Flow-induced vibration of flexible cylinders covered by fixed fairings with different chord-thickness ratios

Abstract

Fairings are important vortex-induced vibration (VIV) suppression devices in the offshore and marine engineering applications. For circular cylinders in the marine structures, both their mean deflection in the in-line direction and the oscillation amplitude in the cross-flow direction can be effectively reduced when the fairings work properly. However, plenty recent investigations report that when the rotational elements of fairings fail to function, the VIV suppression functions may be deprived and severe galloping oscillations may occur. However, very few studies discussed the galloping oscillation mechanism for slender flexible cylinders. We perform a numerical study on the fluid–structure interaction of flexible cylinders covered by fixed fairings with different chord-thickness ratios (c/t) using a self-developed fluid–structure interaction simulation code. The Reynolds number is 1.68×104, which is based on the incoming velocity and the diameter of the cylinder. It is found that large amplitude, low frequency and low order galloping responses are prominent in the cases where the flexible cylinder is covered by fairings with large c/t. The large added mass and the low fluid force oscillation frequency in galloping are found to induce the low vibration frequency and the low order ‘lock-in’ modes in flexible cylinder responses.