Flow Control Around a Finite Square Cylinder by Partial-Span Suction at the Side Leading Edge

This study uses numerical simulation to investigate active flow control on a finite square cylinder, examining the effect of partial-span suction at the side leading edge on the aerodynamic forces and wake structure of the square cylinder at a Reynolds number of 250. The objective is to clarify how the spanwise distribution of suction interacts with the three-dimensional wake dynamics induced by the finite-span geometry. The results show that partial-span suction can effectively reduce the aerodynamic forces acting on the square cylinder and can outperform full-span suction. The best drag control effect occurs at a suction ratio of Γ = 1 (where Γ is the absolute value of the suction velocity divided by the freestream velocity), with a drag coefficient reduction of nearly 20%. The strongest suppression of fluctuating lift occurs at Γ = 0.75, with a reduction of over 85%, which is superior to the full-span suction. Suction forms three vortices near the square cylinder ends, reversing streamwise friction drag from negative to positive. The improved performance of the partial-span arrangement is associated with a different reorganization of the three-dimensional wake, including the downwash-upwash interaction, the side-pressure fluctuation distribution, and the friction-drag response. Proper orthogonal decomposition and principal correlation decomposition analyses indicate that fluctuating lift is mainly related to a few modes representing small-scale flow structures. The essence of the fluctuating lift reduction due to suction is the disruption of small-scale flow structures in the wake.