Performance of two- and three-start helical strakes in suppressing the vortex-induced vibration of a low mass ratio flexible cylinder

Experiments have been conducted on a low mass ratio flexible cylinder with two- and three-start helical strakes, which was free to vibrate in both in-line and transverse water flow directions under the subcritical Reynolds number. The studied parameters included the cross-flow amplitude response, the frequency response in the in-line and cross-flow directions and the mean drag coefficient. The effects of different helical strakes heights (h = 0.05 D, 0.10 D and 0.15 D) and pitches (p = 5 D, 10 D, 15 D) were investigated for five different configurations in two- and three-start helical strakes, respectively. The present study has indicated that the strakes pitch changes the occurrence of the lock-in region, but only the three-start p = 10D model can suppress the high mode frequency. Strakes height is one of the important variables in reducing the amplitude responses of a cylinder while the number of helix is the most significant variable in affecting the performance of helical strakes. The less effectiveness of two-start helical strakes is due to the strakes coverage that is inadequate to disrupt the regular vortices efficiently. The most effective helical strake configuration based on the configurations that are examined in the present study is the three-start p = 10 D h = 0.15 D model.