Abstract
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~hbox {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 approx 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.
Highlights
The roughness sublayer (RSL) which acts from the wall surface up to a certain point above the roughness elements
The inertial sublayer (ISL), which covers a region above the RSL (Raupach et al 1991)
Wind-tunnel experiments were conducted at the University of Surrey on four dense and tall (δ/havg ≈ 3) urban arrays; two canopies were of uniform height and two of varied height, while the average height was kept fixed at havg = 80 mm in both cases
Summary
Understanding flow around urban environments is becoming increasingly important as cities and their populations grow in size (Department of Economic and Social Affairs 2019). Surface energy balance models have recently improved, accurate models for aerodynamic parameters are still poor, for non-conventional roughness geometries—e.g., tall canopies with heterogeneous height, with urban flows that remain poorly described by both theoretical and empirical models (Kanda et al 2013).
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