Dissertations / Theses on the topic 'Fluid flow'

Thesis (Ph. D.) -- University of Maryland, College Park, 2004.
Thesis research directed by: Mathematics. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

Physical and mathematical modelling studies have been performed to investigate liquid flow driven by a horizontally injected gas. The experimental work consisted of water velocity measurements made at 100 locations within a plexiglass tank. Air was introduced into the tank through a series of side-mounted tuyeres, and the effect of air flowrate on water recirculation velocity was observed. The results of the experiments indicate that the maximum water velocity occurs at the water surface. The effect of bubbles coalescing from adjacent tuyeres was observed with increasing air flowrate, and was found to diminish the water recirculation rate. The mathematical model employed a variant of the Marker and Cell (MAC) technique to compute fluid flow with a free surface. The model predictions indicate that the flow in the experimental tank is largely driven by water flowing across the free surface. Based on this knowledge, qualitative predictions of the flow regimes in a Peirce-Smith copper converter and a zinc slag fuming furnace were made.
Applied Science, Faculty of
Mining Engineering, Keevil Institute of
Graduate

Cellulose material is processed to pulp suspensions and MDF boards in order to produce products such as papers, magazines, laminate floors or door skins. A critical stage of these processes is when the cellulose fiber networks are compressed to specific densities and when most of the fluid originally positioned between and inside the fibers is forced to leave the network. The fiber network is then exposed to a drag force generated by the flow. The magnitude of this force is dependent upon how easy the fluid can flow through the network, which is commonly described by its permeability. In addition to the permeability, which relates to the drag on each fiber, there is a solid network force. The response to this force from the fiber network is often termed as the compressibility of it. Hence, to be able to model and predict the compression stage in cellulose material related processes these two material properties must be known. In this thesis two equipments to measure the permeability of MDF networks and pulp suspensions are evaluated and a neat model for a part of the MDF- compression stage is developed. A reference material consisting of spherical particles and relevant fiber networks are used as test objects for the equipments enabling a comparison to theoretical models and existing experimental results. The outcome is that correct enough permeability data are obtained with respective equipment as long as Reynolds number is sufficiently low. The equipments are then used to study different materials showing, for instance, that highly compressed MDF-networks are strongly anisotropic as to permeability and that the tested hardwood pulps have an overall higher permeability compared to the softwood pulps investigated. It was also found that the permeability of the pulps was not influenced by different mechanical treatments of the fiber network, as long as the geometrical dimensions of the fibers were constants.

Godkänd; 2006; 20070109 (haneit)

