On the Effect of Free-Surface Linearization on the Predicted Hydrodynamic Response of Underwater Vehicles Travelling Near the Free-Surface

Abstract

This study presents a comparison of the hydrodynamic responses generated by a boundary element method with linear free-surface boundary conditions and a volume of fluid RANSE CFD method with nonlinear surface capturing for a shallowly submerged prolate spheroid travelling with constant forward velocity. Observing the effects of free-surface proximity and forward velocity on the hydrodynamic forces and wave disturbance between the two numerical methods reveals the influence and importance of considering a nonlinear free-surface. The forces and moments acting on the spheroid using the nonlinear method are generally larger than the linear method showing that nonlinear effects are important in the calculation of wave making resistance. The difference between the linear and nonlinear methods grows as submergence decreases or wave making increases. An additional phenomenon is shown where the difference between the methods displays a trend with respect to Froude number and depth that is similar to the values being compared. The physical response of these nonlinear effects is seen in the steepening of surface waves near the body, which breaks some of the assumptions made in the linear boundary element method.