Urban Boundary Layers Over Dense and Tall Canopies

Wind-tunnel experiments were carried out on four urban morphologies: two tall canopies with uniform height and two super-tall canopies with a large variation in element heights (where the maximum element height is more than double the average canopy height, h(max) = 2.5h(avg)). The average canopy height and packing density are fixed across the surfaces to h(avg) = 80 mm, and lambda(p) = 0.44, respectively. A combination of laser Doppler anemometry and direct-drag measurements are used to calculate and scale the mean velocity profiles with the boundary-layer depth delta. In the uniform-height experiment, the high packing density results in a 'skimming flow' regime with very little flow penetration into the canopy. This leads to a surprisingly shallow roughness sublayer (depth approximate to 1.15h(avg)), and a well-defined inertial sublayer above it. In the heterogeneous-height canopies, despite the same packing density and average height, the flow features are significantly different. The height heterogeneity enhances mixing, thus encouraging deep flow penetration into the canopy. A deeper roughness sublayer is found to exist extending up to just above the tallest element height (corresponding to z/h(avg) = 2.85), which is found to be the dominant length scale controlling the flow behaviour. Results point toward the existence of a constant-stress layer for all surfaces considered herein despite the severity of the surface roughness (delta/h(avg) = 3 - 6.25). This contrasts with the previous literature.

Automated summary

In wind-tunnel experiments comparing uniform-height urban morphologies with heterogeneous-height ones, it was found that despite the same packing density and average height, the flow features were significantly different. Height heterogeneity enhanced mixing, encouraging deeper flow penetration into the canopy. This study suggests the existence of a constant-stress layer for all surfaces considered, despite differences in surface roughness severity.