A Dynamic Test Platform for Evaluating Control Algorithms for a Supercavitating Vehicle

The use of supercavitation to enable marine vehicles to travel at extraordinary speeds is a topic of considerable interest. The control of these vehicles poses new challenges not faced with fully wetted vehicles due to a complex interaction between the vehicle and the cavity that it rides in. Some of the existing models make assumptions that may not be valid for a maneuverable vehicle. Furthermore, since there are various models being suggested for planing forces as well as different ways of obtaining fin and cavitator forces, there is a lack of unity among the equations used to calculate the hydrodynamic forces imparted on such a vehicle. Experimental test platforms have been developed at St. Anthony Falls Laboratory to enable testing and validation of control algorithms and hydrodynamic models. Previous efforts have revealed the destabilization of marginal supercavities by control surfaces, especially when a cavity is being maintained with ventilation [1]. Our latest water tunnel test platform is a body of revolution with an actuated cavitator on the model forebody, actuated fins that protrude through the cavity surface, and variable pitch of the model body, all supported by a six-axis force balance. In this paper we will present a brief description of the forces present in our mathematical model of a supercavitating vehicle, and then present the new experimental test platform that will be used to validate, and expand on this model. INTRODUCTION The phenomenon of supercavitation permits objects to move through water at high speeds because of an order of magnitude reduction in skin-friction drag. Applications of supercavitation include underwater munitions such as torpedoes and projectiles, propellers and pumps, and hydrofoils. In order to capitalize on the phenomenon, operation must either be at high speed or artifical ventilation of the cavity must be resorted to. Figure 1 Diagram of a nominal supercavitating vehicle in longitudinal plane. A) Disk cavitator B) Planing force on afterbody C) Wedge shaped fins. This paper focuses on the operation of a nominal highspeed supercavitating vehicle (HSSV). Only small regions at the nose (cavitator) and on the afterbody are in contact with water. For this nominal HSSV only three primary hydrodynamic forces are present, as shown in Figure 1: cavitator forces, after body planing forces, and fin forces. Any attempt at vehicle stabilization and control must be accomplished using a combination of these three. The absence of lift on the body (unlike a fully wetted vehicle) requires that the body must either be in a planing mode or the control surfaces must provide the necessary lift. Cavity shape is very important when computing the forces acting on the supercavitating vehicle and consideration has to be given to the nonlinear interaction of the control surfaces and the body with the cavity wall. Furthermore, the cavity-vehicle interaction exhibits strong memory effects (cavity shape is a function of the history of the vehicle motion) that must be included in the modeling.