Many environmental free surface flows involve water and sediment transport. The net changes to the surface level of an erodible bed by sediment entrainment and deposition processes have a feedback effect on the local ow hydrodynamics. Bed morphological change is of great socio-economic and environmental importance in that it affects navigation, flood risk management, water quality, species diversity, and overall river sustainability. This thesis describes a mathematical model of the depth-averaged shallow water-sediment equations based on mass and momentum conservation laws. A 2D numerical model is then presented of the fully coupled, variable-density governing equations, which are solved using a Godunov-type HLLC scheme. Dependent variables are specially selected in the numerical model to handle the presence of the variable-density mixture in the mathematical formulation. The model includes suspended sediment, bedload transport, and bed morphological change. The numerical model is verified against benchmark analytical and semi-analytical solutions for complicated, clear water flows, bedload transport and suspended sediment transport. The well-balanced property of the governing equations is verified for a variable-density dam break flow over a bed step. Simulations of an idealised dam-break flow over an erodible bed, in excellent agreement with previously published results, validate the ability of the model to capture complex water-sediment interactions under rapidly-varying flow conditions and a mobile bed, and validate the eigenstructure of the system of variable-density governing equations. The model is then further validated against laboratory based data for complex 2D partial dam breaks over fixed and mobile beds, respectively. The simulations of 2D dam break flows over mobile beds highlight the sensitivity of the results to the choice of closure relationships for sediment transport. To investigate this further, a parameter study is carried out using a variety of commonly used empirical formulae for suspended sediment transport. The numerical model is also used to inform a theoretical model that predicts the flow through and around a porous obstruction in a shallow channel. This problem is relevant to several practical applications, including flow through aquatic vegetation and the performance of arrays of tidal turbines in a finite-width tidal channel. The theoretical model is used to reinterpret the core flow velocities in laboratory-based data for an array of emergent cylinders in a shallow channel. Comparison with experimental data indicates the maximum obstacle resistance for which the theoretical model is valid. In a final application, the theoretical model examines the optimum arrangement of tidal turbines to generate power in a tidal channel, confirming that natural bed resistance increases the power extraction potential for a partial tidal fence.

Thesis (Master of Engineering in Mechanical Engineering)--Cape Peninsula University of Technology, 2018. / Underwater glider wings are the lifting surfaces of unmanned underwater vehicles UUVs depending on the chosen aerofoil sections. The efficiency as well as the performance of an underwater glider mostly depends on the hydrodynamic characteristics such as lift, drag, lift to drag ratio, etc of the wings. Among other factors, the geometric properties of the glider wing are also crucial to underwater glider performance. This study presents an opportunity for the numerical investigation to improve the hydrodynamic performance by incorporating curvature at the trailing edge of a wing as oppose to the standard straight or sharp trailing edge. A CAD model with straight leading edge and trailing edge was prepared with NACA 0016 using SolidWorks 2017. The operating conditions were setup such that the inlet speed varies from 0.1 to 0.5 m/s representing a Reynolds number 27.8 x 10ᵌ and 53 x 10ᵌ. The static pressure at different angles of attack (AOA) which varies from 2 to 16degrees at the increment of 2degrees for three turbulent models (K-Ԑ-standard, K-Ԑ-RNG and K-Ԑ-Realizable), was computed for upper and lower surfaces of the modified wing model using ANSYS Fluent 18.1. Thereafter the static pressure distribution, lift coefficient, drag coefficient, lift to drag ratio and pressure coefficient for both upper and lower surfaces were analysed. The findings showed that the lift and drag coefficient are influenced by the AOA and the inlet speed. If these parameters change the performance of the underwater glider changes as depicted by figure 5.6 and figure 5.7. The hydrodynamics of the underwater glider wing is optimized using the Cʟ/Cᴅ ratio as function of the operating conditions (AOA and the inlet speed). The investigation showed that the optimal design point of the AOA of 12 degrees and a corresponding inlet speed of 0.26m/s. The critical AOA matched with the optimal design point AOA of 12 degrees. It was also observed that Cp varies across the wing span. The results showed the Cp is higher closer to the fuselage while decreasing towards the mid-span and at the tip of the wing. This showed that the wing experiences more stress close to the fuselage than the rest of the wing span which implies that a higher structural rigidity is required close to the fuselage. The results of the drag and lift curves correspond to the wing characteristics typical observed for this type of aerofoil.

