A two dimensional hydrodynamic model describing the solar wind interaction with the local interstellar medium, which surrounds the solar system, is used to study the heliosphere both as a steady-state- and dynamic structure. The finite volume method used to solve the associated system of hydrodynamic equations numerically is discussed in detail. Subsequently the steady state heliosphere is studied for both the case where the solar wind and the interstellar medium are assumed to consist of protons only, as well as the case where the neutral hydrogen population in the interstellar medium is taken into account. It is shown that the heliosphere forms as three waves, propagating away from the initial point of contact between the solar wind and interstellar matter, become stationary. Two of these waves become stationary at sonic points, forming the termination shock and bow shock respectively. The third wave becomes stationary as a contact discontinuity, called the heliopause. It is shown that the position and geometry of the termination shock, heliopause and bow shock as well as the plasma flow characteristics of the heliosphere largely depend on the dynamic pressure of either the solar wind or interstellar matter. The heliosphere is modelled as a dynamic structure, including both the effects of the solar cycle and short term variations in the solar wind observed by a range of spacecraft over the past ~ 30 years. The dynamic model allows the calculation of an accurate record of the heliosphere state over the past ~ 30 years. This record is used to predict the time at which the Voyager 2 spacecraft will cross the termination shock. Voyager 1 observations of 10 MeV cosmic ray electrons are then used in conjunction with a cosmic ray modulation model to constrain the record of the heliosphere further. It is shown that the dynamic hydrodynamic model describes the heliosphere accurately within a margin of error of ±0.7 years and ±3 AU. The model predicts that Voyager 2 crossed the termination shock in 2007, corresponding to preliminary results from observations indicating that the crossing occurred in August 2007. / Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2008.

The presence of hydrodynamically generated air bubbles has been observed to enhance fine particle flotation in a high intensity agitation (HIA) flotation cell. In this study, the cavitation in an HIA cell, used in our laboratory, is studied by hydrodynamic computational fluid dynamics. Different types of impellers are studied to obtain flow characteristics such as velocity and pressure distributions and turbulent dissipation rate in a two-baffled HIA cell. A cavitation model in conjunction with a multiphase mixture model is used to predict the vapor generation in the HIA cell. Cavitating flow is simulated as a function of revolution speed (RPM) and dissolved gas concentration to understand the dependency of hydrodynamic cavitation on these operating parameters. For comparison, cavitation in a pressure driven flow through a constriction is also modeled. A population balance model is used to obtain bubble size distributions of the generated cavities in a flow through constriction. / Chemical Engineering

Estuaries can be considered as vital natural resources and are unique ecosystems at the interface between terrestrial and marine environments. The increase of population density centred on these coastal features and associated anthropogenic activities such as trade, industry, agriculture and recreation can adversely affect these sensitive environments. The Pumicestone Passage, located in northern Moreton Bay, Australia, is one such estuarine environment where there are concerns about degradation of water quality resulting from rapid land use change. These changes are both immediate to the Passage and within its wider catchment. Of notable concern are the outbreaks of Lyngbya (a toxic blue-green algae) in the Passage itself and near its interface with Deception Bay to the south. Other factors of concern are increased suspended and dissolved loads, and maintenance of ecosystem integrity. In this study, numerical modelling, graphical methods and water surface elevation and current velocity parameter calculations are used to describe hydrological processes in the Pumicestone Passage. A hydrodynamic model is developed using the modelling software SMS and RMA2 as a foundation for future hydrodynamic and water quality modelling. In addition, observed data are used to interpret general hydrodynamic behaviour in the passage, and determine various parameters for use in model development and calibration. Tidal prediction is also discussed and used for model calibration. To support the modelling and for preliminary interpretation of hydrodynamic processes within the Passage, measurements were made in the field of (a) water surface elevation variation at 17 sites; (b) tidal current velocities in four of the tributary creeks and at the northern boundary; (c) volumetric flow rates at two cross-sections within the Passage; and (d) cross-sectional bathymetry at sites where tidal current velocities were measured in the creeks. In general, examination of the observational data reveals a number of important processes in the Pumicestone Passage. Almost all sites within Pumicestone Passage and its tributaries are flood dominant indicating that tidal storage and bottom friction effects are significant. Mesotidal ranges occur at sites close to the southern boundary of the passage, however, bottom friction greatly reduces the tidal response at the remaining sites which results in microtidal ranges. The influence of both the southern and northern tides can be seen in the deformation of tidal waveforms in the central passage. Extensive intertidal areas at and inside the northern inlet to the Passage markedly reduce tidal ranges in the northern estuary and its tributary creeks. Issues involved in hydrodynamic model development and performance are discussed. Overall, model results for the southern estuary have satisfactory correlation with observed data whereas model results for the northern estuary are less satisfactory. In addition, water surface elevation variation model results are generally more accurate than tidal current velocity model results. Reasons for the differences between model and observed values are considered and possible solutions given. Factors discussed relate to boundary condition locations, resolution of bathymetric and geographical data, mesh development methods and parameter assignment.

