Control of the aerodynamic forces and the near wake of a 3D wall-mounted square cylinder using pulsing slot suction at its free end

To control the aerodynamic forces and near wake of a 3D wall-mounted square cylinder with an aspect ratio H/d = 5, pulsing slot suction is employed at its free-end leading edge. The present experiment is investigated in a wind tunnel with Reynolds number Re = 2.74 x 10(4). The pulsing suction ratio f*, defined as the ratio of the pulsing suction frequency f(s) to the vortex shedding frequency f(v), ranges from 0 to 1.6, corresponding to a momentum coefficient C-mu range of 0-0.03. The pulsing slot suction can greatly reduce the overall fluctuating drag C-d ' and fluctuating lift C-l' of the cylinder. Aerodynamic suppression effect enhances with increasing f*, and becomes stable for f* >= 0.6 (C-mu >= 0.014). At f* = 0.6, the overall (C-d) over bar, C-d ' and C-l' are reduced by 2.7%, 22.2% and 50.1%, respectively. The pulsing suction control causes periodic reattachment of the free-end shear flow to the cylinder's free end, forming large-scale vortex structures downstream of the cylinder, and enhancing the momentum exchange between wake and free flow. On the other hand, although the strength of spanwise vortex shedding is obviously weakened, its frequency remains unchanged at all tested f*. Results from particle image velocimetry (PIV) show that the turbulent kinetic energy in the cylinder wake is significantly reduced. Analyses using the proper orthogonal decomposition (POD) demonstrate that the periodicity of near-wake vortex structure is significantly suppressed.