Session: Model Testing-1

Paper Number: 164223

164223 - Experimental Study on Vortex-Induced Motion of a Towed FOWT in Currents and Waves 

Abstract: 

In recent years, discussions have advanced regarding the construction methods for fl oating offshore windturbines (FOWT). One of the stages in this process includes towing the FOWT to the on-site wind farm. Duringthe towing of the FOWT, vortex shedding occurs in the wake of the column due to viscous effects of the fl uid,and when the shedding frequency approaches the natural frequency of the fl oating column, Vortex-InducedMotion (VIM) occurs, causing signifi cant vibrations in the sway direction. This phenomenon is often discussedusing the reduced velocity, defi ned as the ratio obtained by dividing the fl ow velocity by the product of thenatural frequency of the fl oater and the characteristic diameter of the column. At reduced velocities near 5 to8, large cross-fl ow vibrations are observed, and increasing the towing speed shifts the resonance to higherfrequencies. However, to reproduce this phenomenon in tank experiments, the effects of viscosity andReynolds number must be carefully considered. In actual sea conditions during FOWT towing, the fl ow istypically in the supercritical or upper transition regimes. However, this study focuses on elucidating the effectsof Reynolds numbers in the subcritical regime (𝑅𝑒<3.0×10^5). To investigate the impact of Reynolds numbers,we conducted tank experiments using two different scaled models. Since the motion of the fl oating structureis dominated by gravity and inertial forces, towing speeds were scaled according to Froude similarity, andmeasurements of the fl oating structure's motion during towing were carried out.The sway motion amplitudewas non-dimensionalized by dividing it by the fl oating structure's characteristic dimension, and responsesfrom the two scaled models were compared. As a result, differences in the fl oating structure's response due tovarying Reynolds numbers were clarifi ed. Additionally, the study examined VIM under wave conditions duringtowing by comparing cases with and without waves at the same towing speed. This approach revealed theimpact of waves on the fl oating structure's response.

Presenting Author: Ueno Daichi The University of Tokyo

Presenting Author Biography: Graduated from Tokyo University of Marine Science and Technology, Faculty of Marine Engineering in 2020.
Entered the University of Tokyo, Graduate School of Frontier Sciences in 2023.

Authors:

Ueno Daichi The University of Tokyo
Shinichiro Hirabayashi The University of Tokyo
Gonçalves Rodolfo The University of Tokyo