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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.

Aeroacoustic whistles are a significant source of customer complaints to automotive manufacturers. Whistles can occur on many such components, but the relative position and configuration of rearview mirrors means they are a more problematic source of tonal noise on vehicles. The low subsonic complex turbulent flow, combined with small cavity scales, determines the possible whistle mechanisms. The one considered to be most problematic, fluid-dynamic cavity resonance, is the topic of this research thesis. The research scope is limited to the automotive environment of external rearview mirrors and the fluid-dynamic resonance mechanism: low subsonic Mach number, M = 0.05 - 0.13; laminar boundary layers; and two-dimensional, acoustically compact cavities. The low unit-cost of rearview mirrors and the desire to have simple identification and prediction schemes, that could be used by production engineers, determined an empirical approach. A search of the existing literature revealed that there were some data on cavities of the above scale in low Mach number flow, but quoted errors in empirical descriptions were large and there was very little research on the effects of flow yaw angle on the chosen resonance mechanism. The research therefore aims to determine whether existing empirical descriptions of fluid-dynamic cavity resonance are suitable for the prediction of the resonance characteristics, with sufficient accuracy to enable unambiguous identification of the presence of the resonance and its mechanism. A second aim is to investigate the effects of a feature of the automotive flow environment, flow yaw angle, on the resonance. Flow yaw angle is determined by those components of the flow in the same plane as the surface in which the cavity is situated. An experimental program was undertaken using a purpose-built aeroacoustic wind tunnel and a simple cavity model. Testing with two types of cavity configurations, as well as flow visualisation, investigated the main features of the resonance in time-averaged yawed flow. Within the scope of this thesis, it is shown that fluid-dynamic cavity resonance characteristics can be accurately identified by a simple empirical model, even in yawed flow. Various descriptors allow identification of the resonance threshold, stage, frequency and relative amplitude in non-yawed flow, while the frequency and stage can also be identified in yawed flow. The relative decrease in resonance amplitude in yawed flow, although identified for these experiments, would depend on the degree of spanwise variation in the boundary layer characteristics for a given cavity configuration. The results also identify significant issues with testing in a free jet tunnel, due to the nature of fluid-dynamic cavity resonance and the fluctuation energy content in free shear layers. Despite this, the thesis aims are achieved, and appropriate design guidelines are produced for automotive designers.

This thesis presents the results of steady, Reynolds-averaged Navier-Stokes (RANS) computations for jet flow emanating from a generic turbofan engine exhaust. All computations were performed with commercial solver FLUENT v6.2.16. Different turbulence models were evaluated. In addition to turbulence modeling issues, a parametric study was considered. Different modeling approaches for turbulent jet flows were explained in brief, with specific attention given to the Reynolds-averaged Navier-Stokes (RANS) method used for the calculations. First, a 2D ejector problem was solved to find out the most appropriate turbulence model and solver settings for the jet flow problem under consideration. Results of one equation Spalart-Allmaras, two-equation standart k-&amp / #949 / , realizable k-&amp / #949 / , k-&amp / #969 / and SST k-&amp / #969 / turbulence models were compared with the experimental data provided and also with the results of Yoder [21]. The results of SST k-&amp / #969 / and Spalart-Allmaras turbulence models show the best agreement with the experimental data. Discrepancy with the experimental data was observed at the initial growth region of the jet, but further downstream calculated results were closer to the measurements. Comparing the flow fields for these different turbulence models, it is seen that close to the onset of mixing section, turbulence dissipation was high for models other than SST k-&amp / #969 / and Spalart-Allmaras turbulence models. Higher levels of turbulent kinetic energy were present in the SST k-&amp / #969 / and Spalart-Allmaras turbulence models which yield better results compared to other turbulence models. The results of 2D ejector problem showed that turbulence model plays an important role to define the real physics of the problem. In the second study, analyses for a generic, subsonic, axisymmetric turbofan engine exhaust were performed. A grid sensitivity study with three different grid levels was done to determine grid dimensions of which solution does not change for the parametric study. Another turbulence model sensitivity study was performed for turbofan engine exhaust analysis to have a better understanding. In order to evaluate the results of different turbulence models, both turbulent and mean flow variables were compared. Even though turbulence models produced much different results for turbulent quantities, their effects on the mean flow field were not that much significant. For the parametric study, SST k-&amp / #969 / turbulence model was used. It is seen that boundary layer thickness effect becomes important in the jet flow close to the lips of the nozzles. At far downstream regions, it does not affect the flow field. For different turbulent intensities, no significant change occurred in both mean and turbulent flow fields.

