Turbulent Flow Visualization over Surface-mounted Finite-height Cylinders and Square Prisms
The study of turbulent flows over surface-mounted, finite-height bluff bodies like cylinders and square prisms have various engineering and industrial applications. The flow field around and in the wake of these bodies is sufficiently complex due to the interactions of the ground plane boundary layer with the separating shear layers from the free end and the sides of these bodies. As such straightforward geometries produce large, complex wakes, there has been increased interest in the literature in the past few decades on examining this flow field experimentally. Recent advancements in computational power have also facilitated the numerical calculation of these flows at higher spatial and temporal resolutions, which were otherwise inestimable by experiments alone. However, the instantaneous flow field in the wake, and specifically above the free end, which is a major contributor to the three-dimensionality of this flow field, has not been well understood. Beyond numerical simulations or experiments, a modern challenge in turbulence research has been to identify and classify the instantaneous energetic structures ensconced in a turbulent flow field. Velocity gradient methods like the swirling strength criterion and the Q-criterion have successfully tendered a mathematical definition to isolate vortex structures embedded in turbulent flow. Another promising approach called proper orthogonal decomposition (POD) has also become popular in the past couple of decades, as it provides for a low-dimensional approximation of a high-dimensional turbulent flow field. Thus, the aim of this thesis is to pursue fundamental studies of the flow topologies above the free end of a surface-mounted cylinder and square prism, as well as in the wake of a surface-mounted square prism, to obtain insightful representations of these flow fields using enhanced post-processing methodologies like the swirling strength criterion, Q-criterion, and POD. The first manuscript (presented as Chapter 2) investigated the flow field obtained from Particle Image Velocimetry (PIV) above the free end of a surface-mounted finite square prism in the vertical symmetry plane at a Reynolds number of Re = 4.2x10^4 for four different aspect ratios AR = 9, 7, 5 and 3. The POD methodology was able to capture the energetic flow features within a small number of energy modes. A qualitative analysis of the energy modes revealed a pair of shear sub-layers from the free end, as well as several vortex structures within these layers, likely evidence of Kelvin-Helmholtz (KH) instabilities. The swirling strength criterion was also used to find other structural features in the near-wake above and behind the free end, and the changes in the flow topologies with aspect ratio were demonstrated. The second manuscript (presented in Chapter 3) investigated the flow field obtained from PIV above a surface-mounted finite cylinder in several horizontal planes close to the free end and parallel to it, at a Reynolds number of Re = 4.2x10^4 for four different aspect ratios AR = 9, 7, 5 and 3. The POD methodology was able to capture the energetic flow features within a small number of energy modes. A qualitative analysis of the energy modes revealed pairs of symmetric, side-tip counter-rotating vortices, a pair of counter-rotating vortices on each side of the midline, as well as evidence of alternate undulation of flow on either side of the midline. Flow topologies were shown to vary with aspect ratio, with a general trend of a vertical compression of the wake at higher aspect ratios due to a stronger entrainment of flow from the outer freestream velocity and a weakening influence of the ground plane. The third manuscript (presented in Chapter 4) investigated the time-averaged three-dimensional flow field obtained from Large Eddy Simulation (LES) around and in the wake of a surface-mounted finite square prism at a Reynolds number of Re = 500 for aspect ratio AR = 3. The dominant flow features include the horseshoe vortex enclosing the base junction, and a pair of counter-rotating tip vortices emerging from the sides of the prism close to the free end. The tip vortices were shown to descend towards the ground plane in the wake owing to downwash effects. Other flow features using streamlines and vorticity were also demonstrated, including the mean recirculation zone (Bt vortex) and the base junction vortex (Nw vortex) in the vertical symmetry plane, a pair of symmetric vortices in the horizontal planes, as well as several separating shear layers and foci around the square prism surfaces. The fourth manuscript (presented in Chapter 5) performed a 3D POD analysis on the instantaneous flow field obtained from Large Eddy Simulation (LES) in the wake of a surface-mounted finite-height square prism at a Reynolds number of Re = 500 for aspect ratio AR = 3. The energetic POD energy modes revealed several dominant streamwise vortex tubes. A bifurcation of a streamwise vortex strand across the vertical symmetry plane was also observed, manifesting evidence of an alternating, upward-inclined streamwise connector strand traversing downstream and culminating in a vortex core. The POD temporal coefficients revealed strong periodicity and were used to obtain the phase information for this flow field. Accordingly, two lower order reconstructions opposite in phase were corroborated with the instantaneous flow field, further substantiating the alternating half-loop structure traversing downstream.
turbulence, proper orthogonal decomposition, square prism, cylinder
Doctor of Philosophy (Ph.D.)