Effect of Time Steps on Accuracy of Indoor Airflow Simulation Using Lattice Boltzmann Method

Abstract:

Lattice Boltzmann method-based large-eddy simulation （LBM-LES） is a new method to solve turbulence problems in recent decades. However， improper time step settings may affect the simulation accuracy of LBM-LES. This paper first analyzed and summarized the impact of time step δt on the results of LBM-LES，theoretically. An oversized δt will cause compressibility error in the velocity field， while a too small δt can lead to the over-relaxation colliding mode， causing the numerical oscillation of velocity field. Subsequently， LBM-LES simulations of an isothermal indoor airflow case were conducted to discuss these errors quantitatively. The results show that a large δt leads to a sharp density change， and the velocity field in the regions where the Mach number （M） in the lattice Boltzmann unit exceeds 0.3 showing that there are obvious compressibility errors. Meanwhile， a too-small δt causes apparent numerical oscillations of both time-averaged and fluctuating velocities. This phenomenon is more significant when the grid resolution is higher. Therefore，it is suggested that δt should be small enough to ensure M<0.3 in the maximum velocity regions， based on which， a larger δt should be utilized to prevent numerical oscillations.