Effects of wall function model in lattice Boltzmann method-based large-eddy simulation on built environment flows

摘要

Bounce-back boundary (BB), the popular wall boundary in lattice Boltzmann method (LBM), corresponds to the no-slip boundary and does not provide an accurate shear drag on walls in some cases. This study discusses a new wall boundary?s effectiveness with a wall function in LBM-based large-eddy simulation (LBM-LES) to predict indoor and outdoor flows in the built environment. ?Wall-function bounce? boundary (WFB, using Spalding?s law) is conducted and compared with BB. Two validation cases of indoor convection in a 9 m ? 3 m ? 3 m room and of turbulent flow around a single 1:1:2 building are employed. Results show that BB provided a lower shear drag accuracy on walls than Spalding?s law in both indoor and outdoor cases, particularly when using coarse grids. WFB compensated for this and yielded a more accurate shear drag. WFB yielded an overall simulation accuracy similar to BB with half-length grids in indoor and outdoor cases and achieved grid independence using a coarser grid resolution. In indoor case, WFB improved the accuracies of both time-averaged and fluctuating velocities in both near-wall and off-wall regions. In outdoor case, LBM-LES with WFB obtained similar timeaveraged flow structures to those with finer-grid BB. WFB also improved the accuracies of the time-averaged velocity and turbulent kinetic energy near roof and ground. This study indicates that a boundary with a wall function (e.g., WFB) is important for LBM-LES in built environment flows because it can yield a better simulation accuracy utilizing coarser grids and reduce the demands of computation.