The capability of simulating multi-fluids flows is essential for the study of many engineering and geophysical processes. A challenge in the multiphase flow simulation is to capture the gas and liquid interface, which can involve strongly nonlinear, violent, and intermittent processes, during which bubbles may be entrapped, sprays generated, and turbulence produced. In our framework of two-fluids simulations, we solve the gas and liquid flows as a coherent system on an Eulerian grid, with the gas-liquid interface captured using a coupled level-set and volume-of-fluid method. The volume-of-fluid method is able to conserve the mass of each phase of fluids precisely, and the level-set method can capture the geometry of free surfaces accurately.
While the large-scale liquid droplets in gas and the gas pockets in liquid can be captured directly by the coupled level-set and volume-of-fluid method, the effect of small-scale sprays and bubbles need to be modeled. In our simulations, the small bubbles are represented by clouds of subgrid-scale bubbles with modeling for the coalescence and breakup processes. A robust and accurate algorithm to identify bubbles and droplets is developed for tracking them and calculating their statistics.