A magnetic field coupling fractional step lattice Boltzmann model for the complex interfacial behavior in magnetic multiphase flows

In the present study, a magnetic field coupling fractional step lattice Boltzmann (FSLB) model is proposed to simulate the complex interfacial behaviors in magnetic multiphase flows with severe interface deformation and large density ratio, which can be recovered to the corresponding macroscopic governing equations with fully second-order accuracy. Since the equilibrium and the non-equilibrium terms are obtained by the Chapman-Enskog (C-E) expansion analysis, the basic idea of the present method is to predict the intermediate variables by updating the various moments of equilibrium distribution functions first, and then correct the physical variables by updating the various moments of non-equilibrium distribution functions. Besides, the present magnetic field coupling FSLB method does not need the evolution of distribution functions compared to the existing LB methods. The present method inherits the excellent performance from the conventional LB method and the relatively better numerical stability from the matured fractional-step method, which is more stable than the original LB method and more efficiency and easy-to-implement than the LB flux solver. Owing to those good properties, we successfully perform the first rigorous numerical simulation on the Rosensweig instability, which accurately captures the spikes forming at the surface of a ferrofluid under an applied magnetic field. Moreover, for the first time we numerically reproduce the falling ferrofluid droplet impacted on the solid surface. Intriguingly, our simulation results reveal that the falling velocity of the ferrofluid droplet is slightly accelerated by the elongation, although the direction of the magnetic field is opposite to that of the gravity. All the results for the interfacial behavior and the magnetic interaction highlights the comparative capability, stability, and accuracy of the present magnetic field coupling FSLB method for simulating the problems of complex fluid dynamics associated with a severe interface deformation and large density ratio in magnetic multiphase flows.