This paper numerically and experimentally studied the Reynolds number independence (Re-independence) of turbulent flow and pollutant dispersion in urban areas. The concept of numerical wind tunnel (NWT) is proposed and validated by prototype wind-tunnel experimental measurements. A new physical quantity: the ratio of relative change, RRC, is proposed to investigate the Re-independence quantitatively. For the given street canyon, numerically predicted variations of RRC vs. building Reynolds number (Re-H) from three k-epsilon turbulence models agree well with each other, and the variation trend shows that there exist two flow regimes in the range of Re-H studied: flow in low Re-H region is strongly affected by Re-H, while that in higher Re-H region is weakly affected by Re-H, giving a strong support to the concept of Re-independence of turbulent flow. A criterion of RRC less than 5% is suggested to determine the value of the critical Reynolds number. For the street canyon studied, such determined critical building Reynolds number (Re-H,Re-crit) is 3.3 x 10(4). Examinations of dimensionless velocity contours, local velocity vectors, and concentration contours demonstrate the feasibility of the suggested critical Reynolds number. (C) 2014 Elsevier Ltd. All rights reserved.