An Experimental Investigation of High Blockage Vortex Shedding for Perturbed Flow

The combined effects of a free stream velocity pulsation of controlled frequencies and a high blockage ratio on the vortex shedding from bluff cylindrical bodies have been studied. This study includes signal quality behind the bluffs, power spectral density, signal to noise ratio, amplitude variation of velocity profile, and Strouhal-Reynolds number correlation. The Reynolds number based on the bluff's characteristic dimension, d is kept in the range 1000 - 6000. Two circular bluffs of dimensions 2 0 and 3 2 : 1 mm and a triangular cylinder of width 1 5 mm were used and positioned vertically in the test section. The effect of moderate frequency amplitude, up to 1 5 Hz, of free stream velocity perturbation on Strouhal number is almost insignificant for low blockage ratios but for higher blockage ratios the Strouhal number dropped by up to 1 0 % around Re approximate to 3000. The velocity amplitude decreased with increasing downstream distance from the bluff bodies. The blockage ratio did not have a significant effect on the SNR as the peak value remained in the range 4 0 - 5 0 d B. However, the Strouhal number increased with increasing tunnel blockage ratio.