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, $h_{max}$=2.5 $h_{avg}$). {The average canopy height and
packing density were 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 were used to calculate and scale the mean velocity
profiles {within the boundary layer depth, $\delta$}. In the uniform-height
experiment, the high packing density resulted in a `skimming flow' regime with
very little flow penetration into the canopy. This led to a surprisingly
shallow roughness sublayer ($z\approx1.15h_{avg}$), and a well-defined inertial
sublayer above it. {In the heterogeneous-height canopies, despite the same
packing density and average height, the flow features were significantly
different.} {The height heterogeneity enhanced mixing thus encouraging deep
flow penetration into the canopy. A deeper roughness sublayer was found to
exist and extend up to just above the tallest element height (corresponding to
$z/h_{avg} = 2.85$)}, which was found to be the dominant lengthscale
controlling the flow behaviour. {Results points toward the existence of an
inertial sublayer for all surfaces considered herein despite the severity of
the surface roughness ($\delta/h_{avg} = 3 - 6.25$)}. This contrasts with
previous literature.