The goal of the present work is a detailed and comprehensive study to assess the accuracy of the numerical simulation of the mixing processes in a lobed mixer flow field via a Reynolds-averaged solution method. To meet this goal, the first objective of the current work was to establish the suitability of various meshing strategies that would allow the complex mixer geometries found in current gas-turbine engine designs to be captured, together with the associated convoluted shear layers. A second objective was targeted at providing further insight and understanding of the capability of eddy-viscosity-based turbulence models in capturing the convoluted shear layers. Simplified mixer configurations selected from the literature were studied under incompressible isothermal flow conditions. Two solution algorithms were employed to model the mixer flow fields. The first consisted of a pressure-based structured grid methodology developed for incompressible flows. A density-based mixed-unstructured grid algorithm for compressible flows was also used, with extensions to low Mach number flows made possible through a low Mach number preconditioner. The effects of turbulence were modelled using ak-e turbulence model. The absence of this model in the code made available for the unstructured algorithm necessitated its implementation as a first step in the current work. The effects of unstructured mesh type on the prediction of flows with internal mixing layers were first assessed for an incompressible planar mixing layer. This simplified case was used as a benchmark case to help understand the effects on the convoluted shear layers arising within the lobed mixer flows. To quantify the capability of a Reynolds-averaged approach in simulating the turbulent mixer flow field, two variants of the two equation k-e model were employed. The first constituted the standard linear high Reynolds number k-e model of Launder and Spalding [62]. The second model was a quadratic non-linear version developed by Speziale [99] for the prediction of secondary flows in non-circular ducts. The relative merits of these two models was assessed through detailed comparisons with experimental data taken from the literature. Of particular importance in the mixer flow was the formation and subsequent evolution of the vorticity field. Consequently, this motivated a detailed study of the evolving vorticity field. The investigations thus far were based on a simplified mixer configuration with no temperature differences between the two streams. Therefore, as a final step, a realistic scarfed mixer was modelled in an attempt to model the temperature mixing. The main contribution of the present work is the assessment of a grid-based Reynolds-averaged solution procedure for the prediction of lobed mixer flows. The study revealed that capturing the initial mixing region proved to be most difficult. Firstly, unstruc-tured meshes employing non-hexahedral elements were very inefficient at simulating the mixing layer in the early stages. Secondly, the initial mixing region presented significant difficulties for the Reynolds-averaged solution method in which neither turbulence model was capable of correctly reproducing the turbulence field. Despite this, global parameters such as momentum thickness and streamwise circulation were well captured in the predictions.

No description available.

Noise induced hearing loss is a large concern for the Department of Defense. Personnel on aircraft carriers are exposed to dangerous noise levels of noise from tactical aircraft, causing hearing damage which results in significant costs for medical care and treatment. Additionally, NASA and the FAA have begun to investigate the viability of reintroducing supersonic commercial transport in the United States and one of the largest problems to address is reducing the noise impact of these aircraft on communities. The overarching goal of jet noise research is to optimize noise reduction techniques for supersonic jets. In order to achieve this, a more complete theoretical framework which links the jet boundary conditions to the turbulence production in the jet plume and the far-field radiated noise must be established. The research presented herein was conducted on the hypothesis that introducing thermal non-uniformities into a heated supersonic jet flow can favorably alter the turbulence structure in the jet shear layer, leading to reductions in radiated noise. To investigate the impact of temperature on the turbulence development in the jet, spatially resolved three-component velocity vectors were acquired using particle image velocimetry (PIV) performed on two small-scale perfectly expanded Mach 1.5 jet flows, one with a uniform temperature profile and another containing a geometrically offset temperature non-uniformity. Using the PIV data, the mean velocities, Reynolds stresses, and correlation coefficients were obtained from both jet flows and compared to analyze changes in the mean turbulence field. Small but significant reductions in the shear layer turbulence were observed in the near nozzle region of the thermally offset jet when compared to the uniform jet case. The changes result in a thickening of the shear layer nearest the location of the cold plume which alters the integral length scales of the coherent turbulent structures in the offset jet in a manner consistent with other techniques presented in the literature that reduce jet noise. Applying quadrant analysis, a conditional averaging technique, to the jet turbulence plume revealed changes in the statistical flow field of Reynolds shear stress structures. The changes provide strong evidence of the presence of intermittent stream-wise vortical structures which serve to reduce the spatial correlation levels of turbulence in the thermally offset jet flow when compared to the uniform baseline jet. / Doctor of Philosophy / Increasingly large and powerful engines are required as the mission requirements for tactical aircraft become more advanced. These demands come at the cost of an increased production of noise which is particularly hazardous to crewpersons operating on Navy aircraft carriers during take-off and landing. Noise-induced hearing loss from extended exposure to high noise levels has become a major medical expenditure for the Navy. To address this issue in tactical aircraft engines, the sources of jet plume noise must be reduced, but doing so requires improved understanding of the connections between nozzle boundary conditions, the jet turbulence plume, and the radiated noise while keeping in consideration system constraints and performance requirements. The current study introduces a novel method for controlling supersonic jet noise induced by turbulence through the introduction of an offset non-uniform temperature perturbation at the nozzle mouth. Non-invasive flow measurements were conducted using stereoscopic particle image velocimetry to obtain high-resolution velocity and turbulence data. Analysis of the flow data indicate that an offset reduced temperature plume introduced at the nozzle exit has a first-order effect on the turbulence evolution which result in small, but significant reductions in jet noise levels. The reductions observed are attributed to a disruption in the coherence of the primary noise generating turbulence structures in the jet plume which are associated with the formation of stream-wise vortical structures induced by the cold plume.

