Effects of flexible films on the aerodynamics of a 5:1 rectangular cylinder

Abstract

The present study demonstrates the aerodynamics of a 5:1 rectangular cylinder with flexible films perpendicularly mounted on its upper surface at both the upstream and downstream ends in wind angle of attack (AOA) up to 20°. Flow and pressure fields around the cylinder are examined by smoke-wire visualizations and pressure measurements in a wind tunnel, respectively. Experimental results show that periodic flapping-induced vortices (FIVs) chiefly caused by the leading-edge flapping film play important roles in the cylinder flow and pressure field properties. The FIVs have a positive effect on the reattachment of the cylinder leading-edge shear layer. For the leading-edge film, the tip of the flapping film would touch down on the cylinder upper surface due to the large amplitude flapping of the film at fD/U∞ ≈ 0.811 (f is the film flapping frequency, D is the model characterized length, and U∞ is the free oncoming flow velocity). Meanwhile, the periodic FIVs dominate the cylinder flow and aerodynamic properties in the range of AoA = [0°, 8°), thus defined as the wall-flapped mode. With an increase in AoA = [8°, 20°], the tip of the flapping leading-edge film would not touch down owing to the small amplitude flapping of the film. Simultaneously, the periodic FIVs have limited effects on the cylinder flow and aerodynamic properties, thus named as the free-flapping mode. Compared to the without flexible films case, the scale of the FIVs is smaller than that of the impinging leading-edge vortices but in a higher frequency, resulting in a quick dissipation of the cylinder impinging shear layer instability, rapid recovery of the pressure on the cylinder upper surface, and reductions in the cylinder drag coefficient, lift coefficient, moment coefficient, and spanwise correlation. For the trailing-edge film, it flaps at fD/U∞ ≈ 0.650 and has a limited effect on the cylinder flow and aerodynamic properties. Finally, the flexible films can significantly mitigate the static stall of the rectangular cylinder, which may stabilize the elongated bluff bodies.