2020 Volume 76 Issue 2 Pages I_481-I_488
We have numerically examined turbulence transport processes in a convective boundary layer developed above a heated roughness, which mimics an urban canopy with buildings and streets by using large-eddy simulations (LESs). We considered two kinds of convective boundary layer flows: one was without an urban canopy and the other was with an urban canopy. The conditions of convective boundary layer and the configurations of urban canopy were set according to previous studies. An open source CFD code, OpenFOAM was used. The present simulation was based on a LES version with PIMPLEfoam of Open-FOAM, in which the option of RANS scheme was turn off. An one-equation subgrid-scale model was adopted to represent the subgrid-scale diffusion. The central differencing scheme and the 2nd-order back