Numerous scenarios exist whereby radioactive particulates are transported between spatially separated points of interest. An example of this phenomenon is, in the aftermath of a Radiological Dispersal Device (RDD) detonation, the resuspension of radioactive particulates from the resultant fallout field. Quantifying the spatial distribution of radioactive particulates allow for the calculation of potential radiation doses that can be incurred from exposure to such particulates. Presently, there are no simulation techniques that link radioactive particulate transport with subsequent radiation field determination and so this thesis develops a coupled Computational Fluid Dynamics (CFD) and Monte Carlo (MC) Radiation Transport approach to this problem. Via particulate injections, the CFD simulation defines the spatial distribution of radioactive particulates and this distribution is then employed by the MC Radiation Transport simulation to characterize the resultant radiation field. GAMBIT/FLUENT are employed for the CFD simulations while MCNPX is used for the MC Radiation Transport simulations. / UOIT

Combustion systems utilizing bluff bodies to stabilize the combustion processes can experience oscillatory heat release due to the alternate shedding of coherent, von Kármán vortices under certain operating conditions. This phenomenon needs to be understood in greater detail, since unsteady burning due to vortex shedding can lead to combustion instabilities and flame extinction in practical combustion systems. The primary objective of this study was to elucidate the influence of combustion process heat release upon the Bénard-von Kármán (BVK) instability in reacting bluff body wakes. For this purpose, spatial and temporal heat release distributions in bluff body-stabilized combustion of liquid Jet-A fuel with high-temperature, vitiated air were characterized over a wide range of operating conditions. Upon comparing the spatial and temporal heat release distributions, the fuel entrainment and subsequent heat release in the near-wake were found to strongly influence the onset and amplitude of the BVK instability. As the amount of heat release in the near-wake decreased, the BVK instability increased in amplitude. This was attributed to the corresponding decrease in the local density gradient across the reacting shear layers, which resulted in less damping of vorticity due to gas expansion. The experimental results were compared to the results of a parallel, linear stability analysis in order to further understand the influence of the combustion processes in the near-wake upon the wake instability characteristics. The results of this analysis support the postulate that oscillatory heat release due to BVK vortex shedding is the result of local absolute instability in the near-wake, which is eliminated only if the temperature rise across the reacting shear layers is sufficiently high. Furthermore, the results of this thesis demonstrate that non-uniform fuelling of the near-wake reaction zone increases the likelihood of absolutely unstable, BVK flame dynamics due to the possibility of near-unity products-to-reactants density ratios locally, especially when the reactants temperature is high.

The problem of bluff body flows and the drag associated with them has been the subject of numerous investigations in the literature. In the two-dimensional case, the flow past a circular cylinder has been most widely studied both experimentally and computationally. As a result, a well documented understanding of the gross features of the near-wake around a circular cylinder exists in the literature. In contrast, very little is understood on the general features of three-dimensional bluff body near-wakes, except that the vortex shedding is known to be less intense. Control or management of bluff body flows, both from the point of view of drag reduction as well as suppressing unsteady forces caused by vortex shedding, has been an area of considerable interest in engineering applications. The basic aim in the different control methods involves direct or indirect manipulation (or modification) of the near-wake structure leading to weakening or inhibition of vortex shedding. Many passive and energetic techniques (such as splitter plates, base and trailing edge modifications and base bleed) have been effective in the two-dimensional case in increasing the base pressure, leading to varying amounts of drag reduction; a large body of this work is centered around circular cylinders because of direct relevance in applications. The present work is an attempt to understand some of the major aspects of the near-wake structure of a sphere and to control the same for drag reduction employing a passive technique. Many of the passive control techniques found useful in two-dimensional flows are not appropriate in the context of a sphere. In this thesis, the effects of natural ventilation on the wake and drag of a sphere at low speeds have been studied experimentally in some detail. Natural bleed into the base is created when the stagnation and base regions of a sphere are connected through an internal duct. Although natural ventilation has features broadly similar to the well known base-bleed