Control of the aerodynamic forces of a finite-length square cylinder with steady slot suction at its free end

A steady slot suction near the free-end leading edge of a finite-length square cylinder was used to control its aerodynamic forces. The aspect ratio (H/d, where H and d are the height and width of the cylinder, respectively) of the tested cylinder was 5. The tested suction ratio Q, which was defined as the ratio of the suction velocity at the slot (U-s) to U-infinity, ranged from 0 to 4. It was found that the overall aerodynamic forces reduce quickly with the increase of Q from 0 to 1, then recovers slightly with Q from 1 to 2, and keep approximately constant with Q >= 2. The maximum reduction of the overall mean drag, fluctuating drag and fluctuating lift occurs at Q = 1, which reaches 3.6%, 17.8% and 45.5%, respectively. The steady slot suction reduces the aerodynamic forces not only near the free end, but also over the entire cylinder span. At Q = 1, the shear flow emanating from the free-end leading edge reattaches on the free end forming a recirculation bubble. The enhanced momentum transport between the free-end shear flow and the wake suppresses the spanwise vortex shedding and aerodynamic forces the most effectively.