Abstract The present thesis investigates the dynamics of wave structures in dense planetary rings by employing hydrodynamic models, along with local N-body simulations of the particulate ring flow. The focus is on the large-scale satellite induced spiral density waves as well as on the free short-scale waves generated by the viscous overstability in Saturn's A and B rings. An analytic weakly nonlinear model is derived by using perturbation theory based on multi-scale methods to compute the damping behavior of nonlinear spiral density waves in a planetary ring subject to viscous overstability. In order to study the complex spatio-temporal evolution of these wave structures, numerical schemes are developed to integrate the hydrodynamical equations in time on large radial domains, taking into account collective self-gravity forces of the ring material, as well as the forcing by an external satellite. The required numerical stability and accuracy is achieved by applying Flux-Vector-Splitting methods aligned with advanced shock-capturing techniques. The free short-scale overstable waves are also investigated with local N-body simulations of the sheared ring flow. In particular, the influence of collective self-gravity between the ring particles as well as the periodic forcing due to a nearby Lindblad resonance on the overstable wave pattern is considered. The linear stability criterion for spiral density waves in Saturn’s rings is found to be identical to the condition for the onset of spontaneous viscous overstability in the limit of long wavelengths and agrees with the stability criterion for density waves derived by Borderies et al. within the streamline formalism. The derived nonlinear damping behavior of density waves can be very different from what has previously been thought. The role of collective self-gravity on the nonlinear evolution of short-scale overstable waves is determined, reconciling the partly contradicting results of previous studies. It is shown that collective self-gravity plays an important role in setting the length-scale on which the nonlinear overstable waves saturate in a planetary ring. A co-existence of spiral density waves and short-scale overstable waves is modeled in terms of one-dimensional large-scale hydrodynamical integrations. Due to the restriction to one space dimension, certain terms in the hydrodynamical equations that arise from the spiral shape of a density wave need to be approximated based on the weakly nonlinear model. These integrations reveal that density waves and spontaneous viscous overstability undergo complex interactions. In particular it is found that, depending on the relative magnitude of the two wave structures, the presence of short-scale overstable waves can lead to a damping of an overstable density wave and vice versa, density waves can suppress overstability. The effect of a density wave on the viscous overstability is also studied in terms of a simplified axisymmetric model of a ring perturbed by a nearby Lindblad resonance. A linear hydrodynamic stability analysis and local N-body simulations of this model system conform the corresponding results of the large-scale hydrodynamical integrations.

L’utilisation d’outils microfluidiques pour la mise en œuvre de procédés sous-pression tels que des réactions chimiques, des synthèses de matériaux nano-structurés, ou en tant qu’outils de détermination de grandeurs physico-chimiques est une thématique de recherche récente. Quelques travaux précédents ont démontré l'intérêt des procédés supercritiques en microcanal pour la chimie organique et la synthèse de nanocristaux. Le développement de ces procédés est concomitant à la mise au point de dispositifs capables de résister à des conditions de pression et température élevées tout en étant compatibles avec l’utilisation de fluides supercritiques. Les avantages de ces fluides pour ce type de procédé sont une faible viscosité et une diffusivité élevée, ce qui offre des conditions de mélange favorables. Cependant, dans le même temps, les propriétés de transport – comme la masse volumique – sont très sensibles aux variations de température et de pression, qui ne peuvent être évités dans ces systèmes où les fluides sont en écoulement. Dans des systèmes diphasiques où le CO2 supercritique (PC = 74 bar) est utilisé en tant que solvant, les transferts thermique et de matière sont fortement influencés par la nature des écoulements. Dans un souci de maîtrise de ces procédés, la compréhension du comportement hydrodynamique, à la fois locale et globale, des fluides supercritiques en microcanal devient fondamentale. Dans cet objectif, un dispositif de microfluidique transparent et résistant à des pressions supérieures à la pression critique du CO2 a été développé. En adaptant une méthodologie propre à la lithographie molle, permettant la fabrication de puces microfluidiques pour des applications à pression atmosphérique, nous sommes parvenus à établir un protocole de fabrication de puces en verre et résine photosensible, viables pour une utilisation à plus de 100 bar en conditions CO2 supercritique. Grâce à ces dispositifs, des expérimentations d’ombroscopie ont pu être réalisées pour observer des écoulements composés de CO2 et d’éthanol dans le microcanal de section carrée de 200 x 200 µm à des pressions comprises entre 40 et 90 bar. Pour identifier et comprendre les phénomènes qui entrent