technique (both involve addition of mass, momentum and energy into the near-wake), there are many significant differences between the two methods; for example, in base bleed, the mass flow injected can be controlled independent of the outer flow, whereas in natural ventilation, it is determined by an interaction between the internal and the external flow around the body. Experiments have been conducted in both wind and water tunnels, which covered a wide range of Reynolds number (ReDj based on the diameter of the sphere) from of 1.7 x 103 to 8.5 x 105 with natural boundary layer transition. The ratio of the frontal vent area to the maximum cross sectional area of the sphere was varied from 1% to 2.25% and the effect of the internal duct geometry, including a convergent and a divergent duct was examined as well. After preliminary force measurements involving different duct geometries and vent areas, it was decided to make detailed measurements with a straight (parallel) duct with a vent area ratio of 2.25%. Extensive flow visualization studies involving dye-flow, hydrogen bubble, surface oil-flow and laser-light-sheet techniques were employed to gain insight into many aspects of the near-wake structure and the flow on the surface of the sphere. Measurements made included model static pressures, drag force using a strain gauge balance and velocity profiles in the near-wake and internal flow through the vent. In addition, wake vortex shedding frequency was measured using a hotwire. In the subcritical range of Reynolds numbers (ReD< 2 x 105), the near-wake of the sphere (without ventilation) was found to be vortex shedding, with laminar separation occurring around a value of0s = 80° (where 0s is the angle between the stagnation point and separation location). In contrast, there was little evidence of vortex shedding in the supercritical range (ReD> 4 x 105), consistent with many earlier observations in the literature; however, flow visualization studies in the near-wake clearly showed the existence of a three-dimensional vortex-like structure exhibiting random rotations about the streamwise axis. In this range of Reynolds numbers, surface flow visualization studies indicated the existence of a laminar separation bubble which was followed by a transitional/turbulent reattachment and an ultimate separation around 0S = 145°. All the above observations are broadly consistent with the results available in the literature. With ventilation at subcritical Reynolds numbers, the pressure distributions on the sphere including in the base region was only weakly altered, resulting in a marginal reduction in the total drag; because of the higher pressure difference between the stagnation and base regions, the mean velocity in the vent-flow was about 0.9 times the free-stream velocity. As may be expected, there was little change in the location of laminar separation on the sphere and the vortex shedding frequency was virtually unaltered due to ventilation. The relatively small effects on pressure distribution and drag suggest weak interaction between the vent-flow and the separated shear layer in the subcritical regime. The time-averaged near-wake flow revealed a stagnation point occurring between the vent-flow and the reverse flow in the near-wake, along with the formation of a torroidal vortex between the stagnation point and the near-wake closure; these features bear some resemblance to those observed with base bleed from a blunt base. With ventilation in the supercritical range of Reynolds numbers (ReD > 4 x 105), significant reduction in the total drag, of as much as 65%, was observed from force measurements. Pressure distributions showed higher pressures in the separated flow zone (consistent with reduced drag) as a result of which the internal mass and the mean velocity of the vent-flow were lower (0.69 times the free-stream velocity) compared to the value in the subcritical flow regime. Flow visualization studies clearly showed that the three-dimensional rotating structure (associated with the wake of the unvented sphere) was significantly modified by ventilation, leading to more symmetric and steady near-wake features. The larger effects on pressure distribution and drag suggest strong interaction between the vent-flow and the separated shear layer, promoted by their close proximity. The comparison of power spectral density of u1 signals in the near-wake showed significant reduction in the amplitude at all frequencies, consistent with observations from flow visualization studies. The time-averaged near-wake flow features a pair of counterrotating ring vortices which are trapped between the outer separated shear layer and the vent-flow shear layer; such a mean flow pattern is qualitatively similar to that behind an axisymmetric base with a central jet with unequal freestream velocities in the jet and outer flow. This study strongly suggests that natural ventilation can provide significant total drag reduction provided the vent-flow is in close proximity of the separated shear layer promoting a strong interaction between them. Drag reduction is associated with more symmetric and relatively steady near-wake features in contrast with the unvented sphere.

