This paper investigates the flow through and over two-dimensional rectangular roughness
elements, arranged in a building-street canyon geometry through a series of
experiments. Geometries of different packing densities of the roughness elements
(λp) were examined and the packing density values ranged from λp = 0.30 to 0.67.
The purpose of the work is: (i) to investigate the flow physics observed both at the
boundary layer scale as well as at the scale within the roughness elements for a range
of packing densities, (ii) to deduce parameterizations of the adjusted rough boundary
layer and their variation with a change in the packing density, and (iii) given
a particular interest in and application to the urban atmosphere, a final aim at the
roughness-element scale is to deduce the variation of the breathability with the packing
density variation. Particle image velocimetery measurements of the velocity flow
field as well as the turbulent kinetic energy and the Reynolds Stress (within and up
to well-above the street canyons) were conducted. The results reveal qualitative flow
features as well as features of the adjusted boundary layer structure—in particular the
roughness and inertial sublayers, which can be associated with the surface roughness
length, zero-plane displacement thickness, and the friction velocity. The lowest friction
velocities are exhibited in the geometries with the highest- and lowest packing
densities while the maximum friction velocities are observed in the medium-packed
geometries. The exchange processes and breathability at the level of the roughness
elements top were characterized and quantified by a mean exchange velocity. The
results show that unlike friction velocity, the normalized exchange velocity (over
the mean bulk velocity) for the most dense and sparse geometries differ by more
than 80%, with the denser-packed geometries exhibiting lower exchange velocities;
this is shown to be related with the thickness of the developed roughness sublayer.