In the first part of this thesis, one-point and two-point statistics of the Navier–Stokes-alpha-beta (NS-alpha-beta) regularization model in homogeneous isotropic turbulence are explored. The results are compared to the limit cases of the Navier–Stokes-alpha (NS-alpha) model and the NS-alpha-beta model without subgrid-scale (SGS) stress, as well as with high-resolution direct numerical simulation (DNS). After reviewing spectra of different energy norms, probability density functions (PDFs) of the filtered and unfiltered velocity increments along with longitudinal velocity structure functions of the regularization models and DNS results are presented. Differences in the statistical properties of the unfiltered and filtered velocity fields entering the governing equations of the NS-alpha and NS-alpha-beta models are highlighted and the usability of both velocity fields for realistic flow predictions is discussed. The influence of the modified viscous term in the NS-alpha-beta model is studied through comparison to the case where the underlying SGS stress tensor is neglected. Whereas the filtered velocity field is found to have physically more viable PDFs and structure functions for the approximation of DNS results, the unfiltered velocity field is found to have flatness factors close to DNS results. In the second part of this thesis, the a priori testing strategy is adopted to study three different alpha regularization models, namely the NS-alpha model, the Leray-alpha model, and the Clark-alpha model. Specifically, high-resolution DNS data of homogeneous isotropic turbulence is used to compute the mean SGS dissipation, the spatial distribution of the SGS dissipation, and the spatial distribution of elements of the SGS stress tensor. Predictions of the three regularization models are compared to the exact values of the SGS stress tensor, as defined in the filtered Navier–Stokes equations. The potential of the three regularization models to provide good approximations is quantified using spatial correlation coefficients. Whereas the Clark-alpha model exhibits the highest spatial correlation coefficients for the SGS dissipation and the SGS stress tensor elements, the Leray-alpha model provides lower correlation coefficients, and the NS-alpha model exhibits the lowest correlation coefficients of the three models. Our results indicate the presence of an optimal choice of the filter parameter alpha depending on the large-eddy simulation grid resolution. In the third part of this thesis, a simple model for simulating flows of active suspensions is investigated. The approach is based on dissipative particle dynamics (DPD). While the model is potentially applicable to a wide range of self-propelled particle systems, the specific class of self-motile bacterial suspensions is considered as a modeling scenario. To mimic the rod-like geometry of a bacterium, two DPD particles are connected by a stiff harmonic spring to form an aggregate DPD molecule. Bacterial motility is modeled through a constant self-propulsion force applied along the axis of each such aggregate molecule. The model accounts for hydrodynamic interactions between self-propelled agents through the pairwise dissipative interactions conventional to DPD. Detailed studies of the influence of agent concentration, pairwise dissipative interactions, and Stokes friction on the statistics of the system are provided. The simulations are used to explore the influence of hydrodynamic interactions in active suspensions. For high agent concentrations in combination with dominating pairwise dissipative forces, strongly correlated motion patterns and a fluid-like spectral distributions of kinetic energy are found. In contrast, systems dominated by Stokes friction exhibit weaker spatial correlations of the velocity field. These results indicate that hydrodynamic interactions may play an important role in the formation of spatially extended structures in active suspensions.
Dans la première partie de cette thèse, les statistiques du modèle de régularisation Navier–Stokes-alpha-beta (NS-alpha-beta) en turbulence homogène et isotrope sont explorées. Les résultats sont comparés aux cas limites du modèle Navier–Stokes-alpha (NS-alpha) et NS-alpha-beta sans contrainte d'échelle inférieure à la maille, ainsi qu'à la simulation numérique directe en haute résolution. Après avoir examiné les spectres de différentes normes énergie, des fonctions de densité de probabilité, des incréments de vitesse filtrés et non-filtrés ainsi que des fonctions de structure de vitesse longitudinales sont présentés. Les différences dans les propriétés statistiques des champs de vitesse non-filtrés et filtrés qui entrent dans les équations principales du modèle NS-alpha et NS-alpha-beta sont mises en évidence et la facilité d'utilisation des deux champs de vitesse à des fins de prévisions réalistes d'écoulement est discutée. L'influence du terme visqueux modifié dans les équations du modèle NS-alpha-beta est étudiée par comparaison avec le cas où le tenseur d'échelle inférieure à la maille sous-jacent est négligé. Le champ de vitesse filtré se trouve à posséder des fonctions de densité de probabilité et les fonctions de structure physiquement plus viables pour l'approximation des résultats de simulations numériques directes. Dans la deuxième partie de cette thèse, la stratégie de test a priori est adoptée pour étudier trois modèles de régularisation alpha différents, à savoir les modèles NS-alpha, Leray-alpha, et Clark-alpha. Les résultats de simulation numériques directs à haute résolution sont utilisés pour calculer la dissipation moyenne d'échelle inférieure à la maille, la répartition spatiale de la dissipation d'échelle inférieure à la maille, et la distribution spatiale des éléments du tenseur de contraintes d'échelle inférieure à la maille. Des prédictions des trois modèles de régularisation sont comparées aux valeurs exactes du tenseur de contraintes d'échelle inférieure à la maille, telles que définies dans les équations de Navier–Stokes filtrées. Le potentiel des trois modèles de régularisation de fournir de bonnes approximations est quantifié à l'aide de coefficients de corrélation spatiale. Nos résultats indiquent la présence d'un choix optimal de paramètre de filtre alpha en fonction de la résolution de la grille de simulation des tourbillons de grande échelle. Dans la troisième partie de cette thèse, un modèle simple de simulation d'écoulements de suspensions actives est étudié. L'approche est basée sur la dynamique des particules dissipatives (DPD). Bien que le modèle soit potentiellement applicable à une large gamme de systèmes de particules automotrices, la classe spécifique de suspensions bactériennes auto-motiles est considérée en tant que un scénario de modélisation. Motilité bactérienne est modélisée grâce à une force d'autopropulsion constante appliquée le long de l'axe de chaque agent. Le modèle tient compte des interactions hydrodynamiques entre les agents automoteurs à travers les interactions dissipatives par paires classiques de la DPD. Des études détaillées de l'influence de la concentration des agents, des interactions dissipatives par paires, et de la friction de Stokes sur les statistiques du système sont fournies. Les simulations sont utilisées pour explorer l'influence des interactions hydrodynamiques dans les suspensions actives. Pour des concentrations élevées de l'agent combinées à des forces dissipatives par paires dominantes, des types de mouvement fortement corrélés et des distributions spectrales d'énergie cinétique analogue à un fluide sont trouvés. En revanche, les systèmes dominés par la friction de Stokes présentent des corrélations spatiales plus faibles du champ de vitesse. Ces résultats indiquent que les interactions hydrodynamiques peuvent jouer un rôle important dans la formation de structures spatialement étendues dans les suspensions actives.