Depuis la découverte des modes asymétriques dans le sillage d'un corps simplifié d'automobile, réminiscents d'une bifurcation à bas nombre de Reynolds, se posent des questions propres au développement aérodynamique des véhicules terrestres telles que l'influence du vent latéral, de l'assiette et du rétreint d'arrière-corps couramment utilisé en phase d'optimisation. Notre travail s'attache à répondre expérimentalement à ces questions pour des géométries simplifiées mais aussi réelles. Les essais sont réalisés en soufflerie industrielle à l'échelle 2/5 pour le corps académique et en pleine échelle pour les monospaces. Nous montrons que le désalignement du véhicule par rapport à l'écoulement incident n'a pour effet que de modifier l'orientation du mode asymétrique sans en changer l'intensité. Nous construisons un modèle simple prédisant non seulement cette orientation mais aussi les conséquences sur les efforts aérodynamiques transverses. La contribution de l'instabilité sur les coefficients aérodynamiques de portance ou d'effort latéral est de l'ordre de 0,02 indépendamment du vent de travers et de l'assiette du véhicule. Les rétreints d'arrière-corps affectent également la dynamique du sillage et son orientation, mais l'instabilité n'est jamais supprimée. Ces résultats sont retrouvés pour des véhicules réels de type monospace dont le sillage est donc également soumis au même mode asymétrique, révélé sans ambigüité par des expériences de sensibilité en assiette. Nos résultats indiquent que, pour tous les véhicules considérés, le mode asymétrique de sillage est systématiquement présent dans l'enveloppe de conduite. Le contrôle ou la suppression de ce mode devrait offrir de nouvelles perspectives d'optimisation des véhicules à culot droit de type monospaces ou SUV. / Since the recent discovery of asymmetric modes in the wake of a simplified vehicle geometry, reminiscent from a bifurcation at low Reynolds numbers, some questions related to the aerodynamic development of ground vehicles such as the influence of lateral wind, pitch and afterbody boat-tail classically used during shape optimization remain unanswered. Our work is devoted to assess those questions experimentally for simplified but also real geometries. The tests are conducted in an industrial wind-tunnel, at the 2/5-scale for the academic body and at the full scale for the minivans. We show that the vehicle's misalignment only modifies the asymmetric mode's orientation without affecting its intensity. We build a model predicting not only this orientation but also the consequences on the cross-flow aerodynamic loading. The contribution of the instability to the lift or side force coefficients is of the order of 0,02 independently of lateral wind or of the vehicle's pitch. Afterbody boat-tails also impact the wake dynamics and its orientation but the instability is never suppressed. These results are retrieved for real vehicles such as minivans, whose wake is then subjected to the same asymmetric mode as well, revealed unambiguously with pitch sensitivity experiments. Our results indicate that, for all considered vehicles, the asymmetric wake mode is systematically present in the driving envelope. The control or the suppression of this mode should offer new optimization's perspectives for blunt based vehicles such as minivans or SUVs.

Lean flame blowout is investigated experimentally within a high-speed combustor to analyze the temporal extinction dynamics of turbulent premixed bluff body stabilized flames. The lean blowout process is induced through fuel flow reduction and captured temporally using simultaneous high-speed particle imaging velocimetry (PIV) and CH* chemiluminescence. The evolution of the flame structure, flow field, and the resulting strain rate along the flame are analyzed throughout extinction to distinguish the physical mechanisms of blowout. Flame-vortex dynamics are found to be the main driving mechanism of flame extinction; namely, a reduction of flame-generated vorticity coupled with an increase of downstream shear layer vorticity. The vorticity dynamics are linked to hydrodynamic instabilities that vary as a function of the decreasing equivalence ratio. Frequency analysis is performed to characterize the dynamical changes of the hydrodynamic instability modes during flame extinction. Additionally, various bluff body inflow velocity regimes are investigated to further characterize the extinction instability modes. Both equivalence ratio and flow-driven instabilities are captured through a universal definition of the Strouhal number for the reacting bluff body flow. Finally, a Karlovitz number-based criterion is developed to consistently predict the onset of global extinction for different inflow velocity regimes.

