Derivation of Hydrodynamics for Multi-Relaxation Time Lattice Boltzmann using the Moment Approach

A general analysis of the hydrodynamic limit of multi-relaxation time lattice Boltzmann models is presented.We examine multi-relaxation time BGK collision operators that are constructed similarly to those for the MRT case,however,without explicitly moving into a moment space representation.The corresponding’moments’are derived as left eigenvectors of said collision operator in velocity space.Consequently we can,in a representation independent of the chosen base velocity set,generate the conservation equations.We find a significant degree of freedom in the choice of the collision matrix and the associated basis which leaves the collision operator invariant.We explain why MRT implementations in the literature reproduce identical hydrodynamics despite being based on different orthogonalization relations.More importantly,however,we outline a minimal set of requirements on the moment base necessary to maintain the validity of the hydrodynamic equations.This is particularly useful in the context of position and time-dependent moments such as those used in the context of peculiar velocities and some implementations of fluctuations in a lattice-Boltzmann simulation.