L'intégration aérodynamique des turboréacteurs à grand taux de dilution nécessite, dès les phases de conception, une connaissance précise des propriétés instationnaires du développement du jet à l'aide de la simulation numérique. Différents niveaux de modélisation sont étudiés dans cette thèse pour évaluer la capacité des méthodes numériques à caractériser le développement du jet. Une configuration de jet moteur double-flux est ici étudiée, en s'appuyant sur une large base de données expérimentale. La modélisation par les équations de Navier-Stokes moyennées démontre une bonne capacité des modèles de turbulence à reproduire les champs moyens de ce type d'écoulement, particulièrement au niveau des couches de mélange. La capture des systèmes de choc développés au sein des jets primaire et secondaire, ainsi que la prévision des niveaux de turbulence des écoulements sont en revanche peu satisfaisantes. Un recalage de la simulation sur les conditions turbulentes expérimentales, à l'aide de différentes configurations, pallie en partie ce défaut. La nécessité de procéder à un calcul et non plus à une modélisation des phénomènes turbulents conduit à l'application d'une méthode de calcul à résolution de turbulence. L'approche hybride de simulation des tourbillons détachés type DES (Detached Eddy Simulation) de cet écoulement montre une précision au moins équivalente pour la prévision des champs moyens et apporte une grande quantité d'informations supplémentaire sur les champs fluctuants et les caractéristiques instationnaires des couches de mélange du jet, en accord avec les données expérimentales. On conclue sur l'applicabilité de cette méthode dans un contexte industriel,son gain de précision rapporté à son temps de calcul et la perspective d'une telle approche pour une configuration de jet installée plus complexe. / The aerodynamic integration of Ultra-High Bypass Ratio turbofans raises the need for an accurate prediction of the unsteady properties of the jet development using Computational Fluid Dynamics, since the design stages. The ability of numerical methods in predicting these phenomena are assessed in this thesis, using different modelling approaches. A dual-stream jet configuration is investigated, using an associated wind-tunnel test campaign. Reynolds-Averaged Navier-Stokes simulations, using several turbulence models, are shown to correctly reproduce the mean flow especially concerning the shear layers. Shock cells and turbulence levels predictions within the primary and secondary jet flows are however perfectible compared to the test results. Turbulence-accounting approaches based upon experimental data and using different configurations partially overcome this issue. An unsteady methodis then applied in order to resolve turbulent phenomena instead of modelling them. The hybrid Detached-Eddy Simulation of the flow demonstrates an at least equivalent accuracy concerning the mean flow and provides additional information on the fluctuating fields and shear layers unsteady properties, in fair agreement with the experimental results. Prospects discuss benefits and consequences of this approach taking into account its cost and the industrial context of this application,as well as its potential use for a more complex, installed jet configuration.

This research focuses on measurements of a convective shear-layer instability seen naturally in quiet hypersonic flow. Experiments were carried out in the Boeing/AFOSR Mach 6 Quiet Tunnel (BAM6QT) at Purdue University. The BAM6QT provides low-disturbance hypersonic flow with freestream noise levels similar to what would be experienced by a flight vehicle. To obtain high-speed, off-the-surface measurements of the instability, a modified focused laser differential interferometer (FLDI) was first designed to work with the contoured Plexiglas windows available in the tunnel.<div><br>A cone-cylinder-flare geometry was then selected to study the instabilities related to an axisymmetric separation bubble at Mach 6. The sharp cone had a 5-degree half-angle, while flare angles of 10 degrees and 3.5 degrees were tested to compare axisymmetric compression with and without separation, respectively. Under quiet flow, laminar separation and reattachment was confirmed by schlieren and surface pressure-fluctuation measurements. Coherent traveling waves were observed. These were attributed to both the second-mode instability, as well as a shear-generated instability from the separation bubble. The symmetry of the bubble was found to be highly sensitive to angle of attack. Additionally, by introducing controlled disturbances on the cone upstream of the separation, larger-amplitude shear-generated waves were measured while the second-mode amplitudes remained unchanged. Therefore, the shear-generated waves were amplified moving through the shear layer, while the second mode remained neutrally stable. These appear to be the first measurements of traveling waves that are generated in the shear layer of a separation bubble in hypersonic flow. <br></div>

