ABSTRACT:

To realistically simulate motions of a floating cylinder in waves, a fully nonlinear model that includes the effects of viscosity is developed. The equations of rigid-body motions are coupled with the Navier-Stokes equations, which are solved by the Free-Surface Random-Vortex Method (Yeung & Vaidhyanathan, 1994). Inviscid results that are of interest for comparison or validation purposes can be recovered by "shutting-off" viscosity in this tbrmulation. This nonlinear model is first applied to study the transient response of a body with initial displacements for pure heave or roll motion, for comparison with some existing inviscid results. Frequency-domain response in waves is obtained by simulating heave motions in the time domain over a range of frequency. For a freely-floating body in waves, the roll amplitude near resonance is observed to be reduced by viscosity by as much as 50%. The fully nonlinear model is validated by comparing the numerical results with those from the measured values of Sen (1993), revealing considerable improvement over existing predictions based on inviscid fluid.

1 INTRODUCTION

Recent advancement in computational power and numerical algorithms encourage the development of time-domain solutions for body motions in waves. Time-domain analysis is preferable when transiedt response and large-amplitude motions of bodies are of concern. For motion involving substantial viscous effects, the time-domain formulation is the only logical approach. One of the first attempts to solve the time-dependent problem was given by Maskell & Ursell (1970), in which the transient response was calculated based on known frequency-domain hydrodynamic coefficients. Chapman (1979) obtained the transient response of a floating body by representing the radiated wave field in terms of a finite sum of harmonics. Yeung (1982) formulated the heave transient response of a floating body via the use of an unsteady Green function that satisfies the linearized free-surface boundary conditions.

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