A numerical study on the issue of Reynolds independence of flow and dispersion within isolated street canyons

Achieving Reynolds number independence is a prerequisite for conducting scale model experiments. This paper presents a numerical model of the two-dimensional isolated street canyon, utilizing the validated reliable k-epsilon turbulence model based on wind tunnel experimental data, to investigate the influence of approaching wind profiles (α) and windward building width (W_B) on achieving Reynolds number independence in the isolated street canyon and compare these characteristics with those observed in urban street canyon. Quantitative criteria, including revised relative change ratios (RRCs) ≤ 20 % and dimensionless concentration relative difference (K_RD) ≤ 5 %, were employed in conjunction with velocity, concentration contours, and streamlines to determine the corresponding critical Reynolds numbers. The results suggest that variations in approaching wind profiles primarily influence the flow field structure within the isolated street canyon after achieving Reynolds number independence, particularly impacting the scale of the counterclockwise vortex. However, these variations do not significantly alter the critical Reynolds number (Re_crit) required for achieving Re-independence, which is estimated to be Re_crit = 2.2 × 10⁴ for the isolated street canyon. Varying W_B affects the formation of counterclockwise vortex within the isolated street canyon. A reduction in W_B facilitates its formation and accelerates reaching Re-independence. The Re_crit is found to be linearly proportional to W_B variation. The flow field structure in urban street canyons remains relatively stable, with a recommended Re_crit = 3.1 × 10⁴. When W_B is 18 cm, the Re_crit for the isolated street canyon aligns with that of the urban street canyon.