Preliminary results are presented from a numerical study of the unsteady hydrodynamics of an AUV thruster. A vortex-lattice, lifting-surface model, developed originally for ship propellers operating under normal design conditions at constant angular velocity, is modified to handle unsteady operating conditions of an AUV thruster during dynamic positioning and maneuvering. The present numerical code applies to an unducted propeller though the method can be modified to handle ducted propellers. The results from sample runs of the thruster undergoing step changes and sinusoidal oscillations in the angular velocity of the propeller are shown.
The motivation of this research is to develop an accurate hydrodynamic model of an AUV thruster during unsteady operations. Such a model could be used to develop controllers for vehicles that operate in the ocean and are subjected to large time-varying forces. For example, the large oscillating wave forces that dominate near-shore and near-surface ocean environments will cause an underwater vehicle to move back and forth with horizontal and vertical amplitudes of the order of meters and with time scales of the order of seconds. To maintain a prescribed course under closed-loop control, the thrusters will have to continually switch from forward to reverse thrust and back again to forward thrust in such a way as to negate the effects of wave and current forces as well as the inertial forces of the vehicle. The correct timing of this control action is possible only if there is an accurate model of the thruster dynamics and hydrodynamics built into the controller. When a dynamic positioning controller commands a certain amount of thrust using present-day methods, the desired thrust does not occur instantaneously because of time delays involving the fluid inertia and velocity of the fluid flowing over the propeller blades.