Rectangular Cylinders in Flows with Harmonic Perturbations

The effects of small amplitude harmonic perturbations on the pressure coefficients and wake development behind rectangular cylinders are examined. Two body configurations with slenderness ratios (streamwise body dimension/transverse body dimension) of $B/D=0.5$ and 2.0 are used. For the $B/D=0.5$ body, with the pulsation frequency equal to twice or four times the natural shedding frequency, and with amplitudes of the order of 1% of the freestream velocity, the mean base pressure shows a decrease of up to 18% when compared with its value in steady flow. Flow visualization shows this decrease in mean base pressure to be associated with increased vortex strength, decreased vortex formation length, and the corresponding increase in shear layer curvature. When the perturbation amplitude exceeds an undetermined threshold value at a frequency corresponding to four times the natural shedding frequency, two vortices are shed simultaneously and symmetrically with respect to the body centerline. For the $B/D=2.0$ body, the primary effect of the pulsations is to enhance the orderly structure of the separated shear layers.