Concomitant vortex-induced vibration experiments: a cantilevered flexible cylinder and a rigid cylinder mounted on a leaf-spring apparatus

This paper presents experimental results of vortex-induced vibrations (VIV), concomitantly carried out in water with a flexible cylinder, rigidly fixed, and with a 'rigid' cylinder, mounted on an elastic apparatus. The experiments were run at IPT towing tank facility, in a side-by-side arrangement. The flexible cylinder is simply fixed at the upper end. For the flexible cylinder, two degrees of freedom (2DOF) are implied for each vibration mode: crosswise and aligned with respect to the incident flow. The elastic support to which the 'rigid' cylinder is mounted is made of two vertical leaf-springs, fixed to two thick horizontal plates, conferring to the cylinder a single degree of freedom (SDOF) to oscillate transversally with respect to the incident flow. The mass ratios of the cylinders are almost the same, around 1.2 and 1.4, respectively, very low values, typical of long ocean pipe structures, as risers and pipelines. The structural damping ratio is also typically low and such as to guarantee high-amplitude responses. Besides usual spectral and statistical analysis, the Hilbert-Huang spectral analysis technique is applied, as, strictly, VIV is a non-stationary oscillation emerged from a nonlinear dynamic system. A discussion is made on the distinct VIV behaviors of the SDOF and the 2DOF systems.