Whole document restricted until 2010, see Access Instructions file below for details of how to access the print copy. / Blind Mexican cave fish (Astyanax fasciatus) lack a functioning visual system, and are known to use self-generated water motion to sense their surroundings; an ability termed hydrodynamic imaging. Nearby objects distort the flow field created by the motion of the fish. These flow distortions are sensed by the mechanosensory lateral line. Little is known about the fluid mechanics involved in hydrodynamic imaging, or how the behaviour of the fish might influence their ability to sense the world around them. Automated image analysis was used to study the effects of swimming kinematics on the ability of the fish to sense their surroundings when introduced into a novel environment. The fish reacted to avoid head-on collisions with a wall at a remarkably short mean distance of 4.0 ± 0.2 mm. The ability of the fish to react, was dependent on whether they were beating their tail as they approached the wall. When following surfaces, such as a wall, the fish changed their swimming kinematics significantly and used both tactile and hydrodynamic information. Measuring the tendency of the fish to follow a tightening curve showed the fish to be moderately thigmotactic. The flow fields around freely swimming fish were experimentally measured using Particle Image Velocimetry (PIV). A new algorithm was developed to calculate the pressure field around the fish based on the velocity field measured using PIV. The algorithm was validated against analytical and computational fluid dynamic (CFD) solutions. The flow fields around gliding fish and the stimuli to the lateral line of the fish were calculated using CFD models, validated against the experimental PIV data. The flow fields changed in characteristic ways as the fish approached a wall head-on or swam parallel to a wall. At 0.10 body lengths from a wall, the stimulus to the lateral line was estimated to be sufficient for the fish to be able to detect the wall, but this decreased rapidly with increasing distance from the wall. The CFD models suggested that the velocity of the fish does not affect the distance at which they detect an object. Hydrodynamic imaging is a short range sensory ability and blind cave fish require their sensitive lateral line and fast reactions in order to be able to use it to sense the world around them and avoid collisions. The information gained about the fluid mechanics of hydrodynamic imaging, and the flow measurement and modelling techniques developed here will be useful for further study of this remarkable ability.

Whole document restricted until 2010, see Access Instructions file below for details of how to access the print copy. / Blind Mexican cave fish (Astyanax fasciatus) lack a functioning visual system, and are known to use self-generated water motion to sense their surroundings; an ability termed hydrodynamic imaging. Nearby objects distort the flow field created by the motion of the fish. These flow distortions are sensed by the mechanosensory lateral line. Little is known about the fluid mechanics involved in hydrodynamic imaging, or how the behaviour of the fish might influence their ability to sense the world around them. Automated image analysis was used to study the effects of swimming kinematics on the ability of the fish to sense their surroundings when introduced into a novel environment. The fish reacted to avoid head-on collisions with a wall at a remarkably short mean distance of 4.0 ± 0.2 mm. The ability of the fish to react, was dependent on whether they were beating their tail as they approached the wall. When following surfaces, such as a wall, the fish changed their swimming kinematics significantly and used both tactile and hydrodynamic information. Measuring the tendency of the fish to follow a tightening curve showed the fish to be moderately thigmotactic. The flow fields around freely swimming fish were experimentally measured using Particle Image Velocimetry (PIV). A new algorithm was developed to calculate the pressure field around the fish based on the velocity field measured using PIV. The algorithm was validated against analytical and computational fluid dynamic (CFD) solutions. The flow fields around gliding fish and the stimuli to the lateral line of the fish were calculated using CFD models, validated against the experimental PIV data. The flow fields changed in characteristic ways as the fish approached a wall head-on or swam parallel to a wall. At 0.10 body lengths from a wall, the stimulus to the lateral line was estimated to be sufficient for the fish to be able to detect the wall, but this decreased rapidly with increasing distance from the wall. The CFD models suggested that the velocity of the fish does not affect the distance at which they detect an object. Hydrodynamic imaging is a short range sensory ability and blind cave fish require their sensitive lateral line and fast reactions in order to be able to use it to sense the world around them and avoid collisions. The information gained about the fluid mechanics of hydrodynamic imaging, and the flow measurement and modelling techniques developed here will be useful for further study of this remarkable ability.

