position affect the inception of cavitation and its development in the nozzle holes. These calculations are made at full needle lift and for various values of injection pressure and back-pressure. The results obtained for injection rate, momentum flux and effective injection velocity at the exit of the nozzles are compared with available CMT-Motores Térmicos in-house experimental data. Also, the cavitation pattern inside the nozzle and its effect on the internal nozzle flow is analyzed. The model predicts with reasonable accuracy the effects of geometry and operating conditions.
Patouna, S. (2012). A CFD STUDY OF CAVITATION IN REAL SIZE DIESEL INJECTORS [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/14723
Palancia

The hotspot temperature of disc windings has a close relation with the transformer age. In oil immersed transformers, oil guides are applied generally to enhance the cooling effects for disc windings. In some cases disc windings without oil guides are used. However, the lack of oil guides is expected to result in a more complicated thermal behavior of the windings, making it more difficult to predict the location and strength of the hotspot temperature (i.e. the hottest temperature in the winding). To get an improved understanding of the thermal behavior, a CFD study has been performed.  This article describes the implementation of CFD simulation for 2D axisymmetry models without oil guides, and then analyzes the results of a series of parametric studies to see the sensitive factors influencing the cooling effects. These parameters include radial disc width, inlet mass flow rate, horizontal duct height, vertical duct width and the inlet/outlet configurations. Three main characteristics, the hotspot temperature, the location of the hotspot and the number of oil flow patterns are detected to describe the thermal performance.

Cable-stayed bridges are massive structures which rely on their structural elements such as deck girder, towers and stay-cables for their stability. The bridge stay-cables can be considered as the most flexible elements of the cable-stayed bridges, and thus their structural stability integrity is verified for several phenomena which might affect them. Wind and wind/rain induced vibrations for bridge stay-cables were comprehensively studied by researchers worldwide; however recent projects have identified a new type of cable vibrations caused by ice accretion formed around the cable circumference. The current research proposed two ice accretion profiles for inclined bridge cables and has experimentally investigated the wind-induced vibrations of the two models for the bridge stay-cables with ice accretion, under different vertical (inclination) and horizontal (yaw) angles, and for different wind speeds. Initially, three models of the bridge cable with 1.0 cm and 2.0 cm ice profile were tested in the wind tunnel of cross-section 61 cm × 90 cm, and a maximum wind speed of 30 m/s. In total 6 cases with 1.0 cm ice thickness and 3 cases with 2.0 cm ice thickness were investigated and the vertical and torsional oscillatory displacements were recorded for wind speeds from 1.5 m/s to 15 m/s at intervals of 1.5 m/s. The wind-induced vibrations were analyzed and were compared with the response reported for cables without ice and with the rain-induced response for stay-cables. Computational Fluid Dynamics (CFD) simulations were performed to observe the drag, lift and pressure coefficients around the surface of the accreted cable models yawed and inclined at α = 0°, β = 0° and α = 60°, β = 15° under the effect of 10 m/s and 15 m/s wind speed applied for both cases. A verification for galloping divergent instability was conducted based on the Den Hartog formulation and the vertical vibrations obtained from the wind tunnel experiment.

This report is the result of a joint collaboration between the two Swedish companies AFRY and Scania AB. The need of CFD analyses for internal flow purposes are continuously increasing in companies like Scania. However the use of CFD software is limited to license costs. The objective and idea of this thesis is to esablish whether OpenFOAM, an open-source CFD software, could be a suitable alternative to the currently used CFD commercial software StarCCM+ when performing total pressure drops analysis on air intake systems. Furthermore, a second objective is to develop a standard method to analyze this class of systems using OpenFOAM. Air intake systems play a significant role in the efficiency of the engine of a bus or a truck, being responsible to drive clean fresh air to the engine with the minimum amount of pressure drops. After an illustrative introduction of the topic and theory principles, the following technical analysis focuses first on a CFD study of the simple Buice and Eaton 2D diffuser, to primarily benchmark OpenFOAM, StarCCM+ and the experimental data in a turbulence models and a mesh independence analyses. For both software the turbulence model RNG k − ε fails almost entirely to detect the separation and wide re-circulation bubble formed in the diffuser, while k − ω SST, being not much more computationally expensive, gives very accurate results. For these reasons, even the velocity profiles, pressure coefficient and skin friction coefficient plots analyzed with RNG k − ε turbulence model show larger discrepancy compared to the experimental data.  Secondly, a total pressure drop CFD study of the Scania air intake system follows. Performing multiple simulations for a range of different mass flow rates, both OpenFOAM and StarCCM+ show very good agreement with the experimental data. This time, due to the lack of big separation zones in the system, k − ω SST and RNG k − ε perform quite similarly, the therefore k − ε relative inexpensiveness in this more complex and heavy system makes the latter the most convenient turbulence model for this kind of study. Looking at different parameters such as user-friendliness, time required, software costs and versatility, OpenFOAM proves itself to be a convenient, accurate and valid alternative to its expensive commercial counterpart. Finally, following these conclusions a user-guide method to analyze not only the single but the entire class of air intake systems for Scania internal purposes is developed and reported.
Denna rapport är resultatet av ett gemensamt samarbete mellan två svenska företag AFRY och Scania AB. Behovet av CFD-analyser för internströmning ökar kontinuerligt i företag som Scania. Användningen av CFD-programvara är dock begränsad av licenskostnader. Syftet med denna avhandling är att fastställa om OpenFOAM, ett öppet källkodsprogram för CFD, kan vara ett lämpligt alternativ till den nuvarande kommersiella CFD-programvaran StarCCM+ när man utför en analys av totala tryckfallet på luftintagssystem. Dessutom är ett andra målatt utveckla en standardmetod för att analysera denna systemklass med hjälp av OpenFOAM. Luftintagssystemet spelar en väsentlig roll för verkningsgraden i bussar och lastbilars motorer. Det ansvarar för att överföra ren luft med minimalt tryckfall till motorn. Efter en illustrativ introduktion av ämnet samt teoriprinciperna fokuserar följande tekniskaanalys först på en CFD-studie av den enkla Buice och Eaton 2D-diffusorn, för att främst jämföra OpenFOAM, StarCCM+ och experimentell data i analyser av turbulensmodeller och nätoberoende. För båda programvarorna misslyckas turbulensmodellen RNG k − ε nästan helt ochhållet att prediktera separationen och den breda återcirkulationsbubblan som bildas i diffusorn, medan k − ω SST, som inte är mycket mer beräkningskrävande, ger mycket exakta resultat. Även hastighetsprofiler, tryckkoefficienter och grafer med ytfriktion som analyserats med RNG k − ε turbulensmodell visar större skillnad jämfört med experimentell data. Sedan följer en CFD-studie av det totala tryckfallet i Scania luftintagssystemet. Efter att ha genomfört flera simuleringar för ett antal olika massflödeshastigheter har det visats att bådeOpenFOAM och StarCCM+ resulterar i mycket bra överensstämmelse med experimentell data. Den här gången, på grund av brist på stora separationszoner i systemet, fungerar k − ω SSToch RNG  k − ε likvärdigt. Detta gör  k − ε som är relativt billig för mer komplexa och tungasystem till den mest lämpliga turbulensmodellen för den här typen av studier. Genom att studera olika parametrar som användarvänlighet, erforderlig tid, mjukvarukostnaderoch mångsidighet visar sig OpenFOAM vara ett bekvämt, korrekt och giltigt alternativ till dess dyra kommersiella motsvarighet. Slutligen, efter dessa slutsatser utvecklas och rapporteras en användarguide för att analysera inte bara denna enskilda utan hela klassen av luftintagssystemför Scanias interna ändamål.

