Experimental investigation of flow past a confined bluff body: Effects of body shape, blockage ratio and Reynolds number

This paper presents a Particle Image Velocimetry (PIV) study of the two-dimensional flow past a bluff body confined in a channel. Three typical shapes of bluff body with the same height, namely, circular cylinder (CC), flat plate (FP), and square cylinder (SC), but with varying blockage ratio (beta = 0.25, 0.3, 0.4 and 0.5) and Reynolds number (Re = 3200, 4600 and 5400) are investigated. It is shown that for each body shape, vortex shedding patterns are relatively insensitive to Reynolds number (most notably for the case of FP), but strongly depend on the blockage ratio. As beta increases, the mean recirculation length of the wake is initially constant and then displays a sharp increase at about beta >= 0.4-0.5, which corresponds to the flow transition from periodic vortex shedding state to steady state. The strength of vortex shedding generally decreases with the increase in beta except for CC at beta = 0.25-0.3. On the other hand, the vortex shedding frequency expressed in terms of dimensionless Stmuhal number (St) increases monotonically with blockage ratio until vortex shedding is suppressed. The critical blockage ratio for the cessation of vortex shedding, beta(Cri), follows the order of beta(Cri_FP) < beta(Cri_SC) < beta(Cri_CC).