Whole document restricted until 2010, see Access Instructions file below for details of how to access the print copy. / Blind Mexican cave fish (Astyanax fasciatus) lack a functioning visual system, and are known to use self-generated water motion to sense their surroundings; an ability termed hydrodynamic imaging. Nearby objects distort the flow field created by the motion of the fish. These flow distortions are sensed by the mechanosensory lateral line. Little is known about the fluid mechanics involved in hydrodynamic imaging, or how the behaviour of the fish might influence their ability to sense the world around them. Automated image analysis was used to study the effects of swimming kinematics on the ability of the fish to sense their surroundings when introduced into a novel environment. The fish reacted to avoid head-on collisions with a wall at a remarkably short mean distance of 4.0 ± 0.2 mm. The ability of the fish to react, was dependent on whether they were beating their tail as they approached the wall. When following surfaces, such as a wall, the fish changed their swimming kinematics significantly and used both tactile and hydrodynamic information. Measuring the tendency of the fish to follow a tightening curve showed the fish to be moderately thigmotactic. The flow fields around freely swimming fish were experimentally measured using Particle Image Velocimetry (PIV). A new algorithm was developed to calculate the pressure field around the fish based on the velocity field measured using PIV. The algorithm was validated against analytical and computational fluid dynamic (CFD) solutions. The flow fields around gliding fish and the stimuli to the lateral line of the fish were calculated using CFD models, validated against the experimental PIV data. The flow fields changed in characteristic ways as the fish approached a wall head-on or swam parallel to a wall. At 0.10 body lengths from a wall, the stimulus to the lateral line was estimated to be sufficient for the fish to be able to detect the wall, but this decreased rapidly with increasing distance from the wall. The CFD models suggested that the velocity of the fish does not affect the distance at which they detect an object. Hydrodynamic imaging is a short range sensory ability and blind cave fish require their sensitive lateral line and fast reactions in order to be able to use it to sense the world around them and avoid collisions. The information gained about the fluid mechanics of hydrodynamic imaging, and the flow measurement and modelling techniques developed here will be useful for further study of this remarkable ability.

Whole document restricted until 2010, see Access Instructions file below for details of how to access the print copy. / Blind Mexican cave fish (Astyanax fasciatus) lack a functioning visual system, and are known to use self-generated water motion to sense their surroundings; an ability termed hydrodynamic imaging. Nearby objects distort the flow field created by the motion of the fish. These flow distortions are sensed by the mechanosensory lateral line. Little is known about the fluid mechanics involved in hydrodynamic imaging, or how the behaviour of the fish might influence their ability to sense the world around them. Automated image analysis was used to study the effects of swimming kinematics on the ability of the fish to sense their surroundings when introduced into a novel environment. The fish reacted to avoid head-on collisions with a wall at a remarkably short mean distance of 4.0 ± 0.2 mm. The ability of the fish to react, was dependent on whether they were beating their tail as they approached the wall. When following surfaces, such as a wall, the fish changed their swimming kinematics significantly and used both tactile and hydrodynamic information. Measuring the tendency of the fish to follow a tightening curve showed the fish to be moderately thigmotactic. The flow fields around freely swimming fish were experimentally measured using Particle Image Velocimetry (PIV). A new algorithm was developed to calculate the pressure field around the fish based on the velocity field measured using PIV. The algorithm was validated against analytical and computational fluid dynamic (CFD) solutions. The flow fields around gliding fish and the stimuli to the lateral line of the fish were calculated using CFD models, validated against the experimental PIV data. The flow fields changed in characteristic ways as the fish approached a wall head-on or swam parallel to a wall. At 0.10 body lengths from a wall, the stimulus to the lateral line was estimated to be sufficient for the fish to be able to detect the wall, but this decreased rapidly with increasing distance from the wall. The CFD models suggested that the velocity of the fish does not affect the distance at which they detect an object. Hydrodynamic imaging is a short range sensory ability and blind cave fish require their sensitive lateral line and fast reactions in order to be able to use it to sense the world around them and avoid collisions. The information gained about the fluid mechanics of hydrodynamic imaging, and the flow measurement and modelling techniques developed here will be useful for further study of this remarkable ability.