The work presented in this thesis can be divided into two main parts: a study of Cellular Automaton (CA) models of incompressible fluid flow and work on the use of Renormalisation Group (RG) methods to derive an effective viscosity for use in subgrid modelling in Large Eddy Simulation of incompressible turbulence. The derivation of hydrodynamic equations for the behaviour of CA models is reviewed in the context of classical statistical mechanics. In computer simulations of such models, velocity and density values are found by calculating averages of appropriate microscopic quantities: the effect of this averaging on noise levels in such simulations is investigated. We verify the expected results that the noise level is proportional to N^-1/2 where N is the number of space cells or time-steps in the average. A new CA model, the '2D multispeed model', is developed by considering the projection of the 4D face-centred hypercubic model into 2D. Optimal collision rules are obtained and computer simulations of flow through a channel are performed, which reproduce the well-known parabolic velocity profile. Kinematic viscosity is calculated as a function of particle density from the velocity profiles and the imposed pressure gradient: the results compare favourably with those of the FHP model in terms of maximum attainable Reynolds number for a given computational effort. After briefly summarising some important aspects of turbulent fluid flow, a conditional averaging procedure is presented, designed to deal with the problems of coupling between low-wavenumber and high-wavenumber modes in the filtered Navier-Stokes equation in k-space. The conditional average is precisely defined in terms of the turbulent ensemble and a method of evaluation is proposed, whereby the conditional average of moments of the velocity field is related to the full-ensemble average of the same quantities, with an explicit error term representing the effect of the coupling. The application of this averaging procedure to the modelling of small-scale motion in homogeneous isotropic turbulence is explained and the derivation of an effective viscosity, due to McComb and Watt (Phys. Rev. Lett. 65 (1990) p.3281) is outlined.

Cette thèse a pour but l'étude de la mise en œuvre de commandes par asservissement visuel pour le contrôle actif d'un écoulement de Poiseuille. D'un point de vue général, le contrôle d'écoulements vise à modifier ou à maintenir l'état de l'écoulement, malgré une éventuelle perturbation extérieure. Une des situations d'intérêt concerne par exemple la transition vers la turbulence où l'écoulement peut devenir turbulent avec la croissance de sa densité d'énergie cinétique. La réduction de la traînée est également une application potentielle dans des problèmes d'ingénierie. Un des buts applicatifs de cette thèse cherchera ainsi à minimiser à la fois la densité d'énergie cinétique et la traînée. Des modèles numériques peuvent être utilisés pour générer un modèle d'état des équations aux dérivées partielles d'un écoulement de Poiseuille. Le modèle d'état considéré dans cette thèse s'appuie sur une représentation spectrale afin de transformer les équations aux dérivées partielles originelles en un système d'équations différentielles ordinaires. Le vecteur d'état rassemble dans notre cas la vitesse et la vorticité. Les signaux de commande dépendent eux de conditions aux limites de type Dirichlet non homogènes qui correspondent à des actions de soufflage/aspiration. Le nombre de degrés de liberté commandé du problème correspond à la dimension du signal de commande. La densité d'énergie cinétique et la traînée sont modélisées en fonction du vecteur d'état et du signal de commande. Dans cette thèse nous avons plus particulièrement considéré un asservissement visuel partitionné. Celui-ci est appliqué au modèle d'état de l'écoulement avec deux degrés de liberté afin de minimiser simultanément la densité d'énergie cinétique et la traînée. La traînée, contrairement à l'énergie cinétique, diminue de façon monotone en fonction du temps. Une augmentation du nombre de degrés de liberté permet d'améliorer la décroissance de la densité d'énergie cinétique. Lorsque le nombre de degré de liberté correspond à la dimension du vecteur d'état, et en s'appuyant sur une commande par asservissement visuel, nous montrons que la densité d'énergie cinétique décroit de façon monotone au cours du temps. Le modèle d'état de l'écoulement de Poiseuille vit dans un espace de très grande dimension. Par conséquent, il est nécessaire d'un point de vue pratique de réduire la dimension du contrôleur. Nous démontrons que la loi de commande s'appuyant sur un modèle réduit peut être appliquée au système complet. Dans ce cas la densité d'énergie cinétique décroit presque de façon monotone au cours du temps en utilisant une commande par asservissement visuel à deux degrés de liberté
The visual servoing control approach is formulated for the flow control of the plane Poiseuille flow. Generally, the flow control can lead the flow from its current state to a desired state. In transition to turbulence, the growth of kinetic energy density can lead the flow to turbulence. Moreover, the drag reduction is a potential application in the engineering applications. Therefore, this thesis aims to minimize the kinetic energy density and the skin friction drag. The governing equations of the plane Poiseuille flow are modeled to a standard form in the automatic control. More precisely, the partial differential equations of the plane Poiseuille flow are transformed to a state space representation by using the spectral method. The streamwise and spanwise directions are discretized based on the Fourier series while the wall-normal direction is discretized based on the Chebyshev polynomials. The state vector involves the wall-normal velocity and vorticity. The control signals depend on the inhomogeneous Dirichlet boundary conditions which correspond to blowing/suction boundary control. The number of independent control signals is called the number of the degree of freedom. Moreover, the skin-friction drag and the kinetic energy density are modeled as a function of the state vector. The goal is to minimize both the skin-friction drag and the kinetic energy density by appropriate methods. The partitioned visual servoing control is used to minimize, simultaneously, the skin-friction drag and the kinetic energy density with two degrees of freedom. As a result, the behavior of the skin-friction drag monotonically decreases in time. However, the behavior of the kinetic energy density does not monotonically decrease in time, the similar results from the other methods such as: PID and LQR controls. Therefore, the number of the degree of freedom increases, which leads to the improvement of the kinetic energy density. In addition, when the number of the degree of freedom equals the number of state vector, the kinetic energy density monotonically decreases in time by using the visual servoing control. The dimension of linearized plane Poiseuille flow is large, therefore, we need to reduce the order of controller. We demonstrate that the control law based on a mode reduction can be applied for the full system. Moreover, the kinetic energy density almost will monotonically decreases in time even using two degrees of freedom when the visual servoing control is designed based on the model order reduction