In order to study three-dimensional unsteady turbulent flow fields such as the wakes of bluff bodies, a Digital Particle Image Velocimetry (DPIV) system was developed. This system allows non-intrusive two-dimensional and time varying velocity measurements. Software and hardware modifications necessary to enhance the capabilities of the system were preformed, resulting in increased frequency resolution. However, due to hardware limitations and limitations inherited from the implementation of the method, space resolution is reduced. Subsequently, digital image processing tools to improve the space resolutions were developed. The advantages and limitations of the method for the study of turbulent flows are presented in detail. The developed system is employed in the documentation of time-varying turbulent flow fields. Initially we study the spanwise variation of the near wake of a low-aspect ratio, surface-mounted, circular cylinder piercing a free surface. The asymmetry of the end conditions combined with the natural unsteadiness of the vortex shedding generates a very complex flow filed which is difficult to study with conventional methods. By employing the aforementioned system we are able to reveal a departure of the two-dimensional character of the flow in the form of oblique vortex shedding. The effect of free surface on the vortex formation length and on the vortex reconnection process is documented. Near the free surface the alternate mode of vortex shedding is suppressed, leading to simultaneous shedding of vortices in the wake. Indications of vortex dislocations and change of the vortex axis in order to reconnect to the free surface are observed. Finally, a novel approach of reconstructing the three-dimensional, time -varying volume of the flow field by obtaining simultaneous measurements of Laser Doppler Velocimetry and Particle Image Velocimetry planes is presented. The same field is investigated with focus on the streamwise structures. Three-dimensional streamwise vortical structures are known to exist due to instabilities of plane shear layers. Similar streamwise vortices, also known as braid vortices have been observed in the past in the wake of circular cylinders with symmetric boundary conditions. The present spatio-temporal analysis demonstrated coexistence of two types of streamwise vortices in the wake, bilge and braid type of vortices. These may be due to the three dimensionality introduced by the free surface. In addition, the sufficient time resolution allowed the detection of the primary Von-Karman vortex through a plane of interrogation normal to the free stream, thus revealing the spanwise variation of the vortex shedding and its evolution at different downstream stations. The combination of the effect of the asymmetric boundary conditions with a free surface is investigated by adding one more source of three-dimensionality in terms of inclination of the cylinder axis. Hydrogen-bubble and particle-flow visualizations are preformed in combination with Laser-Doppler Velocimetry measurements. From both qualitative and quantitative results the effects of inclination and Froude number are documented. It is proved that the vortex shedding is suppressed for high values of the Froude number, however the inclination counteracts the vortex suppression and favors the vortex shedding mechanism. In addition, in the region of the no-slip boundary condition the flow is dominated by the effect of the horseshoe vortex. The case of a three-dimensional separated flow over a surface-mounted prism is investigated using a modified version of the system. The character of the separated from the leading edge corner shear layer and the formed separation bubble are documented in space and time along the mid-plane of symmetry of the body. Three different flows corresponding to different Reynolds numbers are studied. The unsteadiness of the flow is presented indicating a pseudo-periodic character. Large-scale, low-frequency oscillations of the shear layer that have been observed in the past using point measurement methods are now confirmed by means of a whole field velocity measurement, technique allowing a holistic view of the flow. In addition, the unsteadiness of the point of reattachment is associated with the flapping of the shear layer and the shedding of vorticity in the wake. Finally, it is demonstrated that the apparent vortex shedding mechanism of such flows is dependent on the interaction of the primary vortex of the separation bubble with a secondary vortex formed by the separation of the reverse flow boundary layer. By performing measurements with such time and space resolution the inadequacy of time averaged or point measurement methods for the treatment of such complex and unsteady flow fields becomes evident. In final case we employ Particle-Image Velocimetry to show the effect of unsteady excitation on two-dimensional separated flow over a sharp edged airfoil. It is proved that such an approach can be used to effectively control and organize the character of the flow, potentially leading to lift increase and drug reduction of bluff bodies / Ph. D.

<p dir="ltr">Pressure transducer, infrared heat transfer, and schlieren imaging data for a flared cone and cone-cylinder-flare in Mach 6 quiet flow are presented. Flared cone pressure transducer results show second-mode RMS values comparable to that found in prior experimental work. Second-mode frequency is found to linearly increase with increasing freestream unit Reynolds number, and frequency varies little between sensors for a given freestream unit Reynolds number. Turbulent intermittency begins to increase at a freestream unit Reynolds number 2x10⁶/m greater than the unit Reynolds number corresponding to peak second-mode RMS. peak RMS. High-speed schlieren imaging on the downstream section of the flared cone shows the second-mode disturbance following trends in power which correlate with PCB RMS. Infrared heat transfer results contain the azimuthal heating streak pattern observed for the flared cone in prior research, but the hot-cold-hot streak pattern is not seen due to limited model length. Streak heating occurs downstream of second-mode peak RMS over the freestream unit Reynolds number range of 6.4x10⁶/m to 10.4x10⁶/m. The heat transfer of streaks is found to vary significantly from streak to streak, while mean streak heating variation with freestream unit Reynolds number is small.</p><p dir="ltr">PCB results of the cone-cylinder-flare show intermittent turbulence at a freestream unit Reynolds number of 16.0x10⁶/m. Examination of shear-layer and second-mode instabilities show significant increases in RMS moving downstream along the flare and with increasing freestream unit Reynolds number. High-speed schlieren imaging of the shear-layer reattachment region on the flare show the presence of the shear-layer and second-mode instabilities when the model is configured with a sharp nose tip. The instabilities are not present with a blunt 5 mm radius nose tip. Heat transfer is observed to increase along the downstream portion of the flare. The sharp nose tip configuration has higher heat transfer rates than the 5 mm radius nose tip configuration.</p>

