Experimental Investigation into the Effect of Roll Amplitude on Roll Damping

An in-house large eddy simulation(LES)code has been developed in the previous study.In order to validate the code for simulation of roll motion,effect of roll amplitude on forced roll damping is experimentally investigated in a circulating water channel at Shanghai Jiao Tong University(SJTU).KRISO very large crude carrier 2(KVLCC2)is taken as a target ship,and a model with a scale of 1:128.77 is manufactured.Tests are carried out for the ship model at shallow draft with frequencies around its natural roll frequency,where natural roll frequency is attained via free roll decay.Six forced roll tests with roll amplitudes ranging from 1?to 4?are performed experimentally,and three forced roll motions are simulated with the in-house LES code for middle section of the ship model.Discrepancies between the computed roll damping coefficients and the ones from forced roll tests are quite small.By means of particle image velocimetry(PIV),velocity fields in the vicinity of the ship model,and generation and evolution of vortices near the ship bilge are measured in detail.It is shown that roll amplitude has a significant effect on the vortex behavior,and therefore on the magnitude of roll damping coefficient.Further experimental and numerical investigation into roll motions with large amplitudes is planned.