A Study On Drag Reduction Characteristics Of Underwater High Speed Supercavitating Vehicle

For the underwater high speed vehicle, the liquid side of vehicle change to vapor while the zone pressure decrease to saturation pressure of the liquid. When the vapor envelops the whole vehicle, we call the vapor zone supercavity. The viscous drag force of vehicle reduce quickly while it working in the supercavity. In the same way, it is possible to make the cavity around the vehicle by ventilation to reduce the drag force, it is called ventilation supercavity. Since the drag force of vehicle can reduce in the widely range, it is advantaged to develop underwater high speed vehicle, which is useful for coast defence and life saving. However, the vehicle in the supercavity lose the Archimedes force, the imbalance of force around the vehicle make it motion complicatedly. Furthermore, the flow is related to phase change, turbulence, compressive unsteady multiphase flow. So many complicated factors make the flow more complicated. Thus, it is necessary to find the basic disciplinarian before we can utilize the advanced technique.This paper base on theory analysis, experiment and numerical simulation, key works performed in the thesis as follows:Firstly, dynamics equations of supercavitating vehicle are developed based on its characteristic. After revising the shape of supercavity based on half empirical formulae and solving the force around the vehicle, the equations are closure. Then some cases were performed. The results show that for the small projectile with fix mass and shape, the range is shorter if the cavity is big egregiously. In order to get the longest range at the fix launch velocity, the size of cavity should be smaller by change the cavitor, whether optimize the diameter or the shape can get the same longest range.Secondly, unsteady governing equations of single phase were deduced based on the basic conservation equation. The relationship of interface was deduced. Considering the motion of interface and turbulence flow, the equations was averaged base time, time averaged multiphase flow equations were deduced at last. In order to closure the equations, cavitation model by Singhal and RNGk-εturbulence model with standard wall function were employed. The turbulence model was modified for the characteristic of cavitating flow. Then, the equation was discretized with non-orthogonal grid. Boundary setting and solve method of pressure and velocity were given at last.Thirdly, the characteristic of natural supercavitationg was investigation. The experiment about the small supercavitating vehicle was performed. Attach cavitation and supercavitation were simulation by the above model, the results show it is in accordance with the experiment data. The experiment was reworked by the numerical method. Supercavitation has the good ability for the drag reduction after comparison with the motion without supercavitation. Then, volume drag coefficient was brought forward, after the simulation based on several tradition cavitor at different cavitor number, the basic principle of volume drag coefficient was found. The control ability of variable drag cavitor was study by the unsteady simulation, the relationship between drag force and cavity was investigated. The variable cavitor was put forward and simulated, the results indicate that it can supply enough drag force and lift force for control the vehicle.Finally, the purpose and the basic scheme of ventilation supercavity were presented. Two kinds of steady gas leakage manner were analysised. The critical conditions of unsteady ventilated supercavity were analysised. At last, ventilated supercavitation was simulated by large eddy simulation (LES) and volume of fluid (VOF), the same case was simulated by RNGk-εturbulence model, the results show that the difference is obvious for the different turbulence model.A lot of helpful works about supervacitation drag reduction were explored in this thesis. That are all very useful for the design and research in this field.