The worm-LBM, an algorithm for a high number of propagation directions on a lattice Boltzmann grid: the case of phonon transport

TL;DR

The paper introduces the worm-LBM, a novel lattice Boltzmann algorithm with many propagation directions, enhanced by a time-adaptive scheme and grid-MFP correction, to accurately simulate ballistic and diffusive phonon transport without numerical artifacts.

Contribution

It develops the worm-LBM with high-directional propagation, TAS, and grid-MFP correction, overcoming ray effects and anisotropy in phonon transport simulations.

Findings

Accurately simulates ballistic and diffusive phonon transport.

Eliminates numerical smearing and false scattering.

Demonstrates high accuracy in transient cases.

Abstract

The lattice Boltzmann method (LBM) is a numerical approach to tackle problems described by a Boltzmann type-equation, where time, space, and velocities are discretized to describe scattering and advection. Even though the LBM executes advection along a lattice direction without numerical error, its usage in the high Knudsen number regime (ballistic) has been hindered by the ray effect problem (for dimensions greater than 1D). This problem has its origin in the low number of available propagation directions on standard LBM lattices. Here, to overcome this limitation, we propose the worm-lattice Boltzmann method (worm-LBM), which allows a high number of lattice directions by alternating in time the basic directions described within the next neighbor schemes. Additionally, to overcome the velocity anisotropy issue, which otherwise clearly manifests itself in the ballistic regime (e.g. the $\sqrt 2$ higher grid velocity of the D2Q8 scheme along the diagonal direction compared to the axial one), the time-adaptive scheme (TAS) is proposed. The TAS method makes use of pausing advection on the grid, allowing to impose not only isotropic propagation but also arbitrary direction-dependent grid velocity. Last but not least, we propose a grid-mean free path (grid-MFP) correction to correctly handle the aforementioned velocity issue in the diffusive limit, without affecting the ballistic one. We provide a detailed description of the TAS method and the worm-LBM algorithm, and verify their numerical accuracy by using several transient diffusive-ballistic phonon transport cases, including different initial and boundary conditions. Overall, the new, very accurate, and efficient worm-LBM algorithm, free of numerical smearing and false scattering, has the potential to be at the forefront of the numerical solvers to tackle the advective part of different equations in a wide field of applications.