Numerical evaluation of the free surface effect on the hydrodynamics and dynamics of underwater vehicles

Abstract

The present thesis seeks to evaluate the free surface effect on the hydrodynamics and dynamics of a generic underwater vehicle (UV) in the horizontal plane. Accordingly, the captive tests, including the straight-ahead resistance, drift and rotating arm tests, are performed on the bare hull of a UV model by using numerical simulations based on URANS equations with a Reynolds stress turbulence model implemented in the commercial code STARCCM+. These tests are carried out for various submergence depths and proper ranges of UV velocity components. For the purpose of maneuverability assessment, the forces and moments arising from the velocity components obtained from the simulations of the captive tests are implemented in the equations of motion for various submergence depths. Additionally, analytical equations are used to calculate the forces and moments arising from the UV accelerations, thrust and rudder, which all are assumed to remain constant with respect to submergence depth. The obtained results show that, generally, a decrease in submergence depth causes an increase in all the forces arising from the velocity components. The results further show that approaching the free surface has a negligible effect on the lateral force and yaw moment generated by the bow and stern regions. Moreover, it is seen that with a decrease in submergence depth, the region between the UV midlength and the aft shoulder is mainly responsible for the increase or decrease in the lateral force and yaw moment acting on the UV hull. It is also observed that, with a decrease in submergence depth, due to an increase in the UV damping characteristics, the dynamic stability increases remarkably, which leads to a decrease in the UV maneuverability.