This thesis describes research on acoustically excited bluff body flow-fields, motivated by the problem of combustion instabilities in devices utilizing these types of flame-holders. Vortices/convective-structures play a dominant role in perturbing the flame during these combustion instabilities. This thesis addresses a number of issues related to the origin, evolution and the interaction of these structures with the flame. The first part of this thesis reviews the fluid mechanics of non-reacting and reacting bluff body flows. The second part describes the spatio/temporal characteristics of bluff-body flames responding to excitation. The key processes controlling the flame response have been identified as 1) the anchoring of the flame at the bluff body, 2) the excitation of flame-front wrinkles by the oscillating velocity field and 3) flame propagation normal to itself at the local flame speed. The first two processes control the growth of the flame response and the last process controls the decay. The third part of this thesis describes the effect of acoustic excitation on the velocity field of reacting bluff body flows. Acoustic disturbances excite the Kelvin-Helmholtz (KH) instability of the reacting shear layer. This leads to a spatially decaying vorticity field downstream of the bluff body in the shear layers. The length over which the decay occurs was shown to scale with the length of the recirculation zone of the bluff body, i.e. the length over which the velocity profile transitions from shear layer to wake. The flame influences this decay process in two ways. Gas expansion across the flame reduces the extent of shear by reducing the magnitude of negative velocities within the recirculation zone. This combined with the higher product diffusivity reduces the length of the recirculation zone, thereby further augmenting the decay of the vorticity fluctuations. Lastly, these results also revealed phase jitter - a cycle-to-cycle variation in the position of the rolled-up vortices. Close to the bluff-body, phase jitter is very low but increases monotonically in the downstream direction. This leads to significant differences between instantaneous and ensemble averaged flow fields and, in particular, the decay rate of the vorticity in the downstream direction.

Large Eddy Simulation (LES) of bluff body stabilized premixed and partially premixed combustion close to the flammability limit is carried out in this thesis. The LES algorithm has no ad-hoc adjustable model parameters and is able to respond automatically to variations in the inflow conditions. Algorithm validation is achieved by comparison with reactive and non-reactive experimental data. In the reactive flow, two scalar closure models, Eddy Break-Up (EBULES) and Linear Eddy Mixing (LEMLES), are used and compared. Over important regions, the flame lies in the Broken Reaction Zone regime. Here, the EBU model assumptions fail. The flame thickness predicted by LEMLES is smaller and the flame is faster to respond to turbulent fluctuations, resulting in a more significant wrinkling of the flame surface. As a result, LEMLES captures better the subtle effects of the flame-turbulence interaction. Three premixed (equivalence ratio = 0.6, 0.65, and 0.75) cases are simulated. For the leaner case, the flame temperature is lower, the heat release is reduced and vorticity is stronger. As a result, the flame in this case is found to be unstable. In the rich case, the flame temperature is higher, and the spreading rate of the wake is increased due to the higher amount of heat release Partially premixed combustion is simulated for cases where the transverse profile of the inflow equivalence ratio is variable. The simulations show that for mixtures leaner in the core the vortical pattern tends towards anti-symmetry and the heat release decreases, resulting also in instability of the flame. For mixtures richer in the core, the flame displays sinusoidal flapping resulting in larger wake spreading. More accurate predictions of flame stability will require the use of detailed chemistry, raising the computational cost of the simulation. To address this issue, a novel algorithm for training Artificial Neural Networks (ANN) for prediction of the chemical source terms has been implemented and tested. Compared to earlier methods, the main advantages of the ANN method are in CPU time and disk space and memory reduction.

