Suppression of vortex-induced vibration of a cactus-inspired cylinder near a free surface

Abstract

Bionic ribs inspired by those of a cactus are adopted in order to suppress the vortex-induced vibration (VIV) responses of cylinders located near free surfaces. The VIV responses of cactus-inspired cylinders located near free surface are investigated numerically. The range of the Reynolds number is 8.0 × 103 ≤ Re ≤ 5.6 × 104, and the range of the Froude number is 0.08 ≤ Fr ≤ 0.56. The distance between the cylinder and the free surface is 2D (D is diameter of the cylinder), and the cactus-inspired ribs with height ratios (Ks/D, where Ks is the height of the ribs) include 0.0 (M1), 0.025 (M2), 0.05 (M3), and 0.1 (M4). The VIV response of a smooth cylinder (M0) far from a free surface is used as a reference. Different ribs with height ratios have different effects on the VIV responses of cylinders located near free surfaces. The suppression effect of the bionic ribs on the VIV response is largest when Ks/D = 0.1 (M4). The maximum cross-flow amplitude ratio, the maximum in-line amplitude ratio, and the maximum mean drag coefficient of the cactus-inspired cylinder (M4) are 53.57%, 53.13%, and 49.21% lower than that of the smooth cylinder (M1). The cactus-inspired cylinders are easier to desynchronize from the lock-in region. The free surface causes the vortex phase transition to happen earlier. The free surface has a better suppression effect on VIV response in the high Froude number range. When the rib height ratio is small (M2, Ks/D = 0.025), the free surface enhances the VIV response. However, the VIV response is mainly affected by the ribs when the rib height ratio exceeds a certain value (Ks/D ≥ 0.05). When the cactus-inspired rib with height ratio increases, the critical Reynolds number of the vortex phase transition decreases. The free surface and the cactus-inspired ribs are conducive to dissipation of wakes and lead to decreases in the wake turbulence intensity. Because the cactus-inspired ribs can cause the separation line to develop away from the cylinder wall and effectively control the wake intensity, the VIV response of the cactus-inspired cylinder is suppressed effectively. For the cactus-inspired cylinder, galloping responses can be avoided in high-Reynolds-number range, and boundary-layer separation occurs at the top of the ribs. Unlike the boundary-layer separation process of the smooth cylinder (M1), the ribs of the cactus-inspired cylinder destroy the boundary layer, causing smaller vortices to be produced.