Fluid mixing in stirred vessels is widely encountered in a number of industries. In this work, different experimental techniques and the CFD modelling approach are used to measure the mixing of a wide range of fluids in stirred vessels. As the detailed validation is essential for CFD modelling, CFD predictions are compared in detail with different experimental measurements. The capability of CFD modelling of the 3D spatial distribution of velocity and solid concentration within opaque concentrated solid-liquid suspensions with the mean solid concentration up to 40 wt% is assessed by comparing with the experimental data obtained from positron emission particle tracking (PEPT) measurements. Because the impeller configuration is of significant importance to the flow pattern, the performance of different impellers for single-phase mixing of Newtonian and non-Newtonian fluids in stirred vessels is compared. CFD predictions of flow fields generated from different impellers are compared with those measured by the well-established particle image velocimetry (PIV) technique. The capability of CFD modelling of different mixing features of non-Newtonian fluids in stirred vessels are verified by comparing with experimental data obtained from PIV, PEPT, and planar laser induced fluorescence (PLIF) measurements.

Master of Technology: Civil Engineering In the Faculty of Engineering At the Cape Peninsula University of Technology 2014
Notches, particularly rectangular and V shaped are the cheapest and most common devices used to measure the flow rate of water in open channels. However, they have not been used to measure the flow rate of non-Newtonian fluids. These viscous fluids behave differently from water. It is difficult to predict the flow rate of such fluids during transportation in open channels due to their complex viscous properties. The aim of this work was to explore the possibility of extending the application of especially rectangular and V-shaped notches to non-Newtonian fluids. The tests reported in this document were carried out in the Flow Process and Rheology Centre laboratory. Notches fitted to the entrance of a 10 m flume and an in-line tube viscometer were calibrated using water. The in-line tube viscometer with 13 and 28 mm diameter tubes was used to determine the fluid rheology. Flow depth was determined using digital depth gauges and flow rate measurements using magnetic flow meters. Three different non-Newtonian fluids, namely, aqueous solutions of Carboxymethyl Cellulose (CMC) and water-based suspensions of kaolin and bentonite were used as model non-Newtonian test fluids. From these the coefficient of discharge (Cd) values and appropriate non-Newtonian Reynolds numbers for each fluid and concentration were calculated. The experimental values of the coefficient of discharge (Cd) were plotted against three different definitions of the Reynolds number. Under laminar flow conditions, the discharge coefficient exhibited a typical dependence on the Reynolds number with slopes of ~0.43-0.44 for rectangular and V notches respectively. The discharge coefficient was nearly constant in the turbulent flow regime. Single composite power-law functions were used to correlate the Cd-Re relationship for each of the two notch shapes used. Using these correlations, the Cd values could be predicted to within ±5% for the rectangular and V notches. This is the first time that such a prediction has been done for a range of non-Newtonian fluids through sharp crested notches. The research will benefit the mining and food processing industries where high concentrations of non-Newtonian fluids are transported to either disposal sites or during processing.

This thesis presents an effective methodology for the generation of a simulation which can be used to increase the understanding of viscous fluid processing equipment and aid in their development, design and optimisation. The Hampden RAPRA Torque Rheometer internal batch twin rotor mixer has been simulated with a view to establishing model accuracies, limitations, practicalities and uses. As this research progressed, via the analyses several 'snap-shot' analysis of several rotor configurations using the commercial code Polyflow, it was evident that the model was of some worth and its predictions are in good agreement with the validation experiments, however, several major restrictions were identified. These included poor element form, high man-hour requirements for the construction of each geometry and the absence of the transient term in these models. All, or at least some, of these limitations apply to the numerous attempts to model internal mixes by other researchers and it was clear that there was no generally accepted methodology to provide a practical three-dimensional model which has been adequately validated. This research, unlike others, presents a full complex three-dimensional, transient, non-isothermal, generalised non-Newtonian simulation with wall slip which overcomes these limitations using unmatched ridding and sliding mesh technology adapted from CFX codes. This method yields good element form and, since only one geometry has to be constructed to represent the entire rotor cycle, is extremely beneficial for detailed flow field analysis when used in conjunction with user defined programmes and automatic geometry parameterisation (AGP), and improves accuracy for investigating equipment design and operation conditions. Model validation has been identified as an area which has been neglected by other researchers in this field, especially for time dependent geometries, and has been rigorously pursued in terms of qualitative and quantitative velocity vector analysis of the isothermal, full fill mixing of generalised non-Newtonian fluids, as well as torque comparison, with a relatively high degree of success. This indicates that CFD models of this type can be accurate and perhaps have not been validated to this extent previously because of the inherent difficulties arising from most real processes.

