Tomographic PIV Investigation of the Cantilevered Circular Cylinder Wake with Varying Aspect Ratio and Boundary Layer Thickness
Creators: name: Crane, Robert
Publisher: name: Graduate Studies
Year: 2018
Subjects: subject: Vortex Shedding
subject: proper orthogonal decomposition
subject: coherent structure
subject: bluff body
subject: Engineering
subject: Engineering--Aerospace
subject: Engineering--Chemical
subject: Engineering--Environmental
subject: Engineering--Industrial
subject: Engineering--Mechanical
subject: Psychology--Experimental

description: This thesis investigates the wake of a finite wall-mounted circular cylinder of diameter D and height H for aspect ratios (H/D) of 3 &lt; H/D &lt; 7 and boundary layer thickness to diameter (delta/D) ratios, 0.98 &lt; delta/D &lt; 1.25 using Tomographic Particle Image Velocimetry. The Reynolds number based on cylinder diameter is held fixed at Re = 750. The primary focus of the study is to characterize the main topological features of the mean field, and elucidate the wake dynamics leading to the mean topology. The results show that for all cases investigated, the mean field topology contains an arch vortex in the near wake and a quadrupole structure in the far wake characterized by a pair of oppositely signed streamwise vorticity concentrations emanating from both the tip and base. The availability of 3D-3C velocity field data facilitates the first known experimental analysis of the time averaged vorticity transport equation and the streamwise evolution of the full vorticity field. The vorticity transport analysis suggests that streamwise vorticity arises due to a 3-dimensional separation and subsequent re-orientation process which occurs inside the arch vortex. The process involves the tilting of y-vorticity and z-vorticity into the streamwise direction, and streamwise amplification via stretching. Proper Orthogonal Decomposition was applied to the 3D velocity field measurements to classify the wake dynamics associated with the mean field topology and to perform a low order reconstruction. For all values of H/D and delta/D studied, the time averaged field and low order model is characterized by a quadrupole and shed full-loops respectively, in agreement with that observed for the square cylinder. Decreasing H/D resulted in: (i) a reduction in circulation of both the tip and base vortices, (ii) a reduction in circulation of the shed full loop structures, (iii) increased frequency modulation of the shed structures resulting in a broadening of the Strouhal peak, (iv) increased modulation in circulation of the full-loop structures across a sampling of shedding cycles, and (v) a reduction in the relative energy content of the first modal pair associated with the fundamental harmonic of vortex shedding. Decreasing delta/D resulted in a reduction in circulation for the base vortices and an increase in circulation for the tip vortices. In the low order model, decreasing delta/D resulted in the arch vortex developing less curvature at the base which occurred over a smaller portion of the cylinder span before the core of the structure was aligned with the cylinder axis. No critical aspect ratio H/D_crit corresponding to the onset of symmetric arch-vortex shedding was identified across the parameter space of the study.