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

TL;DR

This paper derives the hydrodynamic equations for multi-relaxation time lattice Boltzmann models using a moment-approach without explicitly transforming into moment space, revealing the underlying invariance and flexibility in collision operator choices.

Contribution

It introduces a moment-approach to derive hydrodynamics for MRT lattice Boltzmann models directly in velocity space, explaining invariance despite different orthogonalizations.

Findings

Hydrodynamic limit derived without explicit moment space transformation

Collision operator invariance explains consistent hydrodynamics across MRT implementations

Flexibility in collision matrix choice demonstrated

Abstract

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. Therefore we can explain why MRT implementations in the literature reproduce identical hydrodynamics despite being based on different orthogonalization relations.