Les moteurs de fusées à ergols solides (SRMs) sont sensibles aux instabilités hydrodynamiques qui peuvent déclencher des oscillations auto-entretenues de pression de grandes amplitudes lorsqu’elles se couplent à l’un des modes acoustiques du système. Le moteur de ces instabilités est la formation de structures tourbillonnaires cohérentes synchronisées par des ondes acoustiques longitudinales. Pour certaines conditions de fonctionnement, les ondes acoustiques générées par l’interaction de ces tourbillons avec la tuyère amorcée du moteur renforcent l’oscillation acoustique. L’objectif des travaux menés dans cette thèse est de déterminer l’amplitude et la fréquence des oscillations de pression au cycle limite des instabilités. Celui-ci est atteint par saturation non linéaire des sources, qui est la conséquence de la formation de grosses structures cohérentes. Dans ce cas l’interaction tourbillon tuyère devient insensible à l’amplitude de l’onde du mode acoustique établi dans le foyer. Dans ces conditions, on peut se concentrer sur l’interaction d’un tourbillon avec la tuyère dans le mécanisme de production sonore. En considérant un écoulement incompressible et l’absence de frottement, un premier modèle analytique est développé permettant de déterminer la production sonore d’un tourbillon ingéré par une tuyère bidimensionnelle plane, lorsque le tourbillon est traité comme une ligne vorticité. Des expériences précédentes indiquent que le volume de la cavité autour de l’entrée d’une tuyère intégrée a une grande influence sur l’amplitude des oscillations de pression dans les grands SRMs. On montre que ceci est dû au champ de vitesse acoustique induit par la compressibilité du gaz dans la cavité qui produit une fluctuation de vitesse transverse à la trajectoire du tourbillon. Une seconde alternative au modèle analytique incompressible est développée en considérant toujours l’absence de frottement, mais un modèle compressible de l’interaction tourbillon-tuyère. Celui-ci repose sur un code aéroacoustique pour les écoulements internes basé sur les équations d’Euler (EIA) qui est utilisé ici pour la simulation de l’interaction tourbillon-tuyère. Une étude systématique de cette interaction a été menée pour une tuyère amorcée. Les résultats ont permis de proposer un modèle de sources localisées pour des ondes planes basé sur une analyse théorique des lois d’échelles de ces phénomènes. Les simulations de ces interactions tourbillons-tuyères ont été réalisées pour différents types de tuyères. En employant un bilan énergétique, un modèle avec un seul paramètre de contrôle est formulé, qui permet de reproduire qualitativement le comportement du cycle limite d’oscillations de pression observées dans des expériences réalisées avec des gaz froids décrites dans la littérature. Finalement le modèle Euler est utilisé pour comparer la production de son par interaction tourbillon-tuyère avec celle due à l’ingestion d’une onde d’entropie, appelée aussi tache d’entropie. Contrairement au cas des tourbillons, le bruit produit par ingestion de taches d’entropie n’est pas sensible au volume de la cavité d’une tuyère intégrée. Ces résultats indiquent que le bruit produit par les tourbillons est dominant dans le cas des SRMs étudiés. L’ensemble de ces travaux permet d’améliorer la compréhension des phénomènes d’interaction entre des non-homogénéités de l’écoulement et la tuyère. Elle permet surtout de déterminer quels sont les facteurs de l’écoulement et les éléments géométriques importants qui pilotent le niveau sonore produit par ces interactions. Les modèles développés dans ces travaux, avec divers degrés d’approximation et de complexité permettent d’enrichir la gamme des outils de conception des SRMs. / Solid Rocket Motors (SRMs) can display self-sustained acoustic oscillations driven by coupling between hydrodynamic instabilities of the internal flow and longitudinal acoustic standing waves. The hydrodynamic instabilities are triggered by the acoustic standing wave and results in the formation of coherent vortical structures. For nominal ranges of flow conditions the sound waves generated by the interaction between these vortices and the choked nozzle at the end of the combustion chamber reinforces the acoustic oscillation. Most available literature on this subject focuses on the threshold of instability using a linear model. The focus of this work is on the prediction of the limit-cycle amplitude. The limit-cycle is reached due to nonlinear saturation of the source, as a consequence of the formation of large coherent vortical structures. In this case the vortex-nozzle interaction becomes insensitive to the amplitude of the acoustic standing wave. Hence, one can focus on the sound generation of a vortex with the nozzle. Sound production can be predicted from an analytical two-dimensional planar incompressible frictionless model using the so-called Vortex Sound Theory. In this model the vorticity is assumed to be concentrated in a line vortex. Experiments indicate that the volume of cavities around so-called “integrated nozzles” have a large influence on the pulsation amplitude for large SRMs. This is due to the acoustical field normal to the vortex trajectory, induced by the compressibility of the gas in this cavity. As an alternative to the incompressible analytical model a compressible frictionless model with an internal Euler Aeroacoustic (EIA) flow solver is used for simulations of vortex-nozzle interaction. A dedicated numerical simulation study focusing on elementary processes such as vortex-nozzle and entropy spot-nozzle interaction allows a systematic variation of relevant parameters and yields insight which would be difficult by means of limit cycle studies of the full engine. A systematic study of the vortex-nozzle interaction in the case of a choked nozzle has been undertaken. The results are summarized by using a lumped element model for plane wave propagation, which is based on theoretical scaling laws. From EIA simulations it appears that sound due to vortex-nozzle interaction is mainly generated during the approach phase and that for the relevant parameter range there is no impingement of the vortex on the nozzle wall as has been suggested in the literature. Using an energy balance approach, a single fit-parameter model is formulated which qualitatively predicts limit-cycle observations in cold gas-scale experiments reported in the literature. Finally the Euler model is used to compare the sound production by vortex-nozzle interaction with that due to the ingestion of an entropy non-uniformity also called entropy spot. In addition to insight, this study provides a systematic procedure to develop a lumped element model for the sound source due to non-homogeneous flow-nozzle interactions in SRMs. Such lumped models based on experimental data or a limited number of flow simulations can be used to ease the design of SRMs.

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

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