3

    Cavitation is usually very localized and for the most part is known to have detrimental effects. For example, in bubble cavitation of marine propellers, the rapid formation and collapse of bubbles can be very harmful to the blade of the propeller [Stutz and Reboud 1997]. As a result the blade suffers from fatigue and loss of material. Hence a great incentive lies in predicting cavitation and studying the effect of cavitation with respect to marine propellers. In general, cavitation occurs in various environments like low head pumps, heart valves, venturi meters etc. To model these, analytical methods that employ potential theory have been used with a degree of success [Kunz et al. 2000]. However, cavitation often includes mass transfer, unsteadiness and viscous effects [Stutz and Reboud 1997; Kunz et al. 2000].

                                   Supercavitation

      Supercavitation is a specific cavitating condition wherein a vehicle moving in a liquid is completely enveloped by a gaseous cavity. In a supercavitating condition, only a single gaseous bubble exists that is usually much larger than the body itself and stays attached to the body at all times [Tulin 1961]. A schematic of a supercavitating vehicle is shown in Figure 1-2 [Kam Ng 1999].


Cavitator                                                           Supercavity
           .                   . .;.7





           Figure 1-2 Schematic of a supercavitating vehicle [Kam Ng 1999].

      The incentive to employ supercavitation in underwater vehicles is that the skin friction drag is reduced to a great extent because the viscosity and density of the gas is