In 1999, California Air Resources Board (CARB) implemented a regulation that required all gasoline cars sold in California be fitted with an Onboard Refueling Vapor Recovery System (ORVR). The ORVR system is designed to prevent Volatile Organic Compounds (VOCs) from escaping into the atmosphere during refuelling by storing the gas vapours in a carbon canister. Due to the complex nature of the fuel system, making design changes could have large implications on the ORVR performance of the vehicle. It is therefore desirable to develop a CFD model that can predict the effects of design changes, thereby reducing the need to perform physical tests on each design iteration. This master thesis project was performed at the Fuel Systems department at Volvo Cars in order to help reduce project lead times and product development costs by incorporating CFD as a part of the fuel system development cycle. The CFD results obtained were validated through experimental tests that were also performed as part of this project. In this master thesis project, a CFD model was developed to simulate the refuelling of gasoline for a California specification Volvo XC90 with an OPW-11B pump pistol. The model was set up in STAR-CCM+ using the Eulerian Volume of Fluid model for multiphase flow, the RANS realizable k-epsilon turbulence model and the two layer all y+ wall treatment. The effects of the carbon canister were modelled as a porous baffle interface in the simulations where viscous and inertial resistances of the porous media were adjusted to obtain a desired pressure drop across the canister. This method proved to be a suitable simplification for this study. The effects of evaporation as well as a chemical adsorption model for the carbon canister have been excluded from the project due to time limitations. It was found that the CFD simulations were in good agreement with the experimental results, especially with respect to capturing the overall behaviour of the fuel system during refuelling. It was found that resolving the flow spatially (and temporally) in the filler pipe was a crucial part in ensuring solver stability. A pressure difference between experiment and simulation was also observed as a consequence of excluding evaporation from the CFD model. After the CFD model had been verified and validated, changes to different parts of the fuel system were investigated to observe their effects on ORVR performance. These included changing the recirculation line diameter, changing the carbon canister properties and changing the angle of how the pump pistol was inserted into the capless unit. It was found that the recirculation line diameter is a very sensitive design parameter and increasing the diameter would result in fuel vapour leaking back out into the atmosphere. Similarly, increasing the back pressure by swapping to a different carbon canister would result in the leakage of fuel vapour. On the other hand, insignificant changes in system behaviour were observed when the fuel pistol angle was changed.

In 1999, California Air Resources Board (CARB) implemented a regulation that required all gasoline cars sold in California be fitted with an Onboard Refueling Vapor Recovery System (ORVR). The ORVR system is designed to prevent Volatile Organic Compounds (VOCs) from escaping into the atmosphere during refuelling by storing the gas vapours in a carbon canister. Due to the complex nature of the fuel system, making design changes could have large implications on the ORVR performance of the vehicle. It is therefore desirable to develop a CFD model that can predict the effects of design changes, thereby reducing the need to perform physical tests on each design iteration. This master thesis project was performed at the Fuel Systems department at Volvo Cars in order to help reduce project lead times and product development costs by incorporating CFD as a part of the fuel system development cycle. The CFD results obtained were validated through experimental tests that were also performed as part of this project. In this master thesis project, a CFD model was developed to simulate the refuelling of gasoline for a California specification Volvo XC90 with an OPW-11B pump pistol. The model was set up in STAR-CCM+ using the Eulerian Volume of Fluid model for multiphase flow, the RANS realizable k − ε turbulence model and the two layer all y + wall treatment. The effects of the carbon canister were modelled as a porous baffle interface in the simulations where viscous and inertial resistances of the porous media were adjusted to obtain a desired pressure drop across the canister. This method proved to be a suitable simplification for this study. The effects of evaporation as well as a chemical adsorption model for the carbon canister have been excluded from the project due to time limitations. It was found that the CFD simulations were in good agreement with the experimental results, especially with respect to capturing the overall behaviour of the fuel system during refuelling. It was found that resolving the flow spatially (and temporally) in the filler pipe was a crucial part in ensuring solver stability. A pressure difference between experiment and simulation was also observed as a consequence of excluding evaporation from the CFD model. After the CFD model had been verified and validated, changes to different parts of the fuel system were investigated to observe their effects on ORVR performance. These included changing the recirculation line diameter, changing the carbon canister properties and changing the angle of how the pump pistol was inserted into the capless unit. It was found that the recirculation line diameter is a very sensitive design parameter and increasing the diameter would result in fuel vapour leaking back out into the atmosphere. Similarly, increasing the back pressure by swapping to a different carbon canister would result in the leakage of fuel vapour. On the other hand, insignificant changes in system behaviour were observed when the fuel pistol angle was changed.In 1999, California Air Resources Board (CARB) implemented a regulation that required all gasoline cars sold in California be fitted with an Onboard Refueling Vapor Recovery System (ORVR). The ORVR system is designed to prevent Volatile Organic Compounds (VOCs) from escaping into the atmosphere during refuelling by storing the gas vapours in a carbon canister. Due to the complex nature of the fuel system, making design changes could have large implications on the ORVR performance of the vehicle. It is therefore desirable to develop a CFD model that can predict the effects of design changes, thereby reducing the need to perform physical tests on each design iteration. This master thesis project was performed at the Fuel Systems department at Volvo Cars in order to help reduce project lead times and product development costs by incorporating CFD as a part of the fuel system development cycle. The CFD results obtained were validated through experimental tests that were also performed as part of this project. In this master thesis project, a CFD model was developed to simulate the refuelling of gasoline for a California specification Volvo XC90 with an OPW-11B pump pistol. The model was set up in STAR-CCM+ using the Eulerian Volume of Fluid model for multiphase flow, the RANS realizable k − ε turbulence model and the two layer all y + wall treatment. The effects of the carbon canister were modelled as a porous baffle interface in the simulations where viscous and inertial resistances of the porous media were adjusted to obtain a desired pressure drop across the canister. This method proved to be a suitable simplification for this study. The effects of evaporation as well as a chemical adsorption model for the carbon canister have been excluded from the project due to time limitations. It was found that the CFD simulations were in good agreement with the experimental results, especially with respect to capturing the overall behaviour of the fuel system during refuelling. It was found that resolving the flow spatially (and temporally) in the filler pipe was a crucial part in ensuring solver stability. A pressure difference between experiment and simulation was also observed as a consequence of excluding evaporation from the CFD model. After the CFD model had been verified and validated, changes to different parts of the fuel system were investigated to observe their effects on ORVR performance. These included changing the recirculation line diameter, changing the carbon canister properties and changing the angle of how the pump pistol was inserted into the capless unit. It was found that the recirculation line diameter is a very sensitive design parameter and increasing the diameter would result in fuel vapour leaking back out into the atmosphere. Similarly, increasing the back pressure by swapping to a different carbon canister would result in the leakage of fuel vapour. On the other hand, insignificant changes in system behaviour were observed when the fuel pistol angle was changed.

Atherosclerosis is a term coined to describe a state in which arterial wall thickens due to the accumulation of fatty materials like cholesterol. Though not completely understood, it is believed to occur due to the accumulation of macrophage white blood cells and promoted by low density lipoprotein. Increase in accumulation of plaque leads to enlargement of arteries as arterial wall tries to remodel itself. But eventually the plaque ruptures, letting out its inner content to blood stream. The ruptured plaque clots and heals and shrinks down as well but leaves behind stenosis - narrowing of cross section. Depending on the degree of stenosis blood supply from the artery to its respective organ could decrease and even get blocked completely. Frequently, as the vulnerable plaques rupture, thrombus formed as such could flow through bloodstream towards smaller vessels and block them, leading to a sudden death of tissues fed by that vessel. If the plaques do not rupture and artery gets enlarged to a great extent then it results in an aneurysm. Such blockage of coronary arteries in heart can lead to myocardial infarction - heart attack, in carotid arteries in brain can lead to what is called a stroke, in peripheral arteries in legs can lead to ulcers, gangrene (death of tissue) and hence loss of leg, in renal arteries can lead to kidney malfunction. The most disturbing fact about atherosclerosis is the inability to detect the disease in preliminary stages. As stated by Miller (2001), most of the times coronary artery disease (CAD) gets diagnosed only after 50-75 percent occlusion of arteries.

Nowadays, renewable energy is in full growth. In particular, offshore wind farms will be at the centre of UK energetic strategy in the coming years. However, other types of marine renewable are still at an early development stage. That is the case for tidal energy. Many projects have been undertaken but there is no candidate for competitive commercial applications yet. Deltastream is one of these numerous pioneering projects. It consists of a set of three marine current turbines mounted on a triangular base put down onto the seabed. The device is not moored and no harm is done to the environment. However, that makes the structure more sensitive to water flows. And it is important to ensure that it will remain at its location and not being carried along with the tidal streams. Using CFD, the present study aims to evaluate the drag on the nacelles of the structure and come up with solutions to reduce it as much as possible.

