Vortex formation in the starting flow of rotating, low-aspect-ratio plates
DeVoria, Adam C.
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The three-dimensional vortex formation of a low-aspect-ratio ( AR =1.7), trapezoidal flat plate performing low Reynolds number (order 10 3 ) pure rotation from rest at a 90° angle of attack and is experimentally investigated. The objectives are to characterize the time-dependent, three-dimensional vortex structure as the plate motion progresses, to ascertain the ramifications of the significant root-to-tip flow on the vortex formation, and to determine the presence of and subsequently investigate any evidence of vortex saturation ("formation parameter") effects. The experiments are conducted in a glass water tank facility, and the diagnostics tools used are dye flow visualization and planar digital particle image velocimetry (DPIV). The vortex structures created by the low-aspect-ratio plate are symmetric ring-like vortices comprised mainly of vorticity generated at the tip edge of the plate. For large rotation amplitudes the dye flow visualization shows the shedding of this ring-like vortex end even the generation of a secondary ring-like vortex, all while the plate is still in motion, indicating the saturation of the primary vortex. The time-varying vortex circulation, calculated from the DPIV measurements in the symmetry plane of the flow, provides quantitative evidence of vortex saturation. It was found that the temporal development of the vortex circulation is complex, which obscures the objective determination of physically relevant saturation times. This difficulty is a direct result of an interaction between the evolving formation of the final ring-like vortex via Kelvin-Helmholtz-like instabilities that are inherent in the separated shear layers emanating from the free edges of the plate, and the strong root-to-tip flow, which is induced by the tip-edge- and side-edge-vortices.