Experimental Investigation on the Vortex-Induced Vibrations in Yawed Flexible Cylinders

ABSTRACT

An experimental setup was built to investigate the impact of yaw angle on vortex-induced vibrations of flexible cylinders. The experiment was conducted in a towing tank with a flexible model consisting of a silicone tube filled with steel microspheres to provide a suitable dynamic behavior to obtain vibrations up to the fourth vibration mode. The movements were measured by a submerged optical system that captures positions of reflective targets along the model length. This system was tested in five different yaw angles (θ): 0, 10, 20, 30 and 45 degrees.

Beyond the positions, the model top tension was monitored using a load cell to guarantee the static tension and for consequence the natural frequencies. The results intended, among other objectives, to investigate the independence principle in which the curve of vibration amplitude versus the reduced velocity of the yawed cylinders tends to be the same of the vertical cylinders if the reduced velocity is evaluated considering only the normal component of the current. Previous studies have shown that this principle is valid for rigid cylinders elastically mounted for angles up to 30 degrees and depending on other aspects for up to larger angles. However, there are few studies in the literature investigating the same effect for flexible cylinders. The work concerns in the multimodal behavior of the flexible systems, in which vibrations are present in two or more modes simultaneously. For the signals analysis, the method of modal decomposition was employed in which the movements are projected in each vibration mode, allowing an instantaneous reading of which modes presents vibration along the time. These results intended to fill a gap in the experimental results of VIV, thus providing a database for the improvement of numerical methods and analysis, fundamental for the design of offshore slender structures subject to interaction with the current.

PAPER: ISOPE-I-17-505 – ISBN 978-1-880653-97-5 – Link