MEASUREMENTS OF PRESSURES ON A HEAVING PROLATE SPHEROID

The damping and added mass coefficients for a shiplike form oscillating at a free surface are usually approximated by a 'strip calculation' in which two-dimensional values for shapes similar to sections of the body are summed over the length. For the three-dimensional body the real section values differ from the local values used in this procedure because of the three-dimensional nature of the real flow caused by body geometry and forward speed. This report presents experimental results which show the magnitude of the three-dimensional effect for a prolate spheroid undergoing forced heaving oscillations while moving longitudinally in calm water. Local hydrodynamic forces on a spheroid model were measured by a number of flush pressure gauges distributed around the girth of the model at several sections. The sectional added mass and damping were than obtained by resolving the integral of pressure at each section into in phase and quadrature components. Total constraint forces were measured and checked against the longitudinally integrated pressure forces. Results were compared with strip computations using two-dimensional cylinder data. In general, the greatest discrepancies were found at the ends of the model. At amidships, measured values agreed quite clsosely with two-dimensional results. At high frequencies, forward speed was found to have small effect on added mass. However, at high frequencies and high forward speed, negative damping was found at some sections. (Author)