Numerical investigation on vortex-induced vibration suppression of the cactus-inspired cylinder with some ribs

Vortex-induced vibration (VIV) responses of the cactus-inspired cylinder with some ribs are numerically investigated, and the range of Reynolds number is 8.0 × 103 < Re < 5.6 × 104. The motion equations of VIV response are solved by the Newmark-beta method. The effects of different numbers of ribs (N, N is the number of the ribs) on VIV suppression are investigated in detailed, and six different numbers of ribs (N) include S0 (N = 0), S1 (N = 8), S2 (N = 12), S3 (N = 16), S4 (N = 20) and S5 (N = 24). The peak value of the cross-flow amplitude ratio decreases from 1.50 (S0) to 0.65 (S2), which is reduced by about 56.67%. The peak value of the in-line amplitude ratio decreases from 0.35 (S0) to 0.12 (S2, S3, S4, and S5), which is reduced by about 65.71%. The peak value of the mean drag coefficient decreases from 3.00 (S0) to 2.1 (S2), reduced by about 30.0%. When N = 8, VIV response is enhanced and it can contribute to trigger the large-amplitude or even divergent structural motion, such as galloping. When N ≥ 12, the VIV responses of S2, S3, S4, and S5 are all suppressed. When N ≥ 12, the range of the lock-in region is wider and wider as the number of ribs increase, and the range of lock-in region of S5 (N = 24) is the similar to that of S0 (N = 0). For the cactus-inspired cylinder, the detached boundary-layer is usually formed between two ribs and the smaller vortices are formed between the ribs. And the ribs of the cactus-inspired cylinder destroy the normal boundary-layer separation and reduce the strength of the wake vortex for S2, S3, S4 and S5. However, the reattachment of the boundary layer occurs after the first separation of the boundary layer for S1, which may cause unstable VIV response. Especially, galloping phenomenon may occur in the high Reynolds number for S1. Compared with the separation line of the smooth cylinder (S0), the separation line of the cactus cylinder (S5) obviously develops away from the cylinder and the coupling process of the wake vortex is suppressed.