The flow around a finite square prism has not been studied extensively when compared with an “infinite” (or two-dimensional) square prism. In the present study, the effect of a wake-mounted splitter plate on the flow around a surface-mounted square prism of finite height was investigated experimentally using a low-speed wind tunnel. Of specific interest were the combined effects of the splitter plate length and the prism’s aspect ratio on the vortex shedding, mean drag force coefficient, and the mean wake. Four square prisms of aspect ratios AR = 9, 7, 5 and 3 were tested at a Reynolds number of Re = 7.4×104 and a boundary layer thickness of /D = 1.5. Splitter plate lengths of L/D = 1, 1.5, 2, 3, 5, and 7, were tested, with all plates having the same height as the prism. Measurements of the mean drag force were obtained with a force balance, and measurements of the vortex shedding frequency were obtained with a single-component hot-wire probe. A seven-hole pressure probe was used to measure the time-averaged wake velocity at a Reynolds number of Re = 3.7×104 for AR = 9 and 5 with splitter plates of lengths L/D = 1, 3, 5, and 7. These measurements were carried out to allow for a better understanding of how the splitter plate affects the mean wake of the finite prism. The results show that the splitter plate is a less effective drag-reduction, but more effective vortex-shedding-suppression, device for finite square prisms than it is for infinite square prisms. Significant reduction in the mean drag coefficient was realized only for short prisms (of AR ≤ 5) when long splitter plates (of L/D ≥ 5) were used. A splitter plate of length L/D = 3 was able to suppress vortex shedding for all aspect ratios tested. However, for square prisms of aspect ratios AR ≤ 7, the splitter plate is a less effective vortex-shedding-suppression device when compared to its use with finite circular cylinders, i.e. longer splitter plates are needed for vortex shedding suppression with square prisms. Wake measurements showed distinct wake velocity fields for the two prisms tested. For the prism of AR = 9, a strong downwash flow in the upper part of the wake became weaker towards the ground plane. For the prism of AR = 5, the downwash remained strong close to the ground plane. With splitter plates installed, the downwash became weaker for both prisms. The splitter plate was found to narrow the wake width, especially close to the ground plane, and led to the stretching of the streamwise vortex structures in the vertical direction, and increased entrainment towards the wake centreline in the cross-stream direction.

It is well known that most fluid flows observed in nature or encountered in engineering applications are turbulent and involve separation. Fluid flows in turbines, diffusers and channels with sudden expansions are among the widely observed areas where separation substantially alters the flow field and gives rise to complex flow dynamics. Such types of flows are referred to as internal flows since they are confined within solid surfaces and predominantly involve the generation or utilization of mechanical power. However, there is also a vast variety of engineering applications where the fluid flows past solid structures, such as the flow of air around an airplane or that of water around a submarine. These are called external flows and as in the former case the downstream evolution of the flow field is crucially influenced by separation. The present doctoral thesis addresses both internal and external separated flows by means of direct numerical simulations of the incompressible Navier-Stokes equations. For internal flows, the wall-driven flow in a onesided expansion channel and the pressure-driven flow in a plane channel with a single thin-plate obstruction have been studied in the fully developed turbulent state. Since such geometrical configurations involve spatially developing turbulent flows, proper inflow conditions are to be employed in order to provide a realistic fully turbulent flow at the input. For this purpose, a newly developed technique has been used in order to mimic an infinitely long channel section upstream of the expansion and the obstruction, respectively. With this approach, we are able to gather accurate mean flow and turbulence statistics throughout each flow domain and to explore in detail the instantaneous flow topology in the separated shear layers, recirculation regions as well as the recovery zones. For external flows, on the other hand, the flow past a prolate spheroid has been studied. Here, a wide range of Reynolds numbers is taken into consideration. Based on the characteristics of the vortical structures in the wake, the flow past a prolate spheroid is classified as laminar (steady or unsteady), transitional or turbulent. In each flow regime, the characteristic features of the flow are investigated by means of detailed frequency analysis, instantaneous vortex topology and three-dimensional flow visualizations.

