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

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. © 2022, Editorial Department of Journal of Tongji University. All right reserved.