Motion of a cylinder adjacent to a free-surface: flow patterns and loading

The how structure and loading due to combined translatory and sinusoidal mo tion of a cylinder adjacent to a free-surface are characterized using a cin ema technique of high-image-density particle image velocimetry and simultan eous force measurements. The instantaneous patterns of vorticity and stream line topology are interpreted as a function of degree of submergence beneat h the free-surface. The relative magnitudes of the peak vorticity and the c irculation of vortices formed from the upper and lower surfaces of the cyli nder, as well as vortex formation from the free-surface, are remarkably aff ected by the nominal submergence. The corresponding streamline topology, in terpreted in terms of foci, saddle points, and multiple separation and reat tachment points also exhibit substantial changes with submergence. All of t hese features affect the instantaneous loading of the cylinder. Calculation of instantaneous moments of vorticity and the incremental changes in these moments during the cylinder motion allow identification of those vortices that contribute most substantially to the instantaneous lift and drag. Furt hermore, the calculated moments are in general accord with the time integra ls of the measured lift and drag acting on the cylinder for sufficiently la rge submergence.