Laboratory measurements of vortex-induced vibrations of a yawed flexible cylinder at different yaw angles

Laboratory tests were carried out to characterize the multi-mode vortex-induced vibrations (VIV) of a yawed flexible cylinder in a uniform flow. Five yaw angles in relation to the direction orthogonal to the oncoming flow were adopted, i.e. a = 0°, 15°, 30°, 45° and 60° in this paper. In order to simulate the uniform flow, the flexible cylinder model with a mass ratio of 1.90 and an aspect ratio of 350 was towed by a carriage with the towing velocity varying from 0.05 m/s to 1.0 m/s in a towing tank. The corresponding Reynolds numbers approximately fell in the range of 800-16000. The effects of yaw angle on the multi-mode response of the yawed flexible cylinder subjected to VIV were investigated and discussed based on the experimental results. It can be found that the maximum values of response amplitudes in both cross-flow (CF) and in-line (IL) directions show a slight upward trend when the yaw angle increases from 0° to 60°. The peak value of Strouhal number of the yawed cylinder is about 5% larger than that of the normal-incidence one (a = 0°). The multi-mode VIV response occurs even at a yaw angle as high as 60°. In addition, particular attention was paid to the validity of the independence principle (IP) which assumes that the yawed and normal-incidence cylinder cases are comparable if only the oncoming flow velocity normal component is applied to scale the physical quantities. The experimental results indicate that the IP seems to be a reasonable hypothesis for examining the multi-mode VIV of a yawed flexible cylinder for a⩽30°, and some discrepancies gradually appear as the yaw angle increases from 45° to 60°.