The form and dynamics of ocean turbulence are critical to all marine processes; biological, chemical and physical. The three-dimensional turbulence characteristics of the bottom boundary layer of the coastal ocean are examined using a series of 29,991 instantaneous velocity distributions. These data, recorded by a submersible 3D-PTV system at an elevation of 0.64 m above the seabed, represent conditions typical of moderate tidal flows in the coastal ocean. A complexity associated with submersible 3D-PTV in the coastal ocean is that gaps and noise affect the accuracy of the data collected. To accommodate this, a new Physics-Enabled Flow Restoration Algorithm has been tested for the restoration of gappy and noisy velocity measurements where a standard PTV or PIV laboratory set-up (e.g. concentration / size of the particles tracked) is not possible and the boundary and initial conditions are not known a priori. This is able to restore the physical structure of the flow from gappy and noisy data, in accordance with its hydrodynamical basis. In addition to the restoration of the velocity flow field, PEFRA also estimates the maximum possible deviation of the output from the true flow. 3D-PTV measurements show coherent structures, with the hairpin-like vortices highlighted in laboratory measurements and numerical modelling, were frequently present within the logarithmic layer. These exhibit a modal alignment of 8 degrees from the mean flow and a modal elevation of 27 degrees from the seabed, with a mean period of occurrence of 4.3 sec. These appear to straddle sections of zero-mean along-stream velocity, consistent with an interpretation as packets. From these measurements, it is clear that data collected through both laboratory and numerical experiments are directly applicable to geophysical scales – a finding that will enable the fine-scale details of particle transport and pollutant dispersion to be studied in future. Conditional sampling of the Reynolds shear stress (without using Taylor’s hypothesis) reveals that these coherent structures are responsible for the vertical exchange of momentum and, as such, are the key areas where energy is extracted from the mean flow and into turbulence. The present study offers the first assessment of the magnitude of the errors associated with assuming isotropy on shear-based sensors of the TKE dissipation rate and its consequential effect on the Kolmogorov microscale using 3D-PTV data from the bottom boundary layer of the coastal ocean. The results indicate a high degree of spatial variability associated with the low conditions. The averaged data supports the validity of measurements obtained by horizontal and vertical profilers, however along-stream velocity derivatives underestimate the TKE dissipation rate by more than 40% – a factor of two higher than for the equivalent cross-stream and vertical estimates. This has important implications for the deployment of these sensors and the subsequent interpretation of higher-order statistics. Finally, the data have been processed to test four popular sub-grid scale (SGS) stress models and SGS dissipation rate estimates for Large-Eddy Simulations using these in situ experimental data. When the correlation and SGS model coefficients are assessed, the nonlinear model represents the best stress models to use for the present data, consistent with the substantial anisotropy and inhomogeneity associated with these flows. The detailed measurement and analysis of coherent structures in the coastal ocean undertaken therefore supports the development of numerical models and assists with the understanding of all marine processes.

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