Made available in DSpace on 2014-10-09T12:43:20Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:56:10Z (GMT). No. of bitstreams: 1 06481.pdf: 5013180 bytes, checksum: 7fd69f45c605162fe74bdcf0decbd24d (MD5) / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Hydrodynamic slip is known to suppress vorticity production at the solid-fluid boundary in bluff body flows. This suppression combined with the enhanced vorticity convection results in a substantial reduction in the unsteady vortex shedding and the hydrodynamic loads experienced by the bluff body. Here, using combined theoretical and computational techniques, we investigate the effect of slip on three-dimensional wake dynamics and convective scalar transport from a circular cylinder placed in the uniform cross-flow of a Newtonian incompressible fluid over Reynolds numbers ranging from 0.1 to 1000. We find the wake patterns to be strongly influenced by the degree of the slip, quantified through the non-dimensional slip length in the Naiver slip model, with the asymptotic slip lengths of zero and infinity characterizing no-slip and no-shear boundaries, respectively. With increasing slip length, the wake three-dimensionality, that is observed in the case of a no-slip surface for Re > 190, is gradually suppressed and eventually eliminated completely. For each Reynolds number, we identify the critical slip length beyond which the three-dimensionality is completely suppressed and the wake becomes two-dimensional, on the basis of the total transverse entropy present in the flow field. Over the Reynolds number range considered in this work, we find the critical slip length to be an increasing function of Reynolds number. For sufficiently large slip lengths, we observe suppression of two-dimensional vortex shedding leading to formation of a steady separated wake. Further increments in slip length lead to reduction in the intensity and size of the recirculating eddy pair eventually resulting in its complete disappearance for a no-shear surface for which the flow remains attached all along the cylinder boundary. Next, we quantify the effect of hydrodynamic slip on convective transport from an isothermal circular cylinder placed in the uniform cross flow of an incompressible fluid at a lower temperature. For low Reynolds and high P´eclet numbers, theoretical analysis based on Oseen and thermal boundary layer equations allows us to obtain explicit relationships for the dependence of transport rate on the prescribed slip length. We observe that the non-dimensional transport coefficients follow a power law scaling with respect to the P´eclet number, with the scaling exponent increasing gradually from the lower asymptotic limit of 1/3 for the no-slip surface to 1/2 for a no-shear boundary. Results from our simulations at finite Reynolds number indicate that the local time-averaged transport rates for a no-shear surface exceed the one for the no-slip surface all along the cylinder except in the neighbourhood of the rear stagnation region, where flow separation and reversal augment the transport rates substantially.