A wave cavity resonator (WCR) is investigated to determine the suitability of the device as an energy harvester in rivers or tidal flows. The WCR consists of coupling between self-excited oscillations of turbulent flow of water in an open channel along the opening of a rectangular cavity and the standing gravity wave in the cavity. The device was investigated experimentally for a range of inflow velocities, cavity opening lengths, and characteristic depths of the water. Determining appropriate models and empirical relations for the system over a range of depths allows for accuracy when designing prototypes and tools for determining the suitability of a particular river or tidal flow as a potential WCR site. The performance of the system when coupled with a wave absorber/generator is also evaluated for a range piston strokes in reference to cavity wave height. Video recording of the oscillating free-surface inside the resonator cavity in conjunction with free-surface elevation measurements using a capacitive wave gauge provides representation of the resonant wave modes of the cavity as well as the degree of the flow-wave coupling in terms of the amplitude and the quality factor of the associated spectral peak. Moreover, application of digital particle image velocimetry (PIV) provides insight into the evolution of the vortical structures that form across the cavity opening. Coherent oscillations were attainable for a wide range of water depths. Variation of the water depth affected the degree of coupling between the shear layer oscillations and the gravity wave as well as the three-dimensionality of the flow structure. In terms of the power investigation, conducted with the addition of a load cell and linear table-driven piston, the device is likely limited to running low power instrumentation unless it can be up-scaled. Up-scaling of the system, while requiring additional design considerations, is not unreasonable; large-scale systems of resonant water waves and the generation of large scale vortical structures due to tidal or river flows are even observed naturally. / Graduate / 0547 / 0548 / reaumejd@uvic.ca

In the first part of this dissertation, the phenomenon of self-sustained pressure os-cillations due to the flow past a circular, axisymmetric cavity, associated with inline gate valves, was investigated. In many engineering applications, such as flows through open gate valves, there exists potential for coupling between the vortex shedding from the up-stream edge of the cavity and a diametral mode of the acoustic pressure fluctuations. The effects of the internal pipe geometry immediately upstream and downstream of the shal-low cavity on the characteristics of partially trapped diametral acoustic modes were in-vestigated numerically and experimentally on a scaled model of a gate valve mounted in a pipeline that contained convergence-divergence sections in the vicinity of the valve. The resonant response of the system corresponded to the second acoustic diametral mode of the cavity. Excitation of the dominant acoustic mode was accompanied by pressure oscillations, and, in addition to that, as the angle of the converging-diverging section of the main pipeline in the vicinity of the cavity increased, the trapped behavior of the acoustic diametral modes diminished, and additional antinodes of the acoustic pressure wave were observed in the main pipeline. In addition to that, the effect of shallow chamfers, introduced at the upstream and/or downstream cavity edges, was investigated in the experimental system that con-tained a deep, circular, axisymmetric cavity. Through the measurements of unsteady pressure and associated acoustic mode shapes, which were calculated numerically for several representative cases of the internal cavity geometry, it was possible to identify the configuration that corresponded to the most efficient noise suppression. This arrangement also allowed calculation of the azimuthal orientation of the acoustic modes, which were classified as stationary, partially spinning or spinning. Introduction of shallow chamfers at the upstream and the downstream edges of the cavity resulted in changes of azimuthal orientation and spinning behaviour of the acoustic modes. In addition, introduction of splitter plates in the cavity led to pronounced change in the spatial orientation and the spinning behaviour of the acoustic modes. The short splitter plates changed the behaviour of the dominant acoustic modes from partially spinning to stationary, while the long split-ter plates enforced the stationary behaviour across all resonant acoustic modes. Finally, the evolution of fully turbulent, acoustically coupled shear layers that form across deep, axisymmetric cavities and the effects of geometric modifications of the cavity edges on the separated flow structure were investigated using digital particle image velocimetry (PIV). Instantaneous, time- and phase-averaged patterns of vorticity pro-vided insight into the flow physics during flow tone generation and noise suppression by the geometric modifications. In particular, the first mode of the shear layer oscillations was significantly affected by shallow chamfers located at the upstream and, to a lesser degree, the downstream edges of the cavity. In the second part of the dissertation, the performance of aortic heart valve pros-thesis was assessed in geometries of the aortic root associated with certain types of valve diseases, such as aortic valve stenosis and aortic valve insufficiency. The control case that corresponds to the aortic root of a patient without valve disease was used as a reference. By varying the aortic root geometry, it was possible to investigate corresponding changes in the levels of Reynolds shear stress and establish the possibility of platelet activation and, as a result of that, the formation of blood clots. / Graduate / 0541 / 0546 / 0548 / 0986 / alexbn024@gmail.com