en jeu lors de la création de l’écoulement à haute pression, une approche thermodynamique relative aux équilibres de phase est indispensable. En effet, la connaissance du diagramme de phase permet d’ores et déjà de représenter les zones d’équilibres thermodynamiques (pression, température et composition) pour lesquelles le mélange créé est monophasique liquide ou diphasique liquide-vapeur. L’illustration expérimentale par les séquences d’écoulement obtenues justifie la modélisation thermodynamique du diagramme de phase du binaire d’étude. Le régime d’écoulement de Taylor, obtenu spécifiquement à l’intérieur de la zone d’équilibre diphasique liquide-vapeur, est étudié. Ce régime est caractérisé par des bulles allongées entourées par un film liquide et séparées les unes des autres par une poche liquide. L’évolution des caractéristiques hydrodynamiques de ce régime – longueur de bulle, longueur de slug et vitesse de bulle – est étudiée en fonction des conditions opératoires, des débits et propriétés des fluides. L’objectif étant de repérer les similitudes avec les caractérisations à pression ambiante de la littérature et les particularités résultantes d’une manipulation à haute pression. Ce travail a été à l’origine de plusieurs avancées pour les communautés microfluidique et supercritique. Un nouveau protocole de fabrication à moindre coût de puces microfluidiques compatibles avec l’utilisation de CO2 supercritique et des méthodes de visualisation avancées est présenté. Une modélisation thermodynamique et une étude hydrodynamique expérimentale permettent de construire une carte d’écoulement des régimes biphasiques observés à haute pression ainsi qu’une caractérisation hydrodynamique du régime de Taylor à haute pression en microcanal. / The use of microdevices to run high pressure processes for chemical reaction, nanomaterial synthesis, or as analysis tools for determining physical properties have become of increasing interest in recent years. Several works in the literature have demonstrated the advantages of supercritical microfluidics for organic chemistry and complex nanomaterial synthesis. The development of pressure-resistant microfluidic chips, which also are compatible with the properties of supercritical fluids, is a key step in order to increase knowledge about these processes. Supercritical fluids have low viscosity and high diffusivity, which are advantageous for microprocesses since they facilitate mixing between species. However, the properties of these fluids are also very sensitive with small changes in pressure, temperature and composition. In twophase applications where supercritical CO2 may be used as solvent or reactant, these varying properties can result in very different flow patterns and hydrodynamics with pressure change. Since the hydrodynamics of such systems largely influence heat and mass transfer, the study of flow behavior under supercritical conditions in microchannel is fundamental. In pursuit of this objective, a transparent microdevice, which is suitable for experiments at pressures higher than critical pressure of CO2 (PC = 74 bar), has been developed in this thesis. Using a soft lithography method that is currently used to fabricate microfluidic chips for applications under ambient pressure, a methodology for fabricating a highly resistant chip made from glass and UV-curable polymer was developed. These chips can resist more than 100 bar in supercritical CO2 conditions. The microchips were then used to observe the flow behavior of a CO2-ethanol mixture created in a T junction microchannel (cross section: 200 x 200 µm) for pressures ranging from 40 to 90 bar using high-speed imaging. To identify and interpret phenomena that occur during the flow formation at high pressure, a thermodynamic approach was essential. Depending on the pressure, temperature and composition of the CO2-ethanol mixture, the flow at equilibrium can either be in the single phase liquid domain or in the two-phase liquid vapor domain, according to the phase diagram. Imaging experiments were conducted over the boundaries between the two-phase liquid vapor domain and the single phase liquid and the observed two-phase flow patterns and transitions confirm the predictions of the phase diagram. High-pressure CO2-ethanol Taylor flow, which was obtained in the twophase domain, was then studied. This flow pattern, which is characterized by elongated bubbles surrounded by a liquid film and separated from each other by liquid slugs, is well-known at low pressure and has been widely described in the literature. The objective here was therefore to compare the flow characteristics such as bubble length, slug length and bubble velocity obtained under high pressure operation with the behavior at low pressure. Differences coming from fluid characteristics or operating at high pressure were pointed . This work provides a variety of new results on high pressure microfluidics that will be of interest to both the microfluidics and the supercritical fluids communities. It presents a new protocol to fabricate low cost pressure-resistant microfluidic chips suitable for supercritical CO2 and advanced visualization methods. It also presents new findings obtained with this technology on map flow pattern at high pressure in correlation with thermodynamics approach and characterization of Taylor flow hydrodynamics under high pressure in microchannel.

