  Large-eddy simulation (LES) of a turbulent flow through an array of
building-like obstacles is an idealized model to study transport of
contaminants in the urban atmospheric boundary layer (UABL). A reasonably
accurate LES prediction of turbulence in such an UABL must resolve a
significant proportion of the small but energetic eddies in the roughness
sublayer, which remains prohibitive even though computational power has been
increased significantly. In this article, we present a large-eddy simulation
methodology to study turbulence in UABLs, where the turbulence closure is based
on coupling the eddy viscosity method with the canopy stress method. Unlike the
classical Smagorinsky model that considers only the strain portion of the
velocity gradient tensor, we consider both the strain tensor and the rotation
tensor to compute the eddy viscosity. This allows us to dynamically adapt the
rate of energy dissipation to the scales of the energetic eddies in the
roughness sublayer. Without employing a mesh conforming to the urban roughness
elements, the effect of such solid bodies are represented in the LES model
through a canopy stress method in which the loss of pressure and the sink of
momentum due to the interaction between eddies and roughness elements are
parameterized using the instantaneous velocity field. Simulation results of the
proposed canopy stress method is compared with that of a conventional
Computational Fluid Dynamics (CFD) method employing a block-structured mesh
conforming around the roughness elements. For urban flow simulations, the
results demonstrate that the proposed canopy stress model is accurate in
predicting vertical profiles of mean and variance, as well as the temporal
intermittency of coherent structures.
