Viscous effects are known to have significant influence on hydrodynamic forces on bluff-shape bodies. Ocean structures in long waves and rolldamping arising from bilges of a ship hull are important examples. Some recent efforts to under stand and model such real-fluid effects in the presence of surface waves are described. As a canonical problem for studying ship roll, a surfacepiercing plate is oscillated sinusoidally in the free surface. A Digital Particle Image Velocimetry system (DPIV) was developed at the UC Berkeley Ship-Model Facility to map out the flow details. In one regime, the flow exhibits an asymmetrical vortex shedding pattern. This steady-state pattern is observed to be systematically dependent on the initial conditions. DPIV and other experimental results were used to validate the predictive capabilities of the Free-Surface Random-Vortex Method (Yeung & Vaidhyanathan, 1994). This grid-free Lagrangian method is briefly described. With some calibrations, the method reproduces very well the observed vortical structures, the measured forces and moments, and the wave-generation characteristics. Application of the methodology to a rolling rectangular section with sharp bilges is also considered. The limitations and promise of the method are discussed.
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