| Cavity flow commonly which is often observed in hydraulic machinary, is a form of complex multiphase flow involving many complicated fluidic phenomena, such as vapor-liquid phase change, heat transfer and exchange and pressure wave. Abundant investigations on cavitation by theoretical, experimental and numerical methods have been performed. Various cavitation models have been developed to simulate the cavitating flow. However, the fundamental mechanism of cavitation is still far from understanding. In this work, vapor bubble, which coincides more with the physical condition than others like gas bubble, is chosen as the object of study. And, from the viewpoint of microcosmic dynamics. Direct Numerical Simulations (DNS), combined with the Volume of Fluid (VOF) method, is employed to study the collapse of vapor bubbles driven by higher outer pressure. The used number of vapor bubbles ranges from 1 to 25,50...175. Accordingly, the multi-bubble dynamics has been investigated from the aspects of bubble collapsing shape, pressure distribution, collapse time and void fraction evolution.Considering surface tension, viscosity and compressibility of both vapor and liquid phases, a computational method with a phase-change model is developed to simulate phase change process of vaporization and condensation. The dynamic process of a single vapor bubble under outer pressure is simulated, including the collapse and the rebound to a smaller size. As the results indicate, radius evolution of a single vapor bubble agrees well with analytical results from the Rayleigh Equation. Around the moment the vapor bubble reaches the minimum radius, great gradients of pressure and temperature are found at the bubble surface. The greatest pressure in the liquid is found in the vicinity of bubble. When the bubble collapses to the smallest size, the pressure and temperature at its center reach the highest values, which are significant high pressure and temperature. Ignoring the compressibility of vapor and liquid phase and using an approximate inner pressure, we develop a simplified computational model to simulate the dynamics of vapor bubble. A nice result is also obtained with this simplified model, making it possible to simulate the multi-bubble cases by reducing some calculation cost.The phenomena of multiple bubbles case, due to the interaction between bubbles, are much more complicated than those of the single one. On the basis of single vapor bubble simulation, the computational conditions of bubble clusters-are analyzed and then used in the simulations of dynamics of multi-bubble. It’s found that the collapse time increases with the number of bubbles. The pressure boundary propagates inwards from outside, and comes across the outer layer bubbles to make these bubbles collapse and so on, so that the bubbles collapse layer by layer. Besides, the bubbles furthest away from the boundary change little in the initial period when the concentration of bubbles is over a specific value. However, the slopes of volume profiles of bubbles in different regions tend to be similar. In the last stage of collapse, very high pressure is generated, and strong jet can also be found.The collapse time and void fraction in the multi-bubble system are statistically investigated to analyze the relation of volume fraction rate with the number of bubbles and pressure gap. A modified cavitation model is obtained, which predicts good results coinciding with the data of simulations. For the consideration of viscosity, surface tension and interactions between bubbles, the modified cavitation model is in more accordance with the real cavitation phenomenon. |
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