Focusing on reducing the air consumption of an air flotation rail system, a flowrate-amplified flotation element was recently developed. This new flotation element ulitises the rotational flow to intake extra air via an intake hole, and thus, effectively improves the flotation height. Compared to a conventional flotation element, the flowrate-amplified flotation element can reduce air consumption by approximately 50% for the same load and flotation height. To gain an understanding of the flow phenomenon in the flowrate-amplified flotation element, experiments and CFD simulations are conducted in this study. Based on the results, we found that the flowrate-amplified flotation element could take a part of the kinetic energy of the rotating air to suck in extra air. The intake hole greatly affects the pressure field and velocity field of the flotation element. Additionally, the effects of the variant gap height and supplied flow rate were also discussed. The results indicate that the pressure distribution decreases as the gap height increases and increases as the supplied flow rate increases.

In pulverised coal fired boilers, entrained fly ash particles in the flue gas often leads to erosive wear on metal surfaces along the flow field. This can have a significant effect on the operational life of various sections of boiler (in particular regenerative heat exchanger tubes). In this work, CFD based code FLUENT is used in conjunction with erosion model developed by other researchers for a large-scale furnace to identify the areas likely to be subjected to erosion under various operating conditions.Eulerian- Lagrangian approach is considered to analyse continuum phase and particle tracking for the coal particle. Flow field has been thoroughly examined in terms of velocity, particle and temperature profiles along the gas flow path. The data obtained on particle velocities and trajectories have been utilised to predict the extent of erosion in selected areas of boiler components. Predictions have been found to be in good agreement with the published data as well as plant observations for velocities ranging from 15 to 32 m/s showing a deviation of approximately 4.9 % with 20° impact angle.The results obtained from the present work for understanding erosion pattern in boilers are not only of practical significance but also provides platform for the development of an erosion tool which could assist power utilities in avoiding unnecessary shutdowns and penalties associated with the replacement of boiler components.

With the increase in length of wind turbine blades flutter is becoming a potential design constrain, hence the interest in computational tools for fluid-structure interaction studies. The general approach to this problem makes use of simplified aerodynamic computational tools. Scope of this work is to investigate the outcomes of a 3D CFD simulation of a complete wind turbine blade, both in terms of numerical results and computational cost. The model studied is a 5MW theoretical wind turbine from NREL. The simulation was performed with ANSYS-CFX, with different volume mesh and turbulence model, in steady-state and transient mode. The convergence history and computational time was analyzed, and the pressure distribution was compared to a high fidelity numerical result of the same blade. All the model studied were about two orders of magnitude lighter than the reference in computation time, while showing comparable results in most of the cases. The results were affected more by turbulence model than mesh density, and some turbulence models did not converge to a solution. In general seems possible to obtain good results from a complete 3D CFD simulation while keeping the computational cost reasonably low. Attention should be paid to mesh quality.

This study focuses on the design and development of a new spray applicator design utilizing effects of imposed pressure oscillations in conjunction with cavitation collapse energy to create distribution of fine droplets. An oscillating horn placed inside the nozzle performing high frequency oscillations is envisioned to provide the necessary pressure perturbations on the exiting liquid jet, while the nozzle geometry design in configured to amplify cavitation process. Initially, a two-zone approach modeling the nozzle interior and exterior in a separate fashion and later, a coupled strategy is proposed. Parametric studies describing the effect of horn stroke length, frequency, its position inside the nozzle in combination with different nozzle designs and liquid flow rates are explored to identify their contribution in obtaining desired cavitation characteristics. In this regard, incorporation of a backward facing step profile within the nozzle shows strong capability of generating the required cavitation and flow field distribution at the nozzle exit. The velocity modulations occuring at the nozzle exit due to oscillating horn structure result in a wide gamut of liquid structures specific to the imposed oscillation frequency and modulation amplitude. The disintegration characteristics of these modulated liquid jets are studied using a Volume-of-Fluid (VOF) interface capturing approach based on finite volume methodology employing an interface compression scheme. VOF methods are validated against experimental results and then subsequently used to study scaling parameters governing the modulated liquid jets. To perform coupled interior-exterior nozzle computations with cavitation, two new cavitation models are presented: First, a model based on Homogeneous Equilibrium assumptions for tracking cavitation events in a compressible framework is presented. Owing to its inability to simulate incompressible cavitating flows, a new cavitation event tracking model based on a Cavitation-Induced-Momentum-Defect (CIMD) correction approach is formulated utilizing a scalar transport model for vapor volume fraction with relevant transport, diffusion and source terms. Validations of both the models against experimental observations are detailed. Coupled internal-external liquid flow computations from the proposed atomizer design using a VOF-CIMD strategy shows strong potential for rapid drop formation in the presence of cavitation effects. A prototype model of a new spray applicator design is presented.

The thesis aims to lower the exhaust gas temperature on the coming EU6 Scania buses D7 and Otto gas and is carried out as a final part of the studies in the mechanical engineering program, KTH Stockholm. Euro 6, a new emission standard requirements for heavy duty trucks and buses, puts new demands on the amount of particulate matter and nitrogen oxides that can be emitted. This led to that Scania has developed and improved their after treatment systems. The new after treatment systems generates high exhaust temperatures, and Scania have expressed a desire to reduce these to create a safer environment around the bus.The thesis started with a thorough feasibility study, where the current exhaust systems and its components were studied. Solutions to lower exhaust temperatures were studied, both in the automotive industry and in other fields. The concepts that were developed would be analyzed through CFD simulations, why basic fluid mechanics were studied. Two different exhaust systems were to be analyzed, one with a gas engine and roof outlet and one with a diesel engine and ground outlet. A total of eight concepts were presented, in which five were determined to undergo CFD simulations.A competitor analysis was conducted in which three different diffusers from competing bus and truck manufacturers were CFD simulated.The results showed that the diffusers were superior to the other concepts. The diffuser designed in this project performed well in comparison to the diffusers from competing bus and trucks manufacturers, but it was considered to be expensive and difficult to manufacture. New diffuser designs were suggested, which are believed to have the same good qualities but cheaper to manufacture. The authors recommend Scania to perform field tests of the redesigned diffusers, and also try the ones designed by their competitors. Also, Scania should investigate how a venturi solution can be optimized.
Examensarbetet, vars mål är att sänka avgastemperaturen på Scanias bussar, är genomfört som en avslutande del av studierna på maskinkonstruktionsprogrammet, KTH Stockholm. EuroVI, en ny emissionskravsstandard för tunga fordon, ställer nya krav på hur mycket partiklar och kväveoxider som får släppas ut. Detta har resulterat i att Scania har utvecklat och förbättrat sina efterbehandlingssystem. De nya efterbehandlingssystemen ger upphov till höga avgastemperaturer, och Scania har uttryckt en önskan att sänka dessa för att skapa en säker miljö runt bussen.Examensarbetet började med en grundlig förstudie, där de aktuella avgassystemen och dess komponenter studerades. Lösningar för att sänka avgastemperaturer studerades, både inom fordonsindustrin och inom andra områden. Eftersom koncepten som togs fram skulle analyseras med CFD simuleringar, så studerades även grundläggande strömningsmekanik. Två olika avgassystem skulle analyseras, ett med en gasmotor och takutsläpp, och ett med en dieselmotor och markutsläpp. Totalt togs åtta koncept fram, varav fem ansågs intressanta för CFD simulering. Det gjordes även en konkurrentanalys, där tre olika diffusorer från konkurrerande buss- och lastbilstillverkare CFD simulerades.Resultaten visade att diffusorerna var överlägsna de andra koncepten. Diffusorn som utformats i detta projekt stod sig väl mot konkurrenternas diffusorer, men ansågs dock vara dyr och svår att tillverka. Nya designer togs fram, som anses ha samma temperatursänkande egenskaper men vara enklare att tillverka. Författarna till denna rapport rekommenderar Scania att gå vidare med fysiska tester av de förslagna diffusorerna, och att även testa konkurrenternas diffusorer. Scania bör även undersöka hur en venturilösning kan optimeras.

