Comparison of flow patterns in the near wake of a circular cylinder and a square cylinder placed near a plane wall

The flow patterns in the near wake of a cylinder (either circular or square in shape, D=25 mm) placed in the proximity of a fully developed turbulent boundary layer (thickness δ=0.4 D) are investigated experimentally using particle image velocimetry (PIV). The effects of changing the gap height ( S) between the cylinder bottom and the wall surface, over the gap ratio range S/ D=0.1–1.0, have been investigated. The results show that both the ensemble-averaged and instantaneous flow fields are strongly dependent on S/ D. The flow patterns for the two types of cylinders share many similarities with respect to the change in S/ D, such as the reduced recirculation length and increased velocity fluctuation in the near wake with increasing S/ D, as well as the trend of suppression of vortex shedding at small S/ D and onset of vortex shedding at large S/ D. However, developments of the shear layers, in terms of wake width, flow curvature, etc., are considerably different for these two types of cylinders. In general, the wake development and momentum exchange for the square cylinder are slower those for the circular cylinder at the same gap ratio. Correspondingly, it is shown that the periodic vortex shedding is delayed and weakened in the case of square cylinder, as compared to that of the circular cylinder at the same S/ D.