Modéliser le mouvement des foules humaines est essentiel pour des situations aussi diverses que la prévention de risque dans les lieux publics, la planification d’évènements ou la création d’animations visuelles réalistes. Cependant, la difficulté de mener des expériences quantitatives limite notre compréhension de la dynamique des piétons, et le manque de mesures de référence rend impossible une comparaison poussée des modèles existants. Cette thèse tente d’augmenter notre compréhension des foules humaines par deux approches distinctes. Dans un premier temps, nous avons conduit une étude numérique et théorique pour étudier formation de lignes au sein de flux bidirectionnels d'agents motiles. Nous avons montré qu’une transition de phase critique du second ordre séparait un état mélangé d’un état constitué de lignes géantes le long desquelles se déplacent les agents visants une même direction. Cette séparation est caractéristique des systèmes actifs. Une approche hydrodynamique nous a ensuite permis de prouver que les phases mélangées sont aussi algébriquement corrélées dans la direction longitudinale. Nous avons expliqué et montré que ces fortes corrélations sont génériques de tous systèmes de flux bidirectionnels, qu’ils soient constitués de particules forcées ou de particules actives. Dans un second temps, nous avons mené une campagne expérimentale de grande envergure afin d’établir une expérience de référence des foules humaines. Nous avons pour cela choisi un système modèle, la zone d’attente de marathons. Dans ces foules de dizaines de milliers d’individus, nous avons quantitativement établi que les fluctuations de vitesse se propagent sur de grandes échelles, alors que les variations d’orientation s’évanouissent en quelques secondes. Grâce à ces mesures, nous avons construit une théorie prédictive hydrodynamique des foules polarisées. / Modelling crowd motion is central to situations as diverse as risk prevention in mass events and visual effects rendering in the motion picture industry. The difficulty to perform quantitative measurements in model experiments, and the lack of reference experimental system, have however strongly limited our ability to model and control pedestrian flows. The aim of this thesis is to strengthen our understanding of human crowds, following two distinct approaches.First, we designed a numerical model to study the lane formation process among bidirectional flows of motile particles. We first evidenced the existence of two distinct phases: one fully laned and one homogeneously mixed, separated by a critical phase transition, unique to active systems. We then showed with a hydrodynamic approach that the mixed phase is algebraically correlated in the direction of the flow. We elucidated the origin of these strong correlations and proved that they were a universal feature of any system of oppositely moving particles, active of passive.Second, we conducted a substantial experimental campaign to establish a model experiment of human crowds. For that purpose we performed systematic measurements on crowds composed of tens of thousands of road-race participants in start corrals, a geometrically simple setup. We established that speed information propagates through polarized crowds over system spanning scales, while orientational information is lost in a few seconds. Building on these observations, we laid out a hydrodynamic theory of polarized crowds and demonstrated its predictive power.

