Numerical investigation on underwater towed system dynamics using a novel hydrodynamic model

A new hydrodynamic model is proposed to investigate the dynamic characteristics of an underwater towed system consisting of an unmanned surface vehicle (USV), a towing cable, and a towed vehicle under different operation modes. A depth-tracking control model with a PID algorithm is proposed. The towing cable is divided into a series of discrete elastic catenary segments connected by nodes. The hydrodynamic force acting on the cable is determined by the Morrison equation. The flow field velocity around the towing cable is determined by the local flow field velocity at the nodes. The flow field around the underwater towed system is simulated using computational fluid dynamics method. Experiments were performed to verify the proposed method. The results by numerical simulation show that the draft of the USV increased by 12% under the influence of the towing cable and the vehicle, and sailing resistance of the USV significantly in non-controlled operation. Moreover, the control model is robust in depth tracking operation, the deviation between the prescribed vertical trajectory and the controlled one is acceptable, and the dynamic factors of the underwater towed vehicle are closely related to the longitudinal motion of the shifting weight.