Free-surface interaction of a fully appendaged AUV: An experimental study using the Planar Motion Mechanism method for calculating hydrodynamic coefficients

Understanding the hydrodynamics of an autonomous underwater vehicles (AUV) is critical for developing an efficient control algorithm, and significant research efforts have been made in the past using both numerical and experimental methods. Previous studies have primarily focused on deeply submerged conditions far from the free-surface and seafloor effect regions, where important interactions occur and may influence vehicle's dynamic behaviour. Herein, results from an experimental study focused on the free-surface effects using the planar motion mechanism (PMM) method with a 1.8 m (1:1) model of a fully appendaged lightweight autonomous underwater vehicle (LAUV ‘Fridtjof’) are presented. The forces and moments in surge, sway, yaw, heave, and pitch were measured with speed and submergence depth as control variables, and the hydrodynamic coefficients were calculated from the data. The towing tank at the Marine Technology Centre (Trondheim–Norway) and the hexapod setup were used for the experimental PMM runs, with $F_n$ varying from 0.12 to 0.36 and $h/D$ from 0.5 to 4.0. The results show that the peaks of the lateral and vertical forces, as well as the yaw and pitch moments, increase significantly during the PMM tests for $F_n=0.36$ and $h/D=1.0$, as do the hydrodynamic coefficients $Y_v,Y_{vvv},Y_{vvr},Z_{vv},M_{vv},N_v,N_{vvv},N_{vvr},N_r,N_{rrr}$ at shallow submergence.