In order to extend fluid-based flow control techniques that have been demonstrated at low subsonic speeds to high speed flows, it is necessary to develop actuators having sufficient momentum to control and manipulate high speed flows. Two fluidic actuation approaches are developed where the control jet may reach supersonic velocities and their performance is characterized. The first actuator is a compressible synthetic (zero net mass flux) jet. This is an extension of previous work on synthetic jets with an increase in driver power yielding substantial pressurization of the cavity such that the flow is compressible. The jet is generated using a piston/cylinder actuator, and the effects of variation of the orifice diameter, actuation frequency, and compression ratio are investigated. Operation in the compressible regime uniquely affects the time-dependent cylinder pressure in that the duty cycle of the system shifts such that the suction phase is longer than the blowing phase. The structure of the jet in the near-field is documented using particle image velocimetry and Schlieren flow visualization. In the range investigated, the stroke length is sufficiently long that the jet flow is dominated by a starting jet rather than a starting vortex (which is typical of low-speed synthetic jets). A simple, quasi-static numerical model of the cylinder pressure is developed and is in generally good agreement with the experimental results. This model is used to assess system parameters which could not be measured directly (e.g., the dynamic gas temperature and mass within the cylinder) and for predictions of the actuator performance beyond the current experimental range. Finally, an experiment is described with self-actuated valves mounted into the cylinder head which effectively icrease the orifice area in suction and overcome some of the limitations inherent to compressible operation. The second actuation concept is the combustion-driven jet actuator. This device consists of a small-scale (nominally 1 cc) combustion chamber which is filled with premixed fuel and oxidizer. The mixture is ignited using an integrated spark gap, creating a momentary high pressure burst within the combustor that drives a high-speed jet from an exhaust orifice. At these scales, the entire combustion process is complete within several milliseconds and the cycle resumes when fresh fuel/oxidizer is fed into the chamber and displaces the remaining combustion products. The actuator performance is characterized by using dynamaic measurements of the combustor pressure along with Schlieren flow visualization, limited dynamic thrust measurements, and flame photography. The effects of variation in the following system parameters are investigated: fuel type and mixture ratio, exhaust orifice diameter, chamber aspect ratio, chamber volume, fuel/air flow rate, ignition/combustion frequency, and spark ignition energy. The resulting performance trends are documented and the basis for each discussed. Finally, a proof-of-concept experiment demonstrates the utility of teh combustion-driven jet actuators at low-speed for transitory reattachment of a separated flow over an airfoil at high angles of attack.

It is widely known in oil industry that changes in fluid flow conditions such as water breakthrough or unsteady flow due to well shut-in can lead to sand destabilization, with a possible consequent sand production. In this research, different flow situations are incorporated into stress and stability analysis for the region around a wellbore producing oil from weak or unconsolidated sands, and the analyses involve strength weakening, stress redistribution, and decrease of rock stiffness. Two main mechanisms, chemical reactions of rock with formation water and variations of rock capillary strength, are identified and analyzed to study strength weakening after water breakthrough, both qualitatively and quantitatively. Using theories from particle mechanics, rock mechanics, and interfacial science, four novel capillarity models are developed and verified to analytically capture the physical behaviors of capillary strength at the grain scale. Based on model calculations, significantly better understanding of strength behavior in two-phase fluid environments is achieved. Based on a simplified model that can conservatively but efficiently quantify capillary strength with only two input parameters (i. e. particle radius and water saturation), a verified new method that physically calculates pore pressure in a multiphase environment, and a coupled poro-inelastic stress model, the redistributions of effective stresses with water saturation around a wellbore are solved. In terms of stress changes and growth of a plastic radius defining shear-failure zone, the effects of different stability factors, including capillarity through water-oil menisci, pore pressure changes due to the variations of fluid relative permeabilities, and loss of strength through chemical reactions of water-sensitive cementation materials, are quantified and compared in order to clarify when and how they contribute to sand production after water breakthrough. The nonlinearities of rock elastic properties in stressed and biphasic fluid environments is analytically addressed, based on an improved nonlinear theory that considers both a failure-based mechanism and a confining-stress-based mechanism, the strength model, and the coupled stress model. The calculations demonstrate the redistributions of stress-dependent rock stiffness around a wellbore and its evolution with increase of water saturation, clarify the relative importance of each mechanism in reducing rock stiffness, and fundamentally explain why current predictive technologies are invalid when water appears in a flowing wellbore. To quantify the effect of well shut-down on rock stability, the redistributions of fluid pressure in reservoir are analytically solved and coupled with the stress model, while the water hammer equations provide a boundary condition for the bottom-hole pressure. This approach allows direct solution of the relationships among fluid properties, rock properties and production parameters, within the context of rock stability. The proposed new approaches and models can be applied to evaluate sand production risk in multiphase and unsteady fluid flow environment. They can also serve as points of departure to develop more sophisticated models, or to develop more useful constitutive laws for numerical solutions.

