Numerical simulations of free roll decay are carried out for the DTMB 5512 based on computational fluid dynamics (CFD) theory and adoption of overset mesh technology. The numerical time history of the ship roll shows good quantitative agreement with the experimental time history in the case of Fr = 0.138 with an angle of list of θa0 = 10°. The natural period is well estimated, there is less than 2% error. According to the time history of the roll angle and the corresponding angular velocity, the nonlinear ship roll damping and restoring force of the hull are identified by solving the nonlinear Volterra integral equation of the first kind with Tikhonov's regularization method. The results indicate that the numerical method in the paper is feasible, and the choice of regularization parameter is proper. Additionally, the influences of angle of list, model scale and ship speed on free roll decay are investigated. Roll damping and nonlinear restoring force increase as model scale decreases. Keeping the same scale of ship model, roll damping increases with ship speed while nonlinear restoring force decreases.

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