عنوان مقاله [English]
In this study, numerical simulation of falling incompressible, spherical droplet of a fluid under gravity in other peripheral fluid is done. Phase field model and least square method are used for solving the two dimensional incompressible Cahn-Hilliard and Navier-Stokes equations in so little time step duration for different densities and viscosities between phases. The main performance of the least squares method is the minimization of the residual functional in a least squares manner of parameters. Instead of level-set, large eddy simulation or volume of fluid methods, the phase field models have been successfully implemented to simulate the flow of two or more immiscible fluids whose their densities and viscosities are not the same. The studied geometry includes a spatial domain as a vertical channel [0.2]X[0.08] that its boundaries have no slip and no penetration conditions and a stationary droplet
with 0.005 radius starts falling by gravitational force. The droplet center of mass at the initial time is (0.04, 0.15). The effects of density and viscosity ratios on incompressible droplet dynamics are quantitatively studied. Density and viscosity ratios are defined to generalize the numerical solution by creating the relation between some properties of two different fluids. The chemical potential parameter in Cahn-Hilliard equation depends on the kinematic fluid pressure. The velocity field around droplet and its effect on droplet shape are investigated in this paper. The falling velocity of droplet decreases by density ratio increase, but circularity parameter is approximately constant. The falling velocity of droplet increases a little by viscosity ratio increase, but circularity parameter is decreased rapidly. The falling droplet shape goes from sphere to ellipse after producing vortex loops from its tail, and it leads to circulating region growth at the rear of the droplet. The velocity field makes droplet surface horizontally by growth of vortex sizes.