Passive Control of Flow-Induced Vibration (FIV) by Helical Strakes for Two Staggered Flexible Cylinders

Helical strake is a widely-used device for passive flow-induced vibration (FIV) control of cylindrical structures. It is omnidirectional and can effectively reduce FIV response amplitude. Studies on the passive FIV control for cylindrical structures are mainly concerned with a single isolated cylinder, while the influence of wake interference between multiple cylinders on FIV suppression devices is less considered up to now. In engineering applications, multiple flexible cylinders with large aspect ratios can be subjected to complex flow forces, and the effects of wake interference are obvious. The FIV suppression effect of helical strake of a common configuration (17.5D pitch and 0.25D height, where D is the cylinder diameter) in two staggered cylinders system is still unknown. This paper systematically studied the FIV response of multiple cylinders system fitted with the helical strakes by model tests. The relative spatial position of the two cylinders is fixed at S = 3.0D and T = 8.0D, which ensures the cylindrical structures in the flow interference region. The experimental results show that the helical strakes effectively reduce the FIV response on staggered upstream cylinder, and the suppression efficiency is barely affected by the smooth or straked downstream cylinder. The corresponding FIV suppression efficiency on the downstream cylinder is remarkably reduced by the influence of the upstream wake flow. The wake-induced vibration (WIV) phenomenon is not observed on the staggered downstream cylinder, which normally occurs on the downstream straked cylinder in a tandem arrangement.