  A neutral boundary layer was generated in the laboratory to analyze the mean
velocity field and the turbulence field within and above an array of
two-dimensional obstacles simulating an urban canopy. Different geometrical
configurations were considered in order to investigate the main characteristics
of the flow as a function of the aspect ratio (AR) of the canopy. To this end,
a summary of the two-dimensional fields of the fundamental turbulence
parameters is given for AR ranging from 1 to 2. The results show that the flow
field depends strongly on AR only within the canyon, while the outer flow seems
to be less sensitive to this parameter. This is not true for the vertical
momentum flux, which is one of the parameters most affected by AR, both within
and outside the canyon. The experiments also indicate that, when (i.e. the
skimming flow regime), the roughness sub-layer extends up to a height equal to
1.25 times the height of the obstacles (H), surmounted by an inertial sub-layer
that extends up to 2.7 H. In contrast, for (i.e. the wake-interference regime)
the inertial sub-layer is not present. This has significant implications when
using similarity laws for deriving wind and turbulence profiles in canopy
flows. Furthermore, two estimations of the viscous dissipation rate of
turbulent kinetic energy of the flow are given. The first one is based on the
fluctuating strain rate tensor, while the second is related to the mean strain
rate tensor. It is shown that the two expressions give similar results, but the
former is more complicated, suggesting that the latter might be used in
numerical models with a certain degree of reliability. Finally, the data
presented can also be used as a dataset for the validation of numerical models.