The ingestion of hot mainstream gas into the wheel-space between the rotor and staler discs is one of the most important internal cooling problems for gas turbine designers. To solve this problem, engineers design a rim seal at the periphery of wheel-space and direct a sealing flow from the internal cooling system to prevent ingress. The main aim of this thesis is to build a simple computational model to predict the scaling effectiveness of externally-induced ingress for engine designers. The axisymmetric model represents a gas turbine wheel-space and provides useful information related to the fluid dynamics and heat transfer in the wheel-space. At the same time, this model saves much computation time and cost for engine designers who currently use complex and time-consuming 3D models. The- computational model in this -thesis is called the prescribed ingestion model. Steady simulations are carried out using the commercial CFD code, ANSYS CFX with meshes built using ICEM CFD. Boundary conditions are applied at the ingress inlet of the model using experimental measurements and a mass-based averaging procedure. Computational parameters such as rotational Reynolds number, non-dimensional sealing flow rate and thermal conditions on the rotor are selected to investigate the fluid dynamics and heat transfer at typical experimental rig operating conditions. Different rim seal geometries arc investigated and results are compared with experimental data. In addition to the prescribed ingestion model, two typical axisymmetric rotor-stator system models without ingress arc established. The aim of these rotor-stator models is to investigate the fluid dynamics and heat transfer of the wheel-space in the situation without ingress. The effects of geometry and turbulence model also arc studied in these simulations. Most results from these simulations are in good agreement with experimental data from the literature, which enhances confidence in the prescribed Ingestion model.

As one of the fastest growing renewable energy sources, wind energy is playing an increasingly important part in addressing the climate change and energy crisis issues the world is currently facing. The abundance of wind resource in offshore areas makes them a popular choice for turbine installation. In the past few years, several floating wind projects have emerged where wind turbines are installed far offshore in deepwater sites on moored platforms. Compared to land-based or offshore fixed-bottom wind turbines, an FOWT is a fully coupled system where the wind turbine with flexible blades and the floating platform with its mooring system interact with each other in wind and waves, which makes old design tools inadequate. This work aims to develop a fully coupled high-fidelity aero-hydro-mooring-elastic analysis tool, and to better understand the sophisticated fluid-structure interactions for FOWTs. The numerical tool developed in this work takes advantage of the open source CFD toolbox OpenFOAM to accurately solve wind turbine aerodynamics and floating platform hydrodynamics, and utilises the open source MBD code MBDyn for structural dynamics within a multibody framework while modelling flexible bodies based on a nonlinear beam theory. Coupling of these two solvers is achieved by establishing an interface library to exchange data with the help of the TCP/IP protocol. Additionally, to tackle the complex mesh movement in FOWT simulations, a mesh motion solver is developed in OpenFOAM by combining the sliding mesh technique and the dynamic mesh morphing method. A mooring system analysis module comprising a quasi-static method and a lumped-mass based dynamic approach is also implemented to simulate mooring lines in an FOWT system. A series of test cases is firstly studied to validate the various features of the tool, including basic fluid flow solving, modelling of wind turbine aerodynamics, hydrodynamic analysis of a floating structure with its mooring system, dynamic analysis of a riser or mooring line and coupled analysis of flow induced vibration of a flexible beam. The developed tool is then applied to analyse FSI problems of FOWTs under three different scenarios. Firstly, a coupled aero-hydro-mooring analysis is carried out for the OC4 semisubmersible FOWT under regular waves and uniform wind speed. Blade flexibility is ignored, and mooring lines are solved using the quasi-static method. Interactions between the moored platform and the wind turbine are investigated, focusing on of platform motion on the aerodynamic performance of the wind turbine and the impacts of wind turbine aerodynamics on the responses of the floating platform and its mooring system. Subsequently, an aeroelastic analysis is conducted for the NREL 5-MW offshore wind turbine with flexible blades under uniform wind speed. Effects of blade flexibility on wind turbine aerodynamics and structural responses are studied using the developed CFD-MBD tool. The floating platform supporting the turbine is not directly modelled for simplicity and the influence of platform motion responses on the turbine are analysed via imposing a prescribed surge motion to the turbine base. Fully coupled aero-hydro-mooring-elastic analysis is lastly carried out for the OC4 semi-submersible FOWT under a combined wind/wave condition to demonstrate the capabilities of the developed CFD-MBD tool. Responses of the floating system are investigated in terms of platform hydrodynamics, mooring system dynamics, wind turbine aerodynamics and blade structural dynamics. Interactions between the FOWT and fluid flow are analysed by visualising results obtained via the CFD approach.

Thesis (Ph. D.)--Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Mechanical Engineering; and the Woods Hole Oceanographic Institution), 2009.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Includes bibliographical references (leaves 264-277).
The lateral line system on fish has been found to aid in schooling behavior, courtship communication, active and passive hydrodynamic imaging, and prey detection. The most widely used artificial prey stimulus has been the vibrating sphere, which some fish are able to detect even when the signal velocities to its lateral line are orders of magnitude smaller than background current velocities. It is not clear how the fish are able to extract this signal. This thesis uses a series of computational fluid dynamic (CFD) simulations, matched with recent experiments, to quantify the effects of 3D fish body parts on the received dipole signals, and to determine signal detection abilities of the lateral line system in background flow conditions. An approximation is developed for the dipole induced, oscillatory, boundary layer velocity profile over the surface of a fish. An analytic solution is developed for the case when the surface is a wall, and is accurate at points of maximal surface tangential velocity. Results indicate that the flow outside a thin viscous layer remains potential in nature, and that body parts, such as fins, do not significantly affect the received dipole signal in still water conditions. In addition, the canal lateral line system of the sculpin is shown to be over 100 times more sensitive than the superficial lateral line system to high frequency dipole stimuli. Analytical models were developed for the Mottled Sculpin canal and superficial neuromast motions, in response to hydrodynamic signals. When the background flow was laminar, the neuromast motions induced by the stimulus signal at threshold had a spectral peak larger than spectral peaks resulting from the background flow induced motions.
(cont.) When the turbulence level increased, the resulting induced neuromast motions had dominant low frequency oscillations. For fish using the signal encoding mechanisms of phase-locking or spike rate increasing, signal masking should occur.
by Mark Andrew Rapo.
Ph.D.

This master thesis describes the co-firing concept, benefits and opportunities of pretreated biomass in pulverized coal boilers for industrial use. Burning fossil fuels, i.e. coal is under immense political pressure as European Union (EU) and other countries are trying to bring down the CO2 emission. Biomass combustion is already a proven technology and it plays a greater role in reducing CO2 emission. The main objective of this thesis is the brief study of computational fluid dynamics (CFD) modelling to examine the co-firing of greater amount of pretreated biomass and pulverized coal in a 200MWe pulverized coal boiler. Here, we exchange around 50 % of existing fuel in pulverized coal boiler with torrefied biomass. Torrefied biomass aids to increase the efficiency of existing coal boiler and cut down the CO2 emission. In this work, two cases of co-firing of pretreated biomass and coal have been investigated by CFD. Firstly, an experimental work was done in a laboratory scale to have few different types of torrefied biomass with different degrees of torrefaction. The devolatilization kinetics and char oxidation kinetics were also determined by experiments and other parameters have been calculated. One important aspect of this work has been to evaluate the performance of torrefaction based co-firing. Therefore, co-firing case has been compared to the 100 % coal feed case to understand the performance of torrefaction based co-firing. Furthermore, fluid flow, particles trajectories, heat transfer, and different emission behaviors have been studied. In addition, mechanisms of corrosion during co-firing have been studied and a guideline has been provided for corrosion model for analyzing the characteristics of alkali metals and their effects in co-firing coal boiler. The outcome from the CFD simulation indicated that boiler efficiency increases and the net CO2 emission reduced with increasing the biomass percentage in the co-firing system.