La méthanisation par voie sèche possède différents avantages par rapport à la méthanisation par voie humide. Les réacteurs sont plus petits, les besoins en eau sont moindres, la production de digestat et le prétraitement nécessaire sont également moins importants. Cependant, plusieurs études ont démontré que l'eau favorise l'hydrolyse du substrat et permet le transport des sous-produits d'hydrolyse et des nutriments vers les bactéries. Pour mieux comprendre le rôle de l'eau lors de la méthanisation, des tests de digestion sèche et semi-sèche à partir de substrats organiques complexes (déchets alimentaires, paille de riz, déchets de carotte), avec différentes teneurs en matière sèche de substrat traité ont été réalisées. Les résultats confirment que l'eau joue un rôle essentiel sur le taux de production spécifique de méthane, le rendement final de méthane généré et la dégradation de la matière volatile sèche (MVS). Le rendement final de méthane produit dans des conditions semi-sèches et sèches est respectivement de 51% et de 59% inférieur avec la paille de riz et 4% et 41% de moins pour les déchets alimentaires en comparaison avec des conditions humides. Des tests d'inhibition basés sur l'analyse des acides gras volatils (AGV) ont été menées pour étudier les processus d'inhibition spécifiques qui ont lieu avec les substrats sélectionnés à différentes teneurs en matière sèche. Pour le cas de la méthanisation par voie humide des déchets de carotte, aucune accumulation d'AGV a été trouvé, et toutes les concentrations d'AGV étaient inférieurs aux seuils d'inhibition. Une corrélation directe entre la teneur en matière sèche et la concentration totale d'AGV (AGVtot) a été mise en évidence pour la paille de riz et les déchets alimentaires. Pour la paille de riz, une concentration d'AGVtot maximale de 2,1 g / kg a été trouvé pour la voie sèche, 1 g / kg dans les conditions semi-sèche et 0,2 g / kg dans les conditions humides, alors que pour les déchets alimentaires la concentration d'AGVtot était de 10 g / kg à l'état sec, 9 g / kg dans les conditions semi-sèche et 3 g / kg dans les conditions humides. Un modèle mathématique de la méthanisation de substrats organiques complexes dans des conditions sèches et semi-sèche a été proposé pour simuler l'effet de la teneur en matière sèche sur le processus. Les données obtenues à partir d'expériences en mode batch, en termes de production de méthane et de concentration d'AGV, ont été utilisées pour calibrer le modèle proposé. Les paramètres cinétiques de production et d'élimination d'AGV ont été calibrés à l'aide des données expérimentales, et il a été montré qu'ils sont fortement dépendants de la teneur en matière sèche et différent des valeurs de la littérature concernant la méthanisation par voie humide. Cela est dû à l'accumulation d'AGV dans les conditions sèches, ce qui implique d'utiliser des valeurs plus élevées concernant les constantes d'inhibition introduites dans le modèle. Enfin, comme la méthanisation par voie sèche a généralement lieu dans des réacteurs à écoulement piston, une étude historique et critique de la littérature concernant la compréhension du rôle de l'hydrodynamique dans des bioréacteurs à écoulement piston a été réalisée / Dry Anaerobic Digestion (AD) presents different advantages if compared to wet AD, i.e. smaller reactor size, lesser water addition, digestate production and pretreatment needed, although several studies have demonstrated that water promotes substrate hydrolysis and enables the transfer of process intermediates and nutrients to bacterial sites. To better understand the role of water on AD, dry and semidry digestion tests of selected complex organic substrates (food waste, rice straw, carrot waste), with various TS contents of the treated biomass have been carried out in the present study. The results confirm that water plays an essential role on the specific methane production rate, final methane yield and Volatile Solids (VS) degradation. The final methane yield in semi-dry and dry conditions was 51% and 59% lower for rice straw and 4% and 41% lower for food waste, respectively, if compared with wet conditions. Inhibition tests, based on Volatile Fatty Acid (VFA) analysis, were carried out to investigate the specific inhibition processes that take place with the selected substrates at different TS contents. In wet AD of carrot waste no VFA accumulation was found, and all VFA concentrations were lower than the inhibition limits. A direct correlation between TS content and total VFA (TVFA) concentration was noticed for rice straw and food waste AD. For rice straw a maximum TVFA concentration of 2.1 g/kg was found in dry condition, 1 g/kg in semidry conditions and 0.2 g/kg in wet conditions, whereas for food waste the TVFA concentration was 10 g/kg in dry condition, 9 g/kg in semidry conditions and 3 g/kg in wet conditions. A Mathematical model of complex organic substrate AD in dry and semidry conditions has been proposed to simulate the effect of TS content on the process. The data obtained from batch experiments, in terms of methane production and VFA concentrations, were used to calibrate the proposed model. The kinetic parameters of VFA production and degradation, calibrated using the experimental data, resulted highly dependent on the TS content and different from wet AD literature values. This is due to VFA accumulation in dry conditions, which implies higher values of the inhibition factors introduced in the model. Finally, as dry AD takes usually place in Plug Flow (PF) reactors, an historical and critical review on the role of hydrodynamics in PF bioreactors has been carried out