Numerical Simulation of Atmospheric Turbulence within and above a Cubical Canopy

This study investigated the relationship between the instantaneous flow structures within and above the urban canopy, which is idealized as an array of cubes, under atmospheric conditions by using the large eddy simulation (LES) model. Urban atmospheric boundary layer can be subdivided into three layers of different scales: convective mixed layer (~1000m), inertial sublayer (~100m) and roughness sublayer (~10m for urban case). To simulate the flow in these layers simultaneously, we employed a large domain size (2560m x 2560m x 1796m) with a fine grid spacing (dx=dy=dz=2.5m) to explicitly resolve the flow around the roughness cubes whose height is 40m (=H). The cubes are aligned regularly with the plan area index of 0.25. The mean flow regime in the building cavity of this setting is classified into the skimming flow regime (Oke 1987). The mixed layer convection is invoked by sensible heat flux released from the roof and ground surfaces with a constant value (0.1 K m s-1). The initial potential temperature gradient is 0.08 K m-1 below 800 m and 0.74 K m-1 above 800 m to set the capping inversion. The Rayleigh damping layer is set at ** m to absorb gravity waves. By comparing the horizontal distributions of the instantaneous u, v, w and T structures at heights of 2H, H, 0.5H and 0.125H, we confirmed that the flow structures within and above the canopy layer of this setting are spatially well correlated. The canopy layer flows seem to be controlled by very large coherent streaks of low and high momentum regions. The spatial coherency of these structures extend much larger than the size of cubes. The instantaneous flow structure within a building cavity is not a simple vortex, as may be characterized by its mean structure, but more complex, and looks to follow some semi-periodic modes. We extracted ‘flushing' and ‘cavity eddy' events as conspicuous modes of the cavity flow. Flushing is the upward motion occurring at the entire cavity. Cavity eddy is the vortical motion similar to the mean vortex structure in the cavity. We extracted these events based on numerical criteria. These flushing and cavity eddy events are 10 and 20 % of the whole horizontal domain respectively, however, their contributions to the heat and momentum transports within the canopy layer were much larger than the rate of the occurrences. The locations of the occurrences of these two events were related with the locations of the coherent turbulence structure within the inertial sublayer.