Simulations of binary drop collisions with a multiple-relaxation-time lattice-Boltzmann model

In this paper, we report simulations of drop-drop collisions using a multi-relaxation-time multiphase flow lattice Boltzmann model. Employing a multi-relaxation-time (MRT) model in lieu of the Bhatnagar-Gross-Krook (BGK) model used in the standard lattice-Boltzmann equation enables realization of stable computations of drop collisions at relatively lower fluid viscosities without increasing the lattice resolution to prohibitive levels. Head-on and off-center computations of collisions are carried out using axisymmetric and three-dimensional (3D) versions of the MRT model, respectively. Time-resolved results showing the interactions of the interfaces of drops for different characteristic nondimensional parameters are presented. Computations show that at low Weber numbers, We, coalescence with relatively smaller deformation occurs, sometimes entrapping a stable microbubble. At higher We, head-on collisions lead to reflexive separation with or without the formation of satellite droplets. The size of the satellite droplets appears to increase with increase in the We. The Ohnesorge number, Oh, seems to modulate the transient characteristics and the outcome of collisions. It is found that the greater the Oh, the smaller is the size of the satellite droplets formed. For off-center collisions at a given We, at lower values of the impact parameter permanent coalescence is observed, while higher values result in separation by stretching action. These findings are in satisfactory agreement with experimental observations.