Through the last decades, building designers should deal with reliable design strategies to take advantage of natural resources in order to increase energy efficiency in buildings, as well as to promote sustainable development and add value to the society. This thesis proposes a reliable building design strategy to improve buildings energy efficiency by means of natural ventilation (NV) use. The strategy consists in evaluating the most suitable architectural solution in a particular case study taking into account environmental conditions and building surroundings in order to maximize NV use since the early building design stage. Computational fluid dynamics (CFD) techniques are used to conduct the research. This is a powerful design tool that permits buildings NV behaviour simulation prior to building construction. Therefore, the aim of the thesis is to provide a real case study building in which the NV design strategy is applied to show a reliable example and support building design decisions since the design stage. The design strategy is based on the use of a commercial numerical code that solves the fluid mechanic equations. The CFD software simulates the features that influence NV and predicts its behaviour in the different building configurations prior to building construction. This numerical technique allows, on the one hand, the visualization of air flow paths in buildings. On the other hand, many quantifiable parameters are calculated by the software. Through the analysis and comparison of those parameters, the best architectural solutions are chosen. With regards to all possible architectural decisions, the research is focused on the façade configuration selection and the building location. First of all, the NV design strategy feasibility is analysed in a particular region: the Mediterranean Valencian Coastal area (Spain). The region is characterized by the uniform conditions of the prevailing wind during the warm season. Then, a validated CFD simulation is used to analyse qualitatively and quantitatively the building surrounding influence on wind paths through and around buildings. The objective is to compare different façade opening positions and select the alternative that takes more profit of the NV resources available. Additionally, a general quantification of the ventilated façade contribution to buildings energy efficiency is presented under the frame of the façade configuration selection. Secondly, two simulations are conducted to analyse two different building locations. The assessment of surrounding buildings influence on building NV behaviour is done through validated CFD models. Some parameters and visualizations are proposed to be used in the quantitative and qualitative assessment of each solution respectively. Then, the best location alternative with regards to NV performance is selected. Finally, the research is concluded with the case study building full-scale construction. The indoor CFD simulation used from the beginning is then successfully validated. The NV building behaviour is also successfully verified. Additionally, contrasted performance indexes are used to evaluate indoor comfort conditions: draught risk (DR), predicted mean vote (PMV) and predicted percentage of dissatisfied people (PPD). The results show that comfort conditions can be reached more energy efficiently by means of NV use. Afterwards, it is verified how the comfortable indoor environment conditions are ensured and optimized by the NV use. Although the design strategy is applied to a particular building design, the design strategy potential is that it could be applied to all buildings. Consequently, major potential energy savings could be achieved.
Durante las últimas décadas los agentes involucrados en el diseño de edificios deben de utilizar estrategias fiables de diseño que les permitan aprovechar los recursos naturales del entorno con el objetivo de aumentar la eficiencia energética de los edificios así como promover el desarrollo sostenible y generar valor añadido para la sociedad. Esta tesis propone una estrategia de diseño fiable de edificios para mejorar su eficiencia energética mediante el uso de la ventilación natural (NV por sus siglas en inglés "natural ventilation"). La estrategia consiste en evaluar la solución arquitectónica más adecuada teniendo en cuenta las condiciones ambientales y el entorno de los edificios con el objetivo de maximizar el uso de la ventilación natural desde la fase inicial de su diseño. En esta tesis se aplica la estrategia de diseño a un caso de estudio real y particular. La estrategia de diseño se basa en el uso de un código numérico comercial que resuelve las ecuaciones de la mecánica de fluidos (CFD por sus siglas en inglés "computational fluid dynamics"). El software CFD simula las características que influyen en la ventilación natural y predice su comportamiento en los edificios antes de su construcción. Esta técnica numérica permite la visualización del flujo de aire en los edificios. Además, el software permite calcular parámetros que son analizados y comparados posteriormente para elegir la solución arquitectónica que suponga un mejor comportamiento de la ventilación natural. Con respecto a todas las decisiones arquitectónicas posibles, la investigación se centra en la selección de la ubicación del edificio y de la configuración de los huecos de su fachada. En primer lugar, se analiza la viabilidad de la estrategia de diseño en una región determinada: la zona costera Mediterránea de la Comunidad Valenciana. La región se caracteriza por las condiciones uniformes del viento predominante durante la estación cálida. A continuación, se utiliza una simulación de CFD validada para analizar cualitativamente y cuantitativamente la influencia de los edificios circundantes en los flujos del viento a través y alrededor de los edificios circundantes. El objetivo es comparar distintas posiciones de los huecos de la fachada para seleccionar la alternativa que mejor aproveche los recursos de ventilación natural disponibles. Además, se presenta en el marco de la selección de la configuración de la fachada una cuantificación general de la contribución de la fachada ventilada a la eficiencia energética de los edificios. En segundo lugar, se realizan dos simulaciones para analizar dos ubicaciones diferentes del edificio caso de estudio. La evaluación de la influencia de los edificios circundantes en el comportamiento de la ventilación natural del edificio caso de estudio se realiza mediante la utilización de modelos CFD validados. Se proponen distintos parámetros y visualizaciones para la evaluación cuantitativa y cualitativa de cada solución. A continuación se selecciona la mejor ubicación con respecto al comportamiento de la ventilación natural en el edificio caso de estudio. Finalmente, la investigación concluye con la construcción a escala real del edificio caso de estudio. Se valida con éxito la simulación CFD del interior del edificio utilizada desde la etapa de diseño. También se verifica con éxito el comportamiento de la ventilación natural del edificio. Además, se analizan las condiciones de confort interiores mediante la evaluación de los siguientes índices: riesgo de corrientes de aire (DR por sus siglas en inglés "draught risk"), voto promedio previsto (PMV por sus siglas en inglés "predicted mean vote") y el porcentaje previsto de personas insatisfechas (PPD por sus siglas en inglés "predicted percentage of dissatisfied people"). Los resultados muestran que el uso de la ventilación natural permite alcanzar, de manera más energéticamente eficiente, las
Durant les últimes dècades els agents involucrats en el disseny d'edificis utilitzen estratègies fiables de disseny que els permeten aprofitar els recursos naturals de l'entorn amb l'objectiu d'augmentar l'eficiència energètica dels edificis així com promoure el desenvolupament sostenible i generar valor afegit per la societat. Aquesta tesi proposa una estratègia fiable de disseny d'edificis per a millorar la seva eficiència energètica mitjançant l'ús de la ventilació natural (NV per les sigles en anglès "natural ventilation"). L'estratègia consisteix a avaluar la solució arquitectònica més adequada tenint en compte les condicions ambientals i l'entorn dels edificis amb l'objectiu de maximitzar l'ús de la ventilació natural des de la fase inicial del seu disseny. En aquesta tesi s'aplica l'estratègia de disseny a un cas d'estudi real i particular. L'estratègia de disseny es basa en l'ús d'un codi numèric comercial que resol les equacions de la mecànica de fluids (CFD per les sigles en anglès "computational fluid dynamics"). El programari CFD simula les característiques que influeixen en la ventilació natural i prediu el seu comportament en els edificis abans de la seva construcció. Aquesta tècnica numèrica permet la visualització del flux d'aire en els edificis. A més, el programari permet calcular paràmetres que són analitzats i comparats posteriorment per triar la solució arquitectònica que supose un millor comportament de la ventilació natural. Pel que fa a totes les decisions arquitectòniques possibles, la investigació es centra en la selecció de la ubicació de l'edifici i de la configuració de les obertures de la façana. En primer lloc, s'analitza la viabilitat de l'estratègia de disseny en una regió determinada: la zona costanera Mediterrània de la Comunitat Valenciana. La regió es caracteritza per les condicions uniformes del vent predominant durant l'estació càlida. A continuació, s'utilitza una simulació de CFD validada per analitzar qualitativament i quantitativament la influència dels edificis circumdants en els fluxos del vent a través i al voltant dels edificis circumdants. L'objectiu és comparar diferents posicions dels buits de la façana per seleccionar l'alternativa que millor aprofite els recursos de ventilació natural disponibles. A més, en el marc de la selecció de la configuració de la façana es presenta una quantificació general de la contribució de la façana ventilada a l'eficiència energètica dels edificis. En segon lloc, es realitzen dues simulacions per analitzar dues ubicacions diferents de l'edifici cas d'estudi. L'avaluació de la influència dels edificis circumdants en el comportament de la ventilació natural de l'edifici cas d'estudi es realitza mitjançant la utilització de models CFD validats. Es proposen diferents paràmetres i visualitzacions per a l'avaluació quantitativa i qualitativa de cada solució. A continuació es selecciona la millor ubicació pel que fa al comportament de la ventilació natural a l'edifici cas d'estudi. Finalment, la investigació conclou amb la construcció a escala real de l'edifici cas d'estudi. Es valida amb èxit la simulació CFD de l'interior de l'edifici utilitzada des de l'etapa de disseny. També es verifica amb èxit el comportament de la ventilació natural de l'edifici. A més, s'analitzen les condicions de confort interiors mitjançant l'avaluació dels següents índexs: risc de corrents d'aire (DR per les sigles en anglès "draught risk"), mitjana de vots previstos (PMV per les sigles en anglès "predicted mean vote") i el percentatge previst de persones insatisfetes (PPD per les sigles en anglès "predicted percentage of dissatisfied people"). Els resultats mostren que l'ús de la ventilació natural permet assolir, de manera més energèticament eficient, les condicions de confort.
Mora Pérez, M. (2017). Computational fluid dynamics (CFD) applied to buildings sustainable design: natural ventilation. Case study [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/86208
TESIS

