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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.

A series of experimental tests of passive VIV suppression of an inclined flexible cylinder with round-sectioned helical strakes were carried out in a towing tank. During the tests, the cylinder models fitted with and without helical strakes were towed along the tank. The towing velocity ranged from 0.05 to 1.0 m/s with an interval of 0.05 m/s. Four different yaw angles (a=0°, 15°, 30° and 45°), defined as the angle between the axis of the cylinder and the plane orthogonal of the oncoming flow, were selected in the experiment. The main purpose of present experimental work is to further investigate the VIV suppression effectiveness of round-sectioned helical strakes on the inclined flexible cylinder. The VIV responses of the smooth cylinder and the cylinder with square-sectioned strakes under the same experimental condition were also presented for comparison. The experimental results indicated that the round-Sectioned strake basically had a similar effect on VIV suppression compared with the square-sectioned one, and both can significantly reduce the VIV of the vertical cylinder which corresponded to the case of a=0°. But with the increase of yaw angle, the VIV suppression effectiveness of both round- and square-section strakes deteriorated dramatically, the staked cylinder even had a much stronger vibration than the smooth one did in the in-line (IL) direction.