A variable-density projection method for interfacial flows

General second-order, variable-density, three-step and four-step projection methods are developed to simulate unsteady incompressible interfacial flows. A high-accuracy, variable-density RKCN projection method is presented, in which the three-stage, low-storage Runge-Kutta technique and second-order semi-implicit Crank-Nicholson technique are employed to temporally update the convective and diffusion terms, respectively. To reduce computation cost, a simplified version of the projection method is also presented, in which the pressure Poisson equation (PPE) is solved only at the last substage. The level set approach is employed to implicitly capture the interface for falling droplet flows. Three-dimensional bubble rising flows and two-dimensional falling droplet flows in a small closed channel are studied numerically via the present method. By the definition of the effective pressure, the flow mechanisms for falling droplet flows with different density ratios, viscosity ratios, Weber numbers, and Reynolds numbers are discussed.