Large-eddy simulation of atmospheric boundary layer flows over surface transitions from flat to idealized urban terrains

Accurate prediction of atmospheric boundary layer (ABL) flow and its interaction with urban surfaces is critical for understanding the transport of momentum and scalars within and above cities. This, in turn, is essential for predicting the local climate and pollutant dispersion patterns in urban areas. Large-eddy simulation (LES) explicitly resolves the large-scale turbulent eddy motions and, therefore, can potentially provide improved understanding and prediction of flows inside and above urban canopies. This study focuses on the development and the use of an LES framework to simulate a turbulent boundary layer flow through idealized urban canopies. The LES framework is first validated with wind tunnel experimental data of a turbulent boundary layer flow past a uniform array of cubes. Good agreement between the simulation results and the experimental data are found. The LES framework is then used to simulate ABL flows over surface transitions from a flat homogeneous terrain to aligned and staggered arrays of cubes (with height h) with five different frontal area densities (l_f) ranging from 0.028 to 0.25. An internal boundary layer (IBL) is identified above the arrays and no significant difference in the depth of the IBL among different cases is observed. Within the arrays, the flow is found to adjust quickly and shows similar structure of the wake of the cubes after the second row. At a downstream location where the flow immediately above the cube array is already adjusted to the surface, the spatially averaged velocity is found to have a logarithm profile. The values of the displacement height (d) are found to be quite insensitive to the canopy layout (aligned vs. staggered) and increase roughly from 0.65h to 0.9h as the l_f increases from 0.028 to 0.25. Relatively larger values of the aerodynamic roughness (z₀) are obtained for the staggered arrays, compared with the aligned cases, and a maximum value of z₀ is found at l_f = 0.111 for both configurations. By explicitly calculating the drag exerted by the cubes on the flow and the drag coefficients of the cubes using our LES results, and comparing the results with existing theoretical expressions, we show that the larger values of z₀ for the staggered arrays are related to the relatively larger drag coefficients of the cubes for that configuration compared with the aligned one. The effective mixing length (l_m) within and above different cube arrays are also calculated and a local maximum of l_m within the canopy is found in all the cases, with values ranging from 0.2h to 0.4h. These patterns of l_m are different from those used in existing urban canopy models.