Experimental Studies in the Control of Cavitating Bodies

An overview of some experimental results involving the dynamics of supercavitating vehicles is presented. The focus of the overview is the relationship between component level experiments (involving cavitator control methods, aft control surfaces and tail planing events) and the associated vehicle system scale dynamic behavior. Cavitator control for example may strongly affect the local cavity structure near an aft planing region as well as affecting overall cavity stability. Experiments performed with a 6.75 inch diameter sting mounted pivoting model and an oscillating cavitator showed strong coupling. Understanding observed dynamic interactions and associated cavity stability is imperative for the design of overall system level control methodologies. Background The US Navy is interested in developing high speed weapons and countermeasures that can provide for several marine platform defensive capabilities including: preemptive counter fire; preemptive target evasion; preemptive countermeasure deployment by the target; and disruption of impending attack. However, to achieve high speeds in viscous fluids, the power required to reach such speeds is problematic. The power required to sustain vehicle speed is proportional to its velocity cubed, thereby making drag reduction an important potential performance improvement. For most streamlined shapes, the skin friction or wetted viscous drag amounts to roughly 70%-90% of the overall drag experienced by the vehicle. Supercavitation avoids this drag component completely by generating a gas bubble large enough to encapsulate the vehicle as it flies through the water. The vehicle generates a bow pressure wave with a leading edge cavitator, which with the help of “artificial cavitation” or ventilated cavitation maintains the gas cavity’s interface long enough for the vehicle to traverse through it. Once the gas-water interface is shed from the cavitator, each axial segment of the interface acts like the wake of a ship. The segments movie independently of the vehicle, but are influenced by the ambient pressure field around the vehicle. As long as the wetted area of the vehicle can be minimized aft of the cavitator itself, the drag of the vehicle is also minimized providing for the enhanced high speed performance. Hence generating a cavity that is large enough, either by having enough kinetic energy to vaporize the ambient water, or providing additional gas ventilation from within the cavity, is necessary for supercavitating bodies to operate efficiently. The hydrodynamic forces that control the dynamics of the vehicle are due to wetted contact with the cavitator, the afterbody, and if present, cavity-piercing control fins. The control of a supercavitating vehicle is dependent on understanding these forces in detail under any operating condition. The wetted forces must be specified and understood to determine vehicle maneuverability. Because the wetted contact area is small, unwanted changes in the wetted contact arising from local cavity breakdown area may result in large destabilizing forces and moments for a supercavitating vehicle. Afterbody control fins are fairly well understood hydrodynamically, however their presence can increase the ventilation requirement as much as 30%. These fins may not be needed for control authority in all cases. Understanding cavitator and afterbody tail planing forces are of primary interest. The cavitator’s role in a supercavitating vehicle is crucial for both acoustic sensor and hydrodynamic control requirements. Novel cavitator designs are being developed to accommodate homing sensors. There is relatively little hydrodynamic information available for a wide variety of shapes such as truncated conical cavitators at angle of attack, and similarly not much data on afterbody planning forces at large attack angles. These two critical