Les décollements autour d’un corps en mouvement sont à l’origine de détériorations des performances aérodynamiques, de fatigues structurelles ou de nuisances sonores. La compréhension de ces phénomènes reste encore aujourd’hui l’un des enjeux majeurs de la recherche en aérodynamique. Le développement de systèmes permettant de contrôler l’écoulement et d’altérer ou de réduire les décollements apparaît comme une solution prometteuse en vue d’améliorer les performances aérodynamiques. On distingue les systèmes de contrôles passifs, simples d’utilisation mais incapables de s’adapter aux modifications de l’écoulement, des systèmes actifs qui disposent d’une grande adaptabilité mais nécessitent un apport extérieur d’énergie pour fonctionner. La stratégie du contrôle auto-adaptif s’apparente à un compromis entre ces deux méthodes. En s’inspirant de mécanismes présents dans la nature, elle permet d’associer amélioration des performances aérodynamiques, adaptabilité et autonomie. Ce travail de thèse porte sur l’étude expérimentale du contrôle du sillage turbulent d’un corps épais à l’aide d’actionneurs bio-inspirés avec un double objectif : i. déterminer les paramètres optimaux du dispositif de contrôle qui prend la forme d’un couple de volets flexibles, ii. Identifier les mécanismes physiques d’interactions entre l’actionnement et l’écoulement. Pour mener à bien cet objectif, de nombreux instruments de mesure complémentaires ont été mis en oeuvre. Une étude paramétrique a permis de démontrer l’efficacité du dispositif pour différentes configurations (fixes et auto-adaptatives) et d’identifier des configurations d’intérêt. La caractérisation de l’écoulement autour et dans le sillage du cylindre carré sans et avec contrôle a révélé un allongement de la longueur de recirculation à l’arrière du cylindre et la réduction de l’expansion du sillage. L’un des résultats majeurs de l’étude est que la réduction de traînée obtenue est principalement liée à une action du système sur l’anisotropie des fluctuations de l’écoulement et plus particulièrement sur l’entrainement du fluide dans le sillage de l’obstacle. / Flow separations around moving bodies lead to detrimental effects such as aerodynamic performances loss, structural fatigue and noises production. The understanding of these phenomena remains one of the most challenging issue of modern fluid dynamics. A promising solution to improve aerodynamic performances relies on the development of flow control devices able to prevent or mitigate the effects of separation. One can distinguish the passive flow control strategy, with easy to use devices but unable to adapt to the flow changes, from the active flow control strategy which benefits from a great adaptability but requires external power supply. Self-adaptive flow control appears to be a good compromise between those two strategies. Inspired from mechanisms at play in Nature, it combines good aerodynamic performances, self-adaptability and self-sustainability. This PhD thesis is dedicated to the experimental investigation of the turbulent flow over a bluff-body controlled by means of bio-inspired devices. The objective is two-folds : i. Design the control device which consists of a couple of compliant flaps, ii. Identify the physical mechanisms governing the interactions between the flow and the devices. A great number of complementary measurement techniques have been used in order to achieve these objectives. The efficiency of the devices for different configurations – locked and self-adaptive flaps - has been demonstrated through a parametric study. It has led to the identification of the main parameters involved in the control mechanism. The flow characterization around and in the wake of both uncontrolled and controlled cylinder revealed an increase in the length of the recirculation region and the reduction of the wake width. One of the major findings of this study is that the control essentially modifies the turbulent velocity field leading to a reduction of the lateral flow entrainment in the wake of the obstacle.