Experimental Investigation of Vortex-Induced Vibrations on Yawed and Inclined Flexible Cylinders

Abstract

\n An experimental setup was built to investigate the vortex-induced vibration (VIV) phenomenon on yawed and inclined flexible cylinders, in which five yaw angles θ = 0 deg, 10 deg, 20 deg, 30 deg, and 45 deg and five azimuth angles β = 0 deg, 45 deg, 90 deg, 135 deg, and 180 deg were combined. The experiments were carried out in a towing tank facility at Reynolds numbers from 1800 to 18,000, comprising vibrations up to the eighth natural mode. Time histories of displacements were recorded using a submerged optical system that tracks 17 reflective targets. A modal decomposition scheme based on Galerkin’s method was applied, aiming multimodal behavior investigations. Such an approach allowed the analysis of the modal amplitude throughout time, revealing interesting results for such a class of VIV tests. The flexible cylinder total response is generally a combination of two or more modes. Only for azimuths 0 deg, 90 deg, and 180 deg, a unimodal response was observed for the two first lock-in regimes. The frequency response showed that when the response was multimodal, non-dominant modes can follow the vibration frequency of the dominant one. Assuming a priori the independence principle (IP) valid to define the reduced velocities (Vr), it was observed that the resonance region was restricted to 3 ≤ Vr ≤ ~8 for the tested cases, indicating that the IP can be at least partially applied for flexible structures. As the literature scarcely explores the simultaneous yawed and inclined configurations, the present work may contribute to further code validation and improvements regarding the design of slender offshore structures.