Numerical investigations on Re-independence for the turbulent flow and pollutant dispersion under the urban boundary layer with some experimental validations

Abstract

This paper numerically studied the Re-independence of turbulent flow and pollutant dispersion around five scaled building models. The RANS-based CFD model, hereafter called numerical wind tunnel (NWT) model, was validated by wind-tunnel experiments and applied to investigate the flow-structure independence on building Reynolds number (ReH) and roughness Reynolds number (Rer). The ratio of relative change (RRC) was used to evaluate the Re-independence with 5% selected as the threshold. The results show that the critical ReH (ReH,crit) is strongly affected by the building structure, particularly the building height. However, the value of the critical Rer (Rer,crit) in the pedestrian level plane changes very little with the different building models, which is suggested to be 25 in terms of the RRC criterion. Distributions of dimensionless velocity and concentration in the pedestrian level planes validated this suggested value. For the RANS turbulence models tested in this paper (high Reynolds number model and low Reynolds number model) the predicted value of Rer,crit is independent on the model. This work improves the understanding of Re-independence in the investigation of the flow and pollutant dispersion by using wind-tunnel experiment.