A floating object's drift is governed by its buoyancy, shape, and the wind, waves and currents it experiences. Here, I develop a drift modelling framework for marine mammal carcases in coastal waters. The resulting models were run forwards and backwards in time to provide insights into strategies for environmental monitoring under two scenarios. The first explored the beach search options for carcases resulting from potentially fatal collisions between tidal-stream turbines and marine mammals. The second applied the reverse problem for known-location mass strandings to highlight potential at-sea mortality sites. The drift properties of carcase-like objects were assessed in at-sea experiments. Wave transport was found to be greater than Stokes drift alone and in a complex coastal area could not be represented by a downwind multiplier as many previous models have assumed. A high resolution unstructured grid wave model was set up to complement existing wind and current models for the West Coast of Scotland, and these components were combined to build a carcase drift model. In the forward case, from tidal turbine locations, the drift model showed a wide spread of potential stranding sites, suggesting monitoring a limited number of beaches is unlikely to be fruitful. However, selecting beaches in response to immediate wind direction would improve efficiency. Stranding locations alone can only provide evidence of turbine interactions if the number of animals affected is large. In the reverse case, when applied to a mass stranding in Chile, the drift model showed the ability to exclude areas of origin, even though it could not pinpoint an exact mortality site. This work advances understanding of wave transport of surface floating objects, of carcase drift modelling, and of the feasibility of strandings monitoring. The decomposition rate of carcases is a source of uncertainty in the model where further work should be undertaken.

FIGUEIREDO, David de Oliveira. Dinâmica de colisão de multipartículas: simulando a hidrodinâmica de fluidos complexos através de uma aproximação de partículas. 2014. 78 f. Dissertação (Mestrado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2014. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2014-08-29T19:33:17Z No. of bitstreams: 1 2014_dis_dofigueiredo.pdf: 1288977 bytes, checksum: 5724ec7a504fa2b64c8f07cf0625e1da (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2014-08-29T20:31:51Z (GMT) No. of bitstreams: 1 2014_dis_dofigueiredo.pdf: 1288977 bytes, checksum: 5724ec7a504fa2b64c8f07cf0625e1da (MD5) / Made available in DSpace on 2014-08-29T20:31:51Z (GMT). No. of bitstreams: 1 2014_dis_dofigueiredo.pdf: 1288977 bytes, checksum: 5724ec7a504fa2b64c8f07cf0625e1da (MD5) Previous issue date: 2014 / Simulation techniques with a strategy based on particle dynamics are an interesting alternative approach in describing the behavior of complex fluids. In these systems, phenomena occur typically in the range of mesoscopic size (nanometers to micrometers), where the energies are of the order of the thermal energy kT. In many phenomena the microscopic detail of the interaction between the constituents of the system is crucial for the correct description of the physical processes associated, so that a "coarse-graining" approximation, used in a continuous description based on the Navier-Stokes is not appropriate. It is in this context that the method presented here becomes important. Introduced by Malevanets and Kapral in 1999, the stochastic rotation dynamics, or multiparticle collision dynamics, is a simulation method for mesoscopic fluids; which basically consists of alternating streaming and collisions steps in an ensemble of point particles. The collisions are performed by grouping the particles into cells, in which there is conservation of mass, momentum and energy, in addition to meeting the hydrodynamic equations and taking into account the thermal fluctuations of the system. In this work we aim at presenting the multiparticle collision dynamics, through a discussion of its details, features and how the implementation is done in numerical simulations. Moreover, we present some classical hydrodynamics results, obtained from the method presented in this work. / Técnicas de simulação com uma abordagem fundamentada na dinâmica de partículas são uma alternativa interessante na descrição do comportamento de fluidos complexos. Nesses sistemas, fenômenos ocorrem tipicamente na escala de tamanho mesoscópica (nanometros a micrometros), onde as energias são da ordem da energia térmica kT. Em diversos fenômenos o detalhe microscópico da interação entre os constituintes do sistema é de fundamental importância para a descrição correta dos processos físicos associados, de modo que uma aproximação do tipo "coarse-graining", usada em uma descrição contínua baseada na equação de Navier-Stokes, não é adequada. É neste contexto que o método aqui apresentado se faz importante. Introduzido por Malevanets e Kapral em 1999, a dinâmica de rotação estocástica (stochastic rotation dynamics) ou dinâmica de colisão de multipartículas (multiparticle collision dynamics), é um método de simulação para fluidos mesoscópicos que basicamente consiste em alternar etapas de fluxo (streaming) e colisões num ensemble de partículas pontuais. As colisões são realizadas agrupando as partículas em células, nas quais há conservação de massa, momento linear e energia, além de satisfazer as equações hidrodinâmicas e levar em conta as flutuações térmicas do sistema. Neste trabalho temos como objetivo a apresentação da dinâmica de colisão de multipartículas, através de uma discussão sobre seus detalhes, particularidades e como é feita a implementação em simulações numéricas. Além disso, apresentamos como exemplo alguns resultados clássicos da hidrodinâmica, obtidos a partir do método abordado neste trabalho.