It is shown in this thesis that fluid dynamic forces on unsteadily moving bluff bodies depend on the history of motion as much as on the velocity and acceleration of motion. An empirical relationship between the motion of the body and the resulting force is obtained by analysing the effect of the history of motion on the fluid dynamic force at any instant. The fluid dynamic force, velocity and acceleration are obtained as functions of time, by oscillating test models in water while they are being towed at constant speed. The test models used are: 1. a two-dimensional circular cylinder, 2. a rectangular block with square frontal area and fineness ratio of 3:1, 3. a cruciform parachute canopy with arm ratio of 4:1, and 4. a ring-slot parachute canopy. The functions by which the history of flow affects the future forces, are evaluated by using the Convolution Integral. The results show that the effects due to history of both velocity and acceleration are by no means negligible, that is the velocity and the acceleration at a specific time prior to any instant is so domineering that the fluid dynamic force can approximately be expressed as being delayed by this period of time. This 'time-delay', or time lag (as opposed to phase-lag) in the part of the measured force is found to be independent of the frequency of excitation. In the light of this evidence, a prediction model is suggested for estimating unsteady fluid forces. The data required for the application of this prediction model are obtained experimentally. Chapter One of this thesis gives a brief explanation of the historical background of unsteady fluid dynamics. The effects of acceleration on the fluid dynamic force, in both ideal and real fluids, are discussed in Chapter Two. Explained in Chapter Three are the techniques used for building the force prediction model, and data acquisition. The experimental procedure is explained in Chapter Four. Chapter Five gives the empirical form of the prediction model, and some data that are used in association with this model.

Mould filling is an important part of the casting process and cannot be neglected if an accurate analysis of casting solidification is desired. A numerical technique has been developed to model heat transfer and solidification of metal during and after the filling of a mould. The simulation incorporates a macroscopic fluid flow and heat transfer analysis from the initial stages of filling until filling is completed. Solidification is accounted for as is the temperature dependence of the fluid velocity as solid forms at the mould walls. The computing technique is based on the SOLA-VOF finite difference method for two dimensional mould geometries. Heat conduction and energy transport are modelled using coupled heat conduction-heat convection equations. Limitations of the computing technique are demonstrated.

A series of theoretical models are developed in order to constrain the fundamental underlying controls on low-temperature hydrothermal circulation in seafloor venting systems. In terms of discharge, results of these models predict that (i) some diffuse flow will always exit permeable sulphide mounds or oceanic crust in the vicinity of black smoker vents, (ii) diffuse flow may also occur in the absence of seafloor black smoker venting and (iii) anticorrelated variations in diffuse flow temperature and vertical velocity previously recorded at a sulphide mound on the Juan de Fuca Ridge may have been caused by variations in either the isotropic or vertical permeability of the mound. In terms of recharge, model results predict that the volume flux of seawater entrained into seafloor hydrothermal systems and subsequently discharged as diffuse flow appears to primarily depend on the permeability of the system and on the circulation depth of the entrained seawater. Furthermore, the principal location of such entrainment along the surface of a sulphide mound is determined, to a large extent, by the permeability structure and aspect ratio of the mound. Hydrothermal flow models of subsurface mixing between black smoker fluid and seawater predict that diffuse flow evolves towards a white smoker type fluid as mineral precipitation reduces the permeability of the system. Conversely, low-temperature hydrothermal fluids exiting such systems are predicted to evolve towards a diffuse flow type fluid following permeability-enhancing events. Furthermore, results from sulphide mound models suggest that anhydrite precipitation resulting from low-temperature hydrothermal circulation may act as a self-promoting process.

There have been a number of experimental investigations into the backdraft phenomena. A backdraft occurs in the event of a ventilation source being formed in a compartment, within which a fire has been burning for a sufficiently long enough time to form a deep layer of excess pyrolyzates. The source of fresh air will flow into the compartment in the form of a gravity current. It is the gravity current feature of backdrafts that this research project focuses on. Application of Computational Fluid Dynamics (CFD) to fire problems is expanding, including the development of specific programs for fire engineering applications. The experimental programme that was used in this research project highlights the difficulties of analysing fluid flows by using CFD simulations. The Flow3D program was used to obtain a more detailed understanding of the behaviour of a gravity current, allowing a detailed study of fluid dynamics which cannot be investigated experimentally. The simulations used two different vent configurations, with the CFD model being validated on the experimental results of salt water tank models. The simulations preformed compared well to the experimental data that was used for scaled salt water tank experiments.

