Asymptotic analysis of multiple-relaxation-time lattice Boltzmann schemes for mixture modeling

AbstractA new lattice Boltzmann model for simulating ideal mixtures has been developed by means of the multiple-relaxation-time (MRT) approach. When compared with the previous single-relaxation-time (SRT) formulation of the same model, based on the continuous kinetic theory, the new model offers the possibility to independently tune the mutual diffusivity and the effects of cross collisions on the effective stress tensor. The additional degrees of freedom, due to the increased set of relaxation time constants used for modeling the cross collisions, allow us to match the experimental data on macroscopic transport coefficients. Two different integration rules, i.e. the forward Euler and the modified mid-point integration rule, were used in order to numerically integrate the developed model. Unfortunately the simpler forward Euler integration rule violates the mass conservation and there is no way to fix the problem by changing the definition of the macroscopic velocity. On the other hand, a small correction has been purposely designed for compensating this error by means of the mid-point integration rule. Some numerical simulations are reported for proving the effectiveness of the proposed corrective factor. For the considered application, the asymptotic analysis, recently suggested as an effective tool for analyzing the macroscopic equations corresponding to the lattice Boltzmann schemes, offers a remarkable advantage in comparison with the classical Chapman–Enskog technique, because it easily deals with leading terms in the distribution functions, which are no more Maxwellian

Viscous coupling based lattice Boltzmann model for binary mixtures

A new lattice Boltzmann model for binary mixtures, which can naturally include both the two-fluid approach and the single-fluid approach, is developed. The model is derived from the continuous kinetic model proposed by Hamel, which independently takes into account self-collisions and cross collisions. The original kinetic model is discussed in order to appreciate that cross collisions realize an internal coupling force, proportional to the diffusion velocity, and an additional coupling effect in the effective stress tensor, proportional to the deformation of the barycentric velocity field. For this reason, Hamel’s model is the natural forerunner of all linearized models based on the two-fluid approach and allows us to describe binary mixtures at different limiting regimes consistently. A discrete lattice Boltzmann model, which recovers the original Hamel’s model with second-order accuracy in both time and space, is proposed. This discrete model can analyze ordinary diffusion, pressure diffusion, and forced diffusion