The effects of small bilge keels fitted at the corners of a rectangular cylinder are investigated experimentally and computationally. The freely-floating cylinder is subjected to incident beam waves. Since fluid viscosity plays a major role in the sway and roll motion of the cylinder, a rational prediction of the response of the cylinder requires the solution of the Navier-Stokes equations with additional complexities coming from free-surface effects and free body motion. The fully nonlinear solution is accomplished by using the Free-Surface Random-Vortex Method (k%ung & Vaidhyanathan, 1994), with recent extensions to include the effects of free-body motion (Yeung & Liao, 1999). Frequencydomain response in waves is obtained by simulating this 3DOF system in the time domain over a range of frequency. Comparison with experiments is seen to be rather satisfactory. Both experiments and time-domMn solutions indicate a strong drift velocity, which is relatively insensitive to the presence or size of the keels, but dependent on the wave frequency. As expected, the keels are found to be effective in reducing the resonant modes of motion, heave and roll, near their respective resonant response peaks.
Time-domain solutions are desirable when large-amplitude motion of bodies is of concern. For motion such as roll and sway, influenced substantially by viscous effects, timedomain fornmlation is the only logical approach. Within the context of potential-flow theory, Maskell & Ursell (1970) were one of the first workers to examine the time-dependent problem. Since then, various inviscid models have been developed and presented in Chapman (1979), Yeung (1982), Newman (1985), and more recently, Celebi & Beck (1997) and Bingham et al.(1994) for freely floating bodies, either in the transient state or subjected to some excitation.