An efficient FAS muldgrid solution strategy is presented for the accurate and economic simulation of convection dominated flows. The use of a high-order approximation to the convective transport terms found in the governing equations of motion has been investigated in conjunction with an unsegregated smoothing technique. Results are presented for a sequence of problems of increasing complexity requiring that careful attention be directed toward; the proper treatment of different types of boundary condition. The classical two-dimensional problem of flow in a lid-driven cavity is investigated in depth for flows at Reynolds numbers of 100,400 and 1000. This gives an extremely good indication of the power of a multigrid approach. Next, the solution methodology is applied to flow in a three-dimensional lid-driven cavity at different Reynolds numbers, with cross-reference being made to predictions obtained in the corresponding two-dimensional simulations, and to the flow over a step discontinuity in the case of an abruptly expanding channel. Although, at first sight, these problems appear to require only minor extensions to the existing approach, it is found that they are rather more idiosyncratic. Finally, the governing equations and numerical algorithm are extended to encompass the treatment of thermally driven flows. Ile solution to two such problems is presented and compared with corresponding results obtained by traditional methods.

COORDENAÇÃO DE APERFEIÇOAMENTO DO PESSOAL DE ENSINO SUPERIOR
A superposição de um escoamento circular de Couette e um fluxo com gradiente de pressão axial, através de um espaço anular ocorre em muitas aplicações práticas, tais como: reatores químicos catalíticos, filtros, extratores líquido- líquido, mancais e o fluxo de retorno de lamas de perfuração entre a coluna de perfuração rotatória e a formação rochosa na perfuração de poços produtores de petróleo e gás. As linhas de corrente curvadas do fluxo circular de Couette podem causar uma instabilidade centrífuga que produz vórtices toroidais, conhecidos como vórtices de Taylor. A presença destes vórtices muda as características hidrodinâmicas e a transferência de calor no processo. Em conseqüência, é muito importante ser capaz de prever o aparecimento da instabilidade. A maioria das análises numéricas e experimentais disponíveis na literatura são para fluidos Newtonianos e viscoelásticos (soluções polimericas). Neste trabalho, o efeito das propriedades viscoplásticas de suspensões de altas concentrações neste tipo de escoamento e nas condições críticas para o aparecimento de vórtices são determinadas teoricamente através da solução das equações de conservação. As equações diferenciais foram integradas pelo método de elementos finitos-Galerkin e o sistema de equações algébricas não lineares resultante foi resolvido pelo método de Newton.
The superposition of a circular Couette flow and a pressure- driven axial flow in an annulus occurs in many practical applications, such as catalytic chemical reactors, filtration devices, liquid-liquid extractors, journal bearings, and the return flow of drilling mud between the rotating drill string and the stationary wall in oil and gas well drilling. The curved streamlines of the circular Couette flow can cause a centrifugal instability leading to toroidal vortices, well known as Taylor vortices. The presence of these vortices changes the hydrodynamic and heat transfer characteristics of the process. Therefore, it is very important to be able to predict the onset of instability. Most of the available theoretical and experimental analyses are for Newtonian and viscoelastic (polymeric solutions) liquids. In this work, the effect of the viscoplastic properties of high concentration suspensions on the onset of the Taylor vortices are determined theoretically by solving the conservation equations and searching the critical conditions. The differential equations were solved by the Galerkin / finite element method and the resulting set of non-linear algebraic equations, by Newtons method.

Mapping relative patterns of mineral alteration and metasomatism reveal the visible manifestation of a hydrothermal system, but the visible expression of the hydrothermal system at the distal margins of fluid circulation may be limited owing to significant kinetic barriers to mineralogical and chemical alteration. Light stable isotopes of oxygen and carbon in carbonate minerals can be sensitive indicators of the interaction between hydrothermal fluids and wall rocks that can be used to delineate fluid flow pathways, evaluate the permeability network and source of fluids, estimate integrated fluid fluxes, and define the distal extent of alteration in carbonate-bearing lithologies. Carlin-type Au deposits in northeastern Nevada, USA are predominately sediment-hosted Au deposits formed by large hydrothermal systems. Typically, hydrothermal alteration including silicification, carbonate dissolution, clay alteration, and sulfidation are spatially restricted. The Blue Star-Goldstrike district on the Northern Carlin trend is the largest known occurrence of Carlin-type Au with a reported Au endowment of ~1,960 t. At the northern end of the Goldstrike district, the Banshee Au deposit represents a relatively small Carlin-type deposit primarily hosted within Jurassic lamprophyre dike (west Banshee) and polymict breccia units (east Banshee). Proximal alteration at west Banshee consists of silicification, illitization, and sulfidation with the addition of Au, Cs, Hg, K, Rb, Sb, Tl, and W and depletion in Ba, Ca, Mg, Mn, Na, and Sr. The breccia unit and limestone adjacent to the west Banshee lamprophyre exhibit carbonate dissolution and silicification. Outside of visible alteration, δ18O depletion in wall rock and vein calcite defines a more distal expression of alteration. Analysis of down hole δ¹⁸O in limestone from two transects across the Goldstrike district show that the isotopic alteration footprint surrounding Au mineralization may extend 2 km or more out from the main ore bodies. Reactive transport modeling of δ¹⁸O alteration at Banshee provides a tool for evaluating time integrated fluid flux associated with observed isotopic alteration. The combined datasets form the basis of an alteration model for Carlin-type Au deposits where alteration zones outward from proximal clay alteration, silicification, sulfidation, and carbonate dissolution to distal trace element metasomatism and δ¹⁸O alteration.

