Experimental investigation of flow around a circular cylinder with attached membranes

We study experimentally the reduction of mean force and suppression of force fluctuation on a circular cylinder by attaching flexible membranes to the surface of the cylinder. The experiments were conducted in a wind tunnel. The configuration of a single membrane with a length of two cylinder diameters, i.e. L/D = 2, is found to be optimal for mean force reduction. At the Reynolds number Re = 1.07 x 10(4), with a wide range of attaching angles, 0 <= theta <= 60 and 135 <= theta <= 180, the mean forces are found to be reduced, demonstrating the control effect adaptive to the change of flow directions, would be valuable in real applications. The optimal configuration for suppression of force fluctuation is that with two short membranes 180 apart, which we believe can potentially reduce the vortex induced vibration (VIV). For two short membranes, i.e. L/D = 0.5, the magnitudes of force fluctuation are found to be reduced at almost all attaching angles, with both Reynolds numbers currently studied. We have also investigated the flow wakes by using smoke streaklines and particle image velocimetry (PIV) technique. To understand the physical mechanisms for suppression of force fluctuation with two membranes, we highlight a specific attaching angle, theta = 90. A strong correlation between the flow wake structure and the suppressing effect is revealed. The wake periodicity is observed to be completely inhibited, by this symmetrically configured two membranes, compared to that with a single one, resulting in a remarkable suppression of force fluctuation. (c) 2022 Elsevier Ltd. All rights reserved.