Experimental investigation of the flow past a submarine at angle of drift

When a submarine executes a high-speed turn, the sail experiences a crossflow velocity component, creating a vortex at the sail tip. The flowfield induced by the vortex creates higher pressures on the hull deck and lower pressures on the keel, leading to a nose-up pitching moment. Tests to examine such flows were conducted in the U.S. Navy's William B. Morgan Large Cavitation Channel in Memphis, Tennessee. A submarine model of 6.92-m length was tested at drift angles of 0, 5, and 9.5 deg, with the sail on and off, and at speeds up to 14.9 m/s. Pressure distributions and laser Doppler velocimeter surveys were obtained at two axial locations. Forces and moments were measured using an internal force balance. The pressure distributions were integrated to obtain section force coefficients, and the velocity data were used to estimate the vortex circulation. The pressure distributions were consistent with the hypothesized mechanism by which the vortex created the nose-up pitching moment. The pitching moment and sectional force coefficients demonstrated the nonlinear variations with drift angle predicted by theory. The circulation values were consistently somewhat higher than those obtained by other researchers.