Numerical Research On Ventilated Cavitating Flow For The Supercavitating Vehicles

Drag reduction using cavitation, proposed as a revolutionary method to meet the demand of high speed and long voyage for the future underwater vehicles, is capable of achieving drag reduction over 90% by modifying the flow property and strucure near the vehicle surface. In this paper, the macroscale and multiscale simulation models were established for the ventilated cavitating flow. Systematical and deep research was carried out on the development process of the ventilated cavities, as well as on the interaction between the multiscale cavitating flow and the vehicle body, revealing the evolution process of the macroscale cavity and the dispersion characteristics of the microscale bubbles. The drag reduction mechanisms by the ventilated cavity were further explored, providing theoretical basis for the design of the supercavitating vehicles.The interaction between liquid, continuous gas and dispersed bubbles during the formation of the ventilated cavity was analyzed. The macroscale simulation model for the ventilated cavitating flow was complemented based on the homogeneous multiphase model, integrated with the natural cavitation model and the surface tension force model. The air entrainment model by the re-entrained jet was put forward based on the analysis of the air entrainment mechanisms for the free surface flows. Thereafter, the multiscasle simulation model for the ventilated cavitating flow was established based on the Euler-Euler two fluid model, incorperated with two types of population balance methods and the air entrainment model by the re-entrained jet. The proposed numerical models provide important basis for the numerical research on ventilated cavitating flows.Numerical research on the ventilated cavity in the verticle pipe was carried out using the multiscale simulation model. The variation of the air entrainment rate at the cavity tail was obtained. Three different regions downstream the ventilated cavity including the vortex region, transitional region and the pipe flow region were successfully predicted. The distribution characteristics of the flow field parameters such as the void fration, velocity and bubble size in different regions were obtained. The influence of liquid velocity, ventilation rate and inlet turbulence intensity on the flow field parameters were further discussed. Good agreement was observed between the simulation results and the experimental data, validating the proposed multiscale simulation model and the air entrainment model. The above research sets the theoretical and method basis for the simulation on ventilated partial cavity.Simulations for the ventilated partial cavity was accomplished based on the macroscale model and the multiscale model respectively. The evolution of the cavity shape along with the ventilation rate was successfully captured. The distributions of the flow field parameters including the void fraction, liquid velocity and bubble size in the wake region behind the test body were invesitgated, which revealed the critical elements to the change of the dispersion way and speed of the bubbles. The characteristics of the flow field in the boundary layer of the test body surface were explored. Furthermore, the influences of air entrainment rate and sailing attack angle on the microscale bubbly flow were studied. The drag reduction mechanisms by ventilated partial cavity were better understood, providing technical support for the development of drag reduction method using cavitation.Numerical research was carried out for the high-speed ventilated cavitating vehicles based on the macroscale simulation model. A mathematical model for calculating the cavity shape, applicable for the ventilated cavity with various gas-leakage mechanisms and wide range of Froude number was developed. The model was validated in comparison with the experimental and numerical data. The parameters to evaluate the cavitating vehicle with water inlet pipe were put forward. Subsequently, the performance for the cavitating vehicle with water inlet pipe under various sailing speed and attack angle was studied based on the simulation for the natural cavitating process. The interaction between wedge control fins and the natural cavity was investigated. Obtained results provides theoretical basis for the design of high-speed cavitating vehicles.The research achievements in the thesis will surely promote the development of drag reduction techniques using cavitation, as well as providing support for the advancement of the simulation techniques for the multiphase flows. The research will be of great theoretical value as well as engineering practice meaning for the future supercavitating vehicle research.