Velocity Adjustment and Passive Scalar Diffusion in and Above an Urban Canopy in Response to Various Approach Flows

Abstract

We used wind-tunnel experiments to investigate velocity-field adjustment and scalar diffusion behaviour in and above urban canopies located downwind of various roughness elements. Staggered arrays of rectangular blocks of various heights H and plan area ratios λ_p were used to model the urban canopies. The velocity field in the roughness sublayer (height ${z \lesssim 2H}$) reached equilibrium at distances proportional to ${\sqrt{L_{\rm c}H}}$ where L _c is the canopy-drag length scale determined as a function of λ_p and the block side length L. A distance of about ${20\sqrt{L_{\rm c}H}}$ was required for adjustment at z = H/2 (in the canopy), and a distance of about ${10\sqrt{L_{\rm c}H}}$ was required at z = 2H (near the top of the roughness sublayer). Diffusion experiments from a ground emission source revealed that differences in upwind roughness conditions had negligible effects on the plume growth near the source (up to a few multiples of L from the source) if the source was located at a fetch F larger than about ${10\sqrt{L_{\rm c}H}}$ from the upwind edge of the canopy. However, at locations farther downwind (more than several multiples of L from the source), upwind conditions had considerable effects on the plume growth. For a representative urban canopy, it was shown that a much larger fetch than required for velocity-field adjustment in the roughness sublayer was necessary to eliminate the effects of upwind conditions on plume widths at 24L downwind from the source.