Shear Stress and Fluid-Wall Interaction Force in LBM Simulations of Hydrodynamic and MHD Flows

TL;DR

This paper analyzes shear stress and fluid-wall interaction forces in lattice Boltzmann simulations of hydrodynamic and MHD flows, clarifying artefact effects and proposing consistent computation methods.

Contribution

It provides a detailed Chapman-Enskog analysis of external force implementation in LBM, clarifies artefact term effects, and offers a consistent approach for fluid-wall interaction force calculation.

Findings

Artefact terms are negligible for shear stress calculations.

Fluid-wall interaction force can be computed via a momentum exchange method.

Extra terms affect the global force balance and should be removed.

Abstract

Chapman-Enskog analysis of the lattice Boltzmann method (LBM) is adopted to recover the Navier-Stokes (N-S) equation for the magnetohydrodynamic (MHD) flows driven by external body forces other than the induced Lorentz force. Various numerical schemes are discussed for the implementation of external body forces, leading to different artefact terms. An order-of-magnitude analysis is provided to demonstrate that the artefact terms are negligible in calculating the shear stress and strain rate tensors, which keeps the study of fluid-wall interaction and the implementation of large eddy simulation (LES) simple. This clarifies the confusion in the literature, where both considering and not considering the artefact terms have been adopted without discussion. Additionally, the fluid-wall interaction force can be computed as the intuitive momentum exchange rate using a special distribution function after a half-step propagation, and consistency is obtained with the formula derived using the Chapman-Enskog analysis. The momentum flux tensor for moving boundaries is uncovered as the cause of violating Galilean invariance and the Maxwell stress tensor for Lorentz force is also contained in this intuitively calculated interaction force for MHD flows due to the modification of equilibrium distribution function. Both extra terms should be removed for an interaction force associated with the hydrodynamic pressure and viscous shear stress. The implication of extra terms for the global balance of different physical forces is discussed.