Supersonic hot jet noise causes significant hearing impairment to the military workforce and results in substantial cost for medical care and treatment. Detailed insight into the turbulence structure of high-speed jets is central to understanding and controlling jet noise. For this purpose a new instrument based on the Doppler global velocimetry technique has been developed. This instrument is capable of measuring three-component velocity vectors over ex-tended periods of time at mean data-rates of 100 kHz. As a demonstration of the applicability of the time-resolved Doppler global velocimetry (TR-DGV) measurement technique, statistics of three-component velocity measurements, full Reynolds stress tensors and spectra along the stream-wise direction in a cold, supersonic jet at exit Mach number Mj = 1.4 (design Mach number Md = 1.65) are presented. In pursuance of extending the instrument to planar op- eration, a rapid response photomultiplier tube, 64-channel camera is developed. Integrating field programmable gate array-based data acquisition with two-stage amplifiers enables high-speed flow velocimetry at up to 10 MHz. Incor- porating this camera technology into the TR-DGV instrument, an investigation of the perfectly expanded supersonic jet at two total temperature ratios (TTR = 1.6 and TTR = 2.0) was conducted. Fourth-order correlations which have direct impact on the intensity of the acoustic far-field noise as well as convective velocities on the lip line at several stream-wise locations were obtained. Comprehensive maps of the convective velocity and the acoustic Mach number were determined. The spatial and frequency scaling of the eddy convective velocities within the developing shear layer were also investigated. It was found that differences in the radial diffusion of the mean velocity field and the integral eddy convective velocity creates regions of locally high convective Mach numbers after the potential core. This, according to acoustic analogies, leads to high noise radiation efficiency. The spectral scaling of the eddy convec- tive velocity indicates intermittent presence of large-scale turbulence structures, which, coupled with the emergence of Mach wave radiation, may be one of the main driving factors of noise emission observed in heated supersonic jets. / Ph. D.

No description available.

The interaction between a bluff body and the impinging fluid flow, can involve detached boundary layers, massive flow separations, free shear layers, development of recirculation zones and formation of a highly disturbed and complex region downstream of the bluff body, which can be categorized as wake. The present research aims to experimentally investigate such fluid-structure interaction and provide insight into the wake structure of two bluff bodies. To this end, the airwake over the helicopter platform of a Canadian Coast Guard (CCG) polar icebreaker was studied using high-speed particle image velocimetry (PIV). The experiments were conducted on a scaled model of the polar icebreaker situated on a costume-built and computer-controlled turntable, which provided the ability to accurately change the incidence angle of the impinging flow with a given rate of change for incidence angle. Quantitative flow field data were obtained in several vertical and horizontal planes. The obtained velocity field was then used to calculate the time-averaged flow structure and turbulence metrics over the helicopter platform of the vessel. The present work compared the effects of two types of inflow conditions: (i) a uniform flow and (ii) a simulated atmospheric boundary layer (ABL) on the flow structure over the helicopter platform of the ship. Moreover, for the bluff scaled model, the effects of the Reynolds number on the wake structure and the flow patterns were investigated. The incidence angle (α) between the oncoming flow and the orientation of the ship varied between 0° to 330° with the increment of 30°. It was observed that higher maximum values of the turbulence intensity were associated with the simulated ABL. Moreover, it was found that for both inflow conditions, the incidence angle of 300o corresponded to the highest turbulence levels over the helicopter platform. Building on the results obtained for a stationary vessel in the simulated ABL, this work aimed to quantify the effects of the unsteady change in the direction of the impinging wind, simulated by rotating the model at a certain rate, . It was observed that the increase of the rate of change of the inflow direction resulted in an increase of the turbulent intensity over the helicopter platform. However, an exception was observed for the case of α = 60°, where clockwise rotation of the ship model with respect to the inflow exposed the helicopter platform to increased turbulent velocity fluctuations, while counterclockwise rotation diminished the flow unsteadiness over the helicopter platform. Moreover, aiming to identify the origins of the unsteady forces applied on bluff elongated plates with high chord-to thickness ratio (c/t = 23) at zero incidence, direct force measurement as well as PIV were used to identify the effect of transverse perforations on the flow-induced loading on the flow structure in the near-wake of the plates. The experiments were conducted in a water channel, where the plates were located at the center of channel, parallel to the upstream flow direction. Plates with various characteristic diameter of the perforation as well as a reference case without perforations were considered. The spectra of the trailing-edge vortex shedding and flow-induced forces were compared and it was observed that the vortex shedding frequencies were in very good agreement with those of the measured flow-induced forces for all considered perforation patterns. Thus, it was determined that the trailing-edge vortex shedding was the main mechanism of generating the unsteady loading on the plates. The staggered patterns of the perforations created a three-dimensional flow structure at the vicinity of the trailing edge and in the near wake, which was investigated using PIV at several data acquisition planes. It was found that in the cross-sectional planes corresponding to the close proximity of the perforations to the downstream edge, the periodic trailing-edge vortex shedding were suppressed. Furthermore, it was observed that for small perforations, the velocity fluctuations in the near wake were enhanced. However, further increase of the perforation diameter led to suppression of the velocity fluctuations. / Graduate

No description available.

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