A chevron is a U-shaped rock structure constructed for improving navigation conditions by diverting majority of flow towards main channel. The objective of this study is to improve understanding of how chevrons affect channel flow. For this study, a two-dimensional numerical hydrodynamic model of a two-km-long reach of the Mississippi River was developed; three chevrons have been constructed in the modeled reach. The model was calibrated by adjusting Manning's n to match predicted and observed water surface elevations (WSELs). The model was validated using measured WSEL and velocity data from two events: a low-flow discharge (4,500 m3/s) and high-flow discharge (14,000 m3/s). At reach scale the model performed well in predicting WSELs. Average difference between model prediction and observed WSEL was 0.23 m in low-flow condition and 0.05 in high flow condition. Root mean square of errors (RMSEs) and mean absolute errors (MAEs) were used to measure the degree of agreement between predicted and measured velocities. At the reach scale there was reasonable agreement between predicted and observed velocities (RMSE = 0.416 m/s and 0.425 m/s, respectively, for low-flow and high-flow conditions). Local differences between predicted and observed velocities were up to 1.5 m/s; this is attributed to uncertainties in the velocity measurements. The model's sensitivity of to changes in Manning's n, eddy viscosity and bathymetry were also analyzed. The sensitivity analysis showed that there are specific areas (e.g., near the banks of the river) which are sensitive to changes in Manning's n. This indicates that spatial distribution of Manning's n is required to increase the accuracy in the model's predictions of velocity. Model was found to be stable in a specific range of eddy viscosity values. Eddy viscosity had little effect on velocity predictions but was important for model stability (i.e., the model was stable only for a range of eddy viscosity values). Reach scale changes in bathymetry had minor impacts on RMSE and MAE. However, local changes in channel bathymetry resulted in differences in velocity predictions as much as ±0.4 m/s.

Numerical calculations were performed over a variety of two-dimensional rib roughness configurations in which the ratio of flow depth to roughness height was varied from 1.1 to 40. Periodically fully developed flow was achieved by employing periodic boundary conditions and the effect of turbulence was accounted for by a two-layer model. These calculations were used to test the hypothesis that any rough wall resistance may be reduced to an equivalent wall shear stress located on a plane wall. The position of the plane wall is determined by a novel method of prediction obtained by consideration of strearnwise force moments. The resistance is then determined by three dynamically significant length scales: the first (yo) specifies the position of the equivalent plane wall, the second is the depth of flow h and the third is similar to Nikuradse's sand grain roughness k,,. The latter length scale is however depth dependent and a universal relationship is postulated: ks y,, -,= F/Tk where ksw is the asymptotic value of ks at very large flow depths. For the calculation of friction factor, a resistance equation is proposed of the form typical of fully rough flows. These postulates are supported by the numerical model results though further work including physical experiments is required to confirm them. Before applying the two-layer model to this problem it was tested on smooth rectangular duct flows and Schlichting's (1936) long angle roughness experiments. The opportunity was taken to further explore these flows, and in addition calculations were carried out for Grass et al's (1991) open channel rib roughness experiments. The periodic boundary conditions were also applied to a larninar counter-flow plate-fin heat exchanger. A novel source-sink arrangement for heat flux was developed in order to implement these boundary conditions.

We conduct an investigation into the role that turbulence plays in the formation of stars. In small clouds, with masses of ~ 30 Mʘ and where the turbulence is only injected at the start, we find that the turbulence does not trigger star formation. Instead, the dissipation of the kinetic energy allows the mean Jeans mass of the cloud to control the formation of stars. The equipartition of the kinetic and thermal energies in the final stages before star formation, allows the pre-protostellar clumps to fragment. Binary and multiple systems are thus a natural product of star formation in a turbulent environment. We find that globally unbound clouds can be the sites of star formation. Furthermore the star formation efficiency is naturally less than 100%, thus in part providing an explanation for the low efficiency in star forming regions. Globally unbound GMCs not only form stars, and naturally disperse, within a few crossing times, but also provide a mechanism for the formation of OB associations.