Study On The Stability And Flow Structrue Of Ventilated Supercavitating Flow

Cavitation is a physical phenomenon especially existing in liquid field. It takes place as long as the local pressure drops to lower than the saturated vapor pressure of the liquid.When the vehicle is completely enveloped within the cavity, the drag of the vehicle can be reduced significantly, which is called supercavitation. It can be also achieved by injecting non-condensable gas, when relative speed between vehicle and liquid is not large enough to produce a natural supercavitation.As a revolutionary, efficiency method of drag-reducing, technique of supercavitation can increase the speed of an underwater vehicle significantly. However, ventilated supercavitating flow is a very complex phenomenon. The understanding of its characteristics and mechanism is very limited, especially the stability and the flow field of a ventilated supercavitating flow.In this dissertation, the characteristics and the mechanism of natural cavitation and ventilated supercavitating flow were investigated by using both experimental test and numerical simulation.The flow structure of a flat airfoil and an axial slender vehicle was measured by using PIV technique. The condition of the natural cavitation and ventilated cavitating flow which PIV technique can be used to effectively measure the velocity field is determined. The effect of natural and ventilated cavitation on the flow structure was studied.The flow field around a ventilated supercavitating vehicle with fixed angle of attack in the water tunnel was simulated by using commercial code Fluent. The flow structure of theventilated supercavitating flow was carefully studied, the relationship among the cavity morphology, velocity distribution, pressure distribution and the hydrodynamics was revealed. The simulatin results were in good agreement with experimental results. The effect of the angle of attack and the ventilated rate was also studied.The flow field around a pitching ventilated supercavitating vehicle in a water tunnel was simulated by using dynamic mesh technique. The cavity morphology, the pressure distribution and the hydrodynamics of the vehicle were in good agreement with experimental data. The effects of the pitching amplitude and the pitching frequency were also studied. The flow field around a pitching supercavitating vehicle in the infinite flow field and in a tunnel without support structure was simulated, and the effects of the tunnel wall and the support structure were analyzed.A preconditioned, homogenous, multiphase, unsteady Reynolds Averaged Navier-Stokes(URANS) scheme is dynamically coupled to a six-degree-of-freedom(6DOF) code are presented. The free cruising of a supercavitating vehicle was simulated by using dynamics mesh technique. The generation process and the formation of the supercavity, and the relative motion between the supercavity and the vehicle were investigated.