Numericl Research On Cavitation Under Influence Of Gas-water Mixed Fluid Field Around Launch Tube

Attached cavity of underwater vehicle greatly changes the structure of fluid field and has a important effect on the hydrodynamic performance of underwater vehicle. In the experiments, it is found that the field crossed by underwater vehicle which is launched by high pressure gas isn’t a single aqueous medium but unsteady water-gas mixture fluid field which is mixed by gas jet from launch tube and water around canister. The cavitation of underwater vehicle in water-gas mixture flow field is much different from cavitation in a single aqueous medium: the inception, the attachment and dynamic process of cavity are largely affected by the water-gas mixture flow field, and then cavity would lead to complex changes of the hydrodynamic and trajectory of underwater vehicle. By now most of researches on the cavity or supercavity are studied on cavity in a single aqueous medium but cavity induced by water-gas mixture flow field has not been comprehensively studied yet. So the present study on the cavity induced by water-gas mixture flow field has important application value in this area.Flow field structure of water-gas mixture is complex and diverse, including both blended relatively uniform flow and flow with large and small bubbles, and different structures have different effects on the development of cavity. To study mechanism, this article will simplify the structure of the water-gas mixture into two typical structure: one is homogeneous single-phase structure with water-gas mixture, the second is a large gas bubble structure with clear water-gas interface; and then use numerical methods to investigate separately the inception, the development and hydrodynamic characteristics of the cavity respectively when the underwater vehicle crosses through these two kinds of structures.When the underwater vehicle crosses through the homogeneous water-gas structure, the cavity would be affected by the combined effects of the speed and gas volume fraction. Because of the speed of sound greatly decreased in water-gas mixed media, there may be supersonic flow in the water-gas mixed flow field, in this case body will be affected not only cavity but also shock wave. From the study of interaction between cavitation and shock wave, we get the conclusion as follows: bow shock before caviting region weakens cavitation effects, making caviting region decreases; cavity generated by cavitation results in occurrence of oblique shock; cavity closure point cannot locate on the body surface but on the shock surface, which can be confirmed by the adverse pressure gradient at the end of cavity.When the underwater vehicle crosses through the distent bubble, the bubble would be attached to the underwater vehicle and become cavity under certain conditions. By studying the process that the underwater vehicles of different head type cross the bubble, we find head type has large effect on the attachment: for easy cavitating blunt type, the bubbles are easily attached to the surface of the underwater vehicle and come into cavity. For uneasy cavitating or elongated head type, bubbles are not attached to the underwater vehicle; bubble and underwater vehicle quickly separate after disturbance. In this article preliminary analysis of the mechanism of bubbles attachment conditions has been done. These studies can provide a reference for the exterior design and forecasting the hydrodynamic performance of underwater vehicle.