The Swedish Obligatory Ventilation Control (OVK) was established to ensure that ventilation systemsare clean and work according to design. The control system of today is however not perfect, and thereare many aspects of OVK which could be approached differently for improved efficiency and occupanthealth. A current project in Stockholm is looking at the possibility to use sensors, continuouslymetering volatile organic compounds (VOC), temperature, and relative humidity, in place of the airflowmetering of the traditional OVK solution. One of the first issues encountered was sensor placement, asthe sensors must be discreetly installed on available surfaces while the collected air quality data mustrepresent the air in the occupied zone. A second challenge concerns how the sensor outputs should beinterpreted in terms of indoor air quality.The main purpose of this thesis is to suggest a model that can evaluate the suitability of differentplacements of sensors (such as the ceiling, walls, or lamp fixtures) from a VOC perspective. The idea isto evaluate what areas of the room best represent the average air quality of the occupied zone. This partof the study was approached by combining literature review and computational fluid dynamics (CFD)in two case studies; one office and one apartment. The intent behind the iterated method is to present ageneral CFD model that can be easily interpreted and adapted to accommodate new objects (e.g.building types or rooms). A secondary objective is to discuss how temperature and relative humiditycan be included in the spatial position evaluation. Thirdly, the thesis aims to develop a base for furtherdiscussion regarding a method for how the sensor outputs can be combined into a single indoorenvironment quality index. The two last parts were primarily based on literature review.The conclusions drawn in this study include a general CFD model that can be modified to evaluatedifferent spatial location of VOC sensors, and general guidelines regarding placement of VOC meters inoffices or apartments. Also provided in this report is a base for further discussion concerning indoor airquality estimations by combining the provided sensor-outputs, i.e. total VOC, temperature, and relativehumidity.
Den obligatoriska ventilationskontrollen (OVK) infördes i Sverige för att säkerställa attventilationssystem är rena och fungerar som de är designade att göra. Dagens system är dock inteperfekt och det finns många aspekter av OVK som skulle kunna göras annorlunda för att främjaeffektivitet och personers hälsa. Ett pågående projekt i Stockholm vill undersöka möjligheten attersätta eller komplettera den luftflödesmätning som ingår i dagens OVK med kontinuerlig mätning avflyktiga organiska gaser (VOC), temperatur och relativ luftfuktighet med hjälp av sensorer. En av deförsta utmaningarna för projektet är placeringen av sensorer eftersom att de måste vara diskretinstallerade på befintliga ytor, samtidigt som det som uppmäts bör vara representativt för hur luftenupplevs av personer i byggnaden. En annan utmaning för projektet är hur signalerna från sensorn skakombineras för att utvärdera luftkvalitén.Det huvudsakliga syftet med den här studien är att utveckla en modell som kan användas för attutvärdera lämpligheten av olika sensorplaceringar, till exempel tak, väggar och lamparmaturer, utifrånett VOC-perspektiv. Idén är att ge en uppfattning om vilka placeringar som bäst representerarmedelkvalitén på luften i vistelsezonen. Denna del av arbetet baserades på litteraturstudier ochnumeriska beräkningar med CFD (Computational Fluid Dynamics). Den föreslagna modellenapplicerades i två fallstudier, ett kontor och en lägenhet. Avsikten med modellen är att den enkelt skakunna tolkas och anpassas för olika rumstyper. Ett annat syfte med rapporten är att diskutera hurtemperatur och relativ luftfuktighet kan inkluderas i utvärderingen av sensorplacering. Slutligen är etttredje syfte med studien att påbörja en diskussion för hur de tre mätvärdena från sensorn kankombineras i ett gemensamt inomhusklimatindex. De två sistnämnda delarna baserades främst pålitteraturstudier.Slutsatserna inkluderar en generell CFD-modell och metod som kan modifieras för att utvärdera olikaplaceringar av sensorn, samt riktlinjer för var VOC-sensorer bör placeras i kontor eller lägenheterbaserat på två fallstudier. Också inkluderade i rapporten är en diskussionsbas för hur en utvärdering avluftkvalitet kan göras utifrån de tre värdena från sensorn (VOC, temperatur och relativ luftfuktighet).

In rotogravure printing, engraved cells collect ink from a bath and then release it onto the substrate forming a series of printed dots that comprise the printed image. The purpose of this work was to explore the ink release in the rotogravure process to improve its predictability and scientific understanding. Two complimentary approaches have been investigated, an empirical approach embodied into Artificial Neural Network models and a numerical physical approach based on Computational Fluid Dynamics modelling. The Artificial Neural Network approach was based on a statistical correlation of experimental data on cell geometry with optical properties of the resulting print. The developed A.N.N. models were able to accurately predict the effect of cell geometry on ink release, outperforming traditional modelling techniques such as polynomial regression fitting techniques. The models were found to be practical and suitable to integration into manufacturing environments. The A.N.N. modelling highlighted the need for improved cell geometry data; to facilitate this, new software was developed for the automatic and accurate geometric characterisation of the engraved cells from interferometric profiles. A Computational Fluid Dynamic model of the ink release was successfully developed; the process was described as the evacuation of a Newtonian liquid from an axisymmetric cavity, showing the progressive splitting of the ink and retention of ink in the cell. This model is the first that takes into consideration the dynamic contact angle in the analysis of ink release from a cavity. The reliability of the numerical method and of its dynamic contact angle model was verified by comparing specifically designed models with experimental and literature data. The developed evacuation model shows the importance of the evacuation speed on the dynamics of the process and the critical importance of dynamic contact angle.

Environmental control in agricultural and agro-industrial buildings is a very important and topical subject which falls within the domain of precision agriculture and smart food processing. There is a growing interest in the study of systems able to combine active and passive environmental control techniques and in the development of methodologies for modeling and simulating the environmental conditions in the agro-industrial sector. These ones need a process of validation and experimental calibration, but they can investigate specific aspects and variations of the thermo-fluidynamic phenomena involved in the control of environmental parameters. The computational fluid dynamic application (CFD) can give these opportunities and it has been used to study animal comfort in farms, distribution of temperature and humidity in greenhouses, to define structural improvement of greenhouses and to investigate effective ventilation strategies. This thesis is focused on the ventilation aspects in agricultural buildings, with the aim of considering improvement actions to optimize and act on airflow conditions in an experimental greenhouse and in a cellar, where climate control is extremely important. The natural ventilation of a glass greenhouse has been investigated with a particular focus on the effects of internal shading screens on the internal fluid-dynamic and on the crop growing conditions. A deep focus on the characterization of this type of screens has been carried out, with the aim of identifying applicable methodologies for this purpose. Finally, a smart system has been created to be placed in a cellar, for the improvement of the air flows around the barrels, which would thus prevent the molds formation and avoid air stagnation areas. Different configurations have been analyzed to identify the optimal design of the system. In conclusion, CFD approach has allowed to reach conclusions on possible decisions or strategies to improve the ventilation for improving the production and food quality conditions.

A numerical study of a proposed innovative jet interaction configuration is presented. This work aimed at improving present-day jet interaction configurations in their applications as control thrusters on hypersonic vehicles. Jet thrusters are a useful control system for fast-moving vehicles flying in the upper layers of the atmosphere because of their effectiveness and responsiveness. They produce a strong and responsive lateral force on the vehicle through the interaction of two main mechanisms. The first mechanism comes from the momentum of the injectant itself, basically the thrust of the jet. The second and subtler contribution comes from the jet interaction flowfield, the interaction of the expanding injectant with the crossflow. This interaction produces areas of high pressure ahead of the injector and areas of low pressure in the region aft of the jet. The combination of the high-pressure regions in front of and low-pressure regions aft of the injector produces an undesirable nose-down pitching moment on the vehicle. In order to counterbalance the nose-down attitude, modern-day thruster designs include a large secondary injector far aft of the center of gravity of the vehicle. The thrust of this second injector acting far aft of the primary injector neutralizes the nose-down pitching moment. This is not an efficient method to obviate the problem since it requires the vehicle to be designed to carry two large thrusters and double the quantity of fuel necessary for one thruster. In light of these considerations, this study aimed at developing a jet interaction configuration that can dispense from the need of a large secondary injector to compensate for the nose-down pitching moment. The cases studied here were first a primary jet alone and then a primary jet with pairs of smaller jets. This configuration was based on the notion that the interaction of the secondary jets, conveniently located immediately aft of the thruster, with the barrel shock and the wake of the primary jet can drastically reduce the nose-down pitching moment. Because of the complexity of the jet interaction flowfield the investigation of the feasibility and the assessment of the efficiency of the new jet interaction configurations combined the present numerical effort with experimental studies of jet interaction flowfields performed in the supersonic wind tunnel at Virginia Tech. During the present numerical study the jet interaction flowfield associated with the sonic injection of a gas into a high-speed crossflow was simulated by numerically solving the Reynolds Averaged Navier Stokes (RANS) equations. Turbulence was modeled through a first-order model, the Wilcox's 1988 k-w turbulence model. The computations made use of the finite volume code General Aerodynamic Simulation Program (GASP) Version 4. For simplicity and to keep the study general, the jet interaction flowfield was studied on a flat plate instead of a body of revolution as on a vehicle. Calculations were run for a number of jet interaction configurations consisting of a primary jet alone, a primary jet and one pair of secondary jets, and a primary jet and two pairs of secondary jets. The flow conditions of the simulations ranged from a Mach number of 2.1 up to a Mach number of 4.5 and jet total pressure to freestream static pressure ratios of 14 to 680. A large effort was dedicated to the development of an efficient computational grid that could capture most of the flow-physics with a minimum number of cells. To this end , Chimera or overset grids were employed in the simulation of the secondary injectors. Grid convergence was shown to be achieved for the case of single injection by conducting a thorough convergence study. The discretization error was calculated through a modified Richardson extrapolation to be low. The numerical solutions were compared to the experimental results in order to assess the capability of RANS equations and of first-order turbulence models to properly simulate the complex flowfield. The k-w turbulence model proved to be reliable and robust and the results it provided for this type of flowfield were accurate enough from an engineering standpoint to make informed decisions about the configuration layout. In spite of the overall good performance, the k-w turbulence model failed to correctly predict the flow in the regions of strong adverse pressure gradients. Comparisons with experimental results showed that the separation region was often under-predicted thus highlighting the need to employ better turbulence models for more accurate results. The RANS equations were found accurate enough to provide physical mean-flow solutions. Further, the numerical simulations provided information about the detailed physics of the flowfield that is impossible to obtain through experimental work. The analysis of the numerical solutions highlighted the existence of a complex system of counter-rotating trailing vortices that are responsible for the mixing of the injectant with the freestream. The typical features of the flowfield created by an under-expanded jet exhausting in a quiescent medium were visible in the jet interaction flowfield with the difference of the existence of a crossflow and a non-uniform back-pressure. The region of low pressure aft of the injector was shown to be generated by the detachment of the barrel shock from the surface of the flat plate that leaves a large volume to be filled by the surrounding fluid. The simulations showed that the innovative configuration with one primary jet and an array of smaller secondary jets can effectively decrease the nose-down pitching moment by as much as 160%. In some cases, it also increased the total normal force acting on the flat plate (namely the thrust) by as much as 3%. This effect was found to be caused by the reduction in size and intensity of the low-pressure region aft of the primary injector.
Ph. D.

Includes abstract.
Includes bibliographical references.
Spark Ignition (SI) engine fuels' anti-knock properties are measured in the Co-operative Fuel Research (CFR) engine under two different test conditions as prescribed by the American Society for Testing and Materials (ASTM) for Research Octane Number (RON) and Motor Octane Number (MON) ratings. Recent research has been focused on determining whether the numerical difference between RON and MON, known as Octane Sensitivity (OS), is a result of the chemical or physical properties of the fuel. The present research examined the effect that the operating environment has on fuel evaportion, and thus OS, in the CRF engine.

A miniature centrifugal compressor is a key component of a reverse Brayton cycle cryogenic cooling system. The system is commonly used to generate a low cryogenic temperature environment for electronics to increase their efficiency, or generate, store and transport cryogenic liquids, such as liquid hydrogen and oxygen, where space limit is also an issue. Because of space limitation, the compressor is composed of a radial inlet guide vane, a radial impeller and an axial-direction diffuser (which reduces the radial size because of smaller diameter). As a result of reduction in size, in order to obtain the required static pressure ratio/rise, the rotating speed of the impeller is as high as 313 KRPM, if Helium is used as the working fluid. Two main characteristics of the compressor – miniature and high-speed, make it distinct from conventional compressors. Higher compressor efficiency is required to obtain a higher COP (coefficient of performance) system. Even though miniature centrifugal compressors start to draw researchers' attention in recent years, understanding of the performance and loss mechanism is still lacking. Since current experimental techniques are not advanced enough to capture details of flow at miniature scale, numerical methods dominate miniature turbomachinery study. This work numerically studied a high speed miniature centrifugal compressor. The length and diameter are 7 cm and 6 cm, respectively. The study was done on the same physical compressor but with three different combinations of working fluid and operating speed combinations: air and 108 KRPM, helium and 313 KRPM, and neon and 141 KRPM. The overall performance of the compressor was predicted with consideration of interaction between blade rows by using a sliding mesh model. It was found that the specific heat ratio needs to be considered when similarity law is applied. But Reynolds number effect can be neglected. The maximum efficiency observed without any tip leakage was 70.2% for air 64.8% for helium 64.9% for neon. The loss mechanism of each component was analyzed. Loss due to turning bend was found to be significant in each component, even up to 30%. Tip leakage loss of small scale turbomachines has more impact on the impeller performance than that of large scale ones. Use of 10% tip gap was found to reduce impeller efficiency from 99% to 90%. Because the splitter was located downstream of the impeller leading edge, any incidence at the impeller leading edge leads to poorer splitter performance. Therefore, the impeller with twenty blades had higher isentropic efficiency than the impeller with ten blades and ten splitters. Based on numerical study, a four-row vaned diffuser was used to replace a two-row vaned diffuser. It was found that the four-row vaned diffuser had much higher pressure recovery coefficient than the two-row vaned diffuser. However, most of pressure is found to be recovered at the first two rows of diffuser vanes. Consequently, the following suggestions were given to further improve the performance of the miniature centrifugal compressor. 1. Redesign inlet guide vane based on the numerical simulation and experimental results. 2. Add de-swirl vanes in front of the diffuser and before the bend. 3. Replace the current impeller with a twenty-blade impeller. 4. Remove the last row of diffuser.
Ph.D.
Department of Mechanical, Materials and Aerospace Engineering;
Engineering and Computer Science
Mechanical Engineering

The understanding of secondary flow behavior has become an important aspect in the design of modern gas turbines. Secondary flow gives rise to aerodynamic losses, distorts the thermal field and affects the flow conditions at the exit of a passage negatively. Therefore, reducing secondary flow is a major concern for efficiency improvement. Many passive control-methods have been suggested by turbine designers and researchers, and one very promising modification is blade leading edge contouring near the endwall. At the Division of Heat and Power Technology KTH, Stockholm, a detailed experimental investigation of three filleted nozzle guide vanes in an annular sector cascade has been performed, providing excellent experimental data for numerical validation of complex turbine flows. Based on the above, a numerical study and aerodynamic investigation for a leading edge filleted vane and baseline vane has been performed. The potential effect of the leading edge fillet on flow structure and secondary losses has been evaluated based on a number of flow parameters, and computational predictions have been compared to experimental results. The numerical investigation has shown some differences in the flow behavior between the filleted and baseline case. All results indicate that the fillet affects the flow structure in regions close to the hub endwall. It shifts the position of vortices and loss core. However, the overall effect on reducing secondary losses downstream of the passage is insignificant. Additionally, the numerical results show good qualitative agreement with experimental results.

