Critical geometry of oscillating bluff bodies

Measurements are presented of the mean pressures around rectangular and D-section cylinders, with a flat front face normal to the flow, forced to oscillate transversely at an amplitude of 10% of the length of the front face. The ratio of depth (streamwise dimension) to height (cross-stream dimension) of the cross-section ranges from 0.2 to 1.0 for rectangular cylinders and from 0.5 to 1.5 for D-section cylinders. The range of reduced velocities investigated, 3 to 11, includes the vortex-resonance region. When increasing the depth, an oscillating bluff cylinder shows a critical depth where base suction attains a peak. The value of a critical depth is lowered with decreasing reduced velocity. In particular, an extraordinarily low critical depth with a very high base suction is obtained on cylinders oscillating at vortex resonance. For cylinders with depths beyond the critical, a reattachment-type pressure distribution is established on the afterbody due to the shear-layer/edge direct interaction. The shear-layer/edge direct interaction can also occur on oscillating cylinders with a fixed splitter plate. At low reduced velocities, the reattachment-type pressure distributions on cylinders with and without a splitter plate are similar except for the mean level. A remark is made on the critical geometry of bluff bodies under various flow conditions.