Optimal Design of Supercavitating Vehicles Based on Trimmed Flight Performance

Supercavitating vehicles exploit supercavitation as a means to reduce drag and increase their underwater speed. The dynamic behavior of this class of vehicles is very complex as a result of the coupling between vehicle and cavity dynamics and of their strong interactions. In addition, the hydrodynamic stability of supercavitating vehicles needs to be carefully addressed as the loads are completely different from those on conventional submerged bodies. As a supercavity completely envelops the vehicle, most of its surface is acted upon by almost negligible water-vapor forces, and only a small percentage of it is wetted. As a result, the center of pressure is always ahead of the center of mass, thus violating a fundamental principle of hydrodynamic stability. To this date, a number of basic issues need to be addressed regarding configurational design and operating conditions of supercavitating vehicles. This paper presents a preliminary investigation, where the vehicle configuration is evaluated for optimal performance during trim, level flight. The dynamic behavior of the vehicle is investigated through a 6 degrees of freedom rigid body model. In the considered formulation, the vehicle is acted upon by gravity and propulsion and by loads generated by the control surfaces of the vehicle and by the interactions with the cavity. The control surfaces include 4 fins and the cavitator disk at the nose, which produces the cavity. Trimmed conditions for assigned velocity and vehicle configurations are evaluated by solving a nonlinear system of equations resulting from the vehicle’s equation of motion. The vehicle performance is defined in terms of achievable range of operation, which is used as an objective function in an optimization problem, where basic configurational and operational parameters are considered as design variables. The presented results show that proper selection of the vehicle configuration can significantly improve the selected performance index. The simulation of the vehicle dynamic behavior in trim conditions and its response to small perturbations with respect to the trim values show the challenges associated to the limited stability of the vehicle and provide indications for future investigations.