The labyrinth seal is widely used in turbo machines to reduce leakage flow. The stability of the rotor is influenced by the labyrinth seal because of the driving forces generated in the seal. The working fluid usually has a circumferential velocity component before entering the seal; the ratio of circumferential velocity and shaft synchronous surface velocity is defined as pre-swirl rate. It has been observed that pre-swirl rate is an important factor affecting driving forces in the labyrinth seal thus affecting the stability of the rotor. Besides the pre-swirl, the eccentricity, the clearance, and the configuration of tooth locations are all factors affecting the rotordynamic properties of the labyrinth seal. So it is of interest to investigate the exact relationships between those factors and the sealâ s rotordynamic properties. In this research, three types of labyrinth seals have been modeled: the straight eye seal, the stepped eye seal, and the balance drum seal. For the straight eye seal, a series of models were built to study the influence of eccentricity and clearance. The other two seals each have only one model. All models were built with Solid Works and meshed with ANSYS-ICEM. Flows in those models were simulated by numerically solving the Reynolds-Averaged Navier-Stokes (RANS) equations in the ANSYS-CFX and then rotordynamic coefficients for each seal were calculated based on the numerical results. It had previously been very difficult to generate a pre-swirl rate higher than 60% in a numerical simulation. So three ways to create pre-swirl in ANSYS-CFX were studied and finally the method by specifying the inlet velocity ratio was employed. Numerical methods used in this research were introduced including the frame transfer, the k-ε turbulence model with curvature correction, and the scalable wall function. To obtain the optimal mesh and minimize the discretization error, a systematical grid study was conducted including grid independence studies and discretization error estimations. Some of the results were compared with previous bulk-flow or experimental results to validate the numerical model and method. The fluid field in the labyrinth seal must be analyzed before conducting rotordynamic analysis. The predicted pressure distributions and leakages were compared with bulk-flow results. A second small vortex at the downstream edge of each tooth was found in the straight eye seal. This has never been reported before and the discovery of this small vortex will help to improve seal designs in the future. The detailed flows in discharged region and in chambers were also discussed. Radial and tangential forces on the rotor were solved based on the fluid field results. It is shown that the traditional first-order rotordynamic model works well for low pre-swirl cases but does not accurately reflect the characteristics for high pre-swirl cases. For example compressor eye seals usually have pre-swirl rates bigger than 70% and the second order model is required. Thus a second-order model including inertia terms was built and applied to the rotordynamic analysis in this research. The influence of pre-swirl, eccentricity and clearance were studied using the straight eye seal model. The rotordynamic characteristics of the stepped eye seal and the balance drum seal were studied considering high pre-swirl rates. Some relationships between influencing factors and the four rotordynamic coefficients were concluded. The results also showed that for all the three seals higher pre-swirl leads to higher cross-coupled stiffness which is one of the main factors causing rotor instability. The rotor stability analysis was conducted to study the influence of drum balance seal on the stability. The rotor was designed with typical dimensions and natural frequencies for a centrifugal compressor rotor. The parameters for bearing and aerodynamic force were also set according to general case in compressors to minimize the effects from them. The result shows that the high pre-swirl rate in balance drum seal leads to rotor instability, which confirmed the significant effect of pre-swirl on the seal and the rotor system.
Ph. D.

We have studied gas flow and particle deposition in a realistic three-dimensional model of the bronchial tree, extending from the trachea to the segmental bronchi (7th airway generation for the most distal ones) using Computational Fluid Dynamics (CFD). The model is based on the morphometrical data of Horsfield et al. [J. Appl. Physiol. 31: 207-217, 1971] and on bronchoscopic and CT images, which give the spatial 3D-orientation of the curved ducts. It incorporates realistic angles of successive branching planes. Steady inspiratory flow varying between 50cm³/s and 500cm³/s was simulated as well as deposition of spherical aerosol particles (1 to 7&
Doctorat en sciences appliquées
info:eu-repo/semantics/nonPublished

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Architecture, 2005.
Includes bibliographical references (p. 150-154).
Natural ventilation is widely applied in sustainable building design because of its energy saving, indoor air qualify and indoor thermal environment improvement. It is important for architects and engineers to accurately predict the performance of natural ventilation, especially in the building design stage. Unfortunately, there is not any good public tool available to predict the natural ventilation design. The integration of the multi-zone model and the computational fluid dynamics (CFD) simulation provides a way to assess the performance of natural ventilation in whole buildings, as well as the detailed thermal environmental information in some particular space. This work has coupled the multi-zone airflow model with the thermal model. A new program, called MultiVent, has been developed with a web-server that can provide online calculation for the public. The MultiVent program can simultaneously simulate the indoor air temperature and airflow rate with known indoor heat sources for buoyancy dominated, buoyancy-wind combined and wind dominated cases. To properly apply the MultiVent program to the natural ventilation design, two configurations in naturally ventilated buildings should be carefully studied: the atrium and large openings between the zones. A criterion has been set up for dividing the large opening and the connected atrium space into at least two sub-openings and sub-zones. The results of the MultiVent calculation can provide boundary conditions to the CFD simulation for some particular zone. In order to correctly simulate the particular space with CFD, the location and conditions at the integrating surface (boundary surface) have been studied. This work suggested that the simulation zone should include part of the connected atrium space when
(cont.) the occupied room is simulated with CFD. There are two options to integrate the MultiVent and CFD simulation through different boundary conditions: velocity (mass) integration and pressure integration. The case studies of this work showed that both of them can generate good CFD simulation results.
by Gang Tan.
Ph.D.

This research investigates the effect of surface roughness on Thermo- Elastohydrodynamic Lubrication (TEHL) by Computational Fluid Dynamics (CFD). Traditionally, the Reynolds equation has been used to describe the flow of a lubricant for the TEHL problem, but this approach has some limitations. To overcome these, CFD is used in this research, as an alternative to solving the Reynolds equation. The commercial software packages ANSYS ICEM CFD 13.0 and ANSYS FLUENT 13.0 are employed to solve the Navier-Stokes equations. User-defined functions (UDFs) for the heat generated in the lubricant film, the density and the viscosity of lubricant, and the elastic deformation of the cylindrical roller bearing are created for this particular research. For viscosity, the lubricant is modelled as a non-Newtonian fluid based on the Ree-Eyring model. A number of CFD models are created under different conditions to predict the flow characteristics in the TEHL line contact problem, including the pressure distribution, the temperature distribution, the film thickness, and the friction coefficient. The effect of surface roughness is considered in the CFD models. The predicted results from the CFD models and the Reynolds equation are compared. The pressure distribution and the film thickness of both models are found to be in agreement. The simulation results show that the surface roughness affects significantly for the behaviour of fluid film lubrication problems, especially in the thin film case. It is found that the pressure profile at the centre of the contact area directly relates to the roughness amplitude. Furthermore, the CFD models can model the elastic deformation of cylinders of different materials, which is another advantage of the CFD approach over the Reynolds equation.