The thesis is concerned with the flow of polymeric fluids and their response to deformations. The current state of research into the rheology of polymers is reviewed and an introduction to non-Newtonian fluid mechanics is given. A novel numerical algorithm for simulating the flow of non-Newtonian fluids is presented, in which the fluid is represented by a set of Lagrangian particles embedded in it. Each particle carries a velocity and a stress with it as it moves through the flow geometry. The velocity gradient tensor and the divergence of the stress tensor are calculated for each particle by averaging over the neighbouring particles. The velocity is changed according to a discretised form of the equation of conservation of momentum and the stress is updated according to the constitutive equation for the fluid. Extra algorithms are presented to deal with the boundary conditions. The simulation is used to study the two-dimensional flow of a co-rotational Maxwell fluid past an array of cylinders between two walls. In the second part of the thesis, a computer simulation is developed which will allow the constitutive equation for a polymer melt to be replaced by a numerical version. Previous computer simulations of polymers are reviewed and a new, real space, reptation model is presented. This model is shown to have the correct behaviour for a reptating chain and is used to study the stress response to a step shear deformation. The long-term behaviour agrees with reptation theory, but the short-time behaviour is also found.

Thesis (MTech (Civil Engineering))--Cape Technikon, Cape Town, 1999
Pipe roughness is known to greatly increase the turbulent flow friction factor for Newtonian fluids. The well-known Moody diagram shows that an order of magnitude increase in the friction is possible due to the effect of pipe roughness. However, since the classical work of Nikuradse (1926 -1933), very little has been done in this area. In particular, the effects that pipe roughness might have on non-Newtonian turbulent flow head loss, has been all but totally ignored. This thesis is directed at helping to alleviate this problem. An experimental investigation has been implemented in order to quantify the effect that pipe roughness has on non-Newtonian turbulent flow head loss predictions. The Balanced Beam Tube Viscometer (BBTV), developed at the University of Cape Town, has been rebuilt and refined at the Cape Technikon and is being used for research in this field. The BBTV has been fitted with pipes of varying roughness. The roughness of smooth P\'C pipes was artificially altered using methods similar to those of Nikuradse. This has enabled the accumulation of flow data in laminar and turbulent flow in pipes that are both hydraulically smooth and rough Newtonian and non-Newtonian fluids have been used for the tests. The data have been subjected to analysis using various theories and scaling laws. The strengths and problems associated with each approach are discussed and It is concluded that roughness does have a significant effect on Newtonian as well as non-Newtonlan flow.

Measurements of the response to inline flow oscillations of vortex shedding from certain bluff bodies have been made. Four cylinders with fixed separation points were exposed to a mean stream with controlled sinusoidal oscillations at defined frequency ratios and amplitudes. Attention was concentrated on the highly sensitive reduced velocity regime around the inverse of twice the Strouhal number, 1/2S. Synchronisation of vortex formation was first established; then, for conditions at which synchronisation occured, threshold amplitude was measured. In order to determine the common and distinguishing features of bluff body shape the response from cylinders having 'zero' and finite afterbody were investigated. Finally, for a selected cylinder, the influence, in the presence of oscillations, of turbulence intensity (of defined scale), solid blockage and aspect ratio to the synchronous range were examined. Of particular interest were the changes that occur in the characteristic period of vortex formation and base pressure, relevant to the design and application of vortex flowmeters and self induced excitations of structures in general. The experiments were carried out in two separate blower tunnels under various oscillatory flow conditions in the Reynolds number range (0.5-5.0) X 104 and amplitudes of velocity fluctuation (+/-AU/U) of up to 0.3. It was found that amplitudes of oscillations of the order of 0.025 were sufficient to induce frequency lock-in when the reduced velocity was close to 1/2S and, provided the amplitude was sufficiently large, limited synchronisation could also be induced near the upper and lower reduced velocities of 1/S and 1/4S. Synchronisation is accompanied by enhanced vortex shedding which, in turn, can lead to increased base suction (and therefore increase in drag force). Flow visualisation revealed that the near wake vortex arrangement can vary a great deal depending on the ratio of Strouhal numbers at the forced and self-excited frequencies (N/no). It was concluded that the behaviour of the base pressure reflected this situation and that the range of synchronisation depended strongly on the stability of the prevailing mode of vortex shedding. The precise details of the flow were found to be highly dependent on body geometry and the frequency ratio N/no. The production of oscillatory flow in the wind tunnels did not proved to be an easy task. A review of the various methods that have been used in the past is presented. The advantages and disadvantages of different techniques are highlighted and details are given of a further method developed for use with the present open circuit blower tunnels of differing sizes. In the smaller tunnel, having a working section size of 0.3x0.3m, it was possible to produce sinusoidal variations of the working section flow, having peak to peak amplitude of up to 60% of the mean flow speed and frequencies up to, typically, that corresponding to the acoustic quarter-wavelength frequency determined by the tunnel length. Over the viable working range, the device is shown to produce high quality periodic flow with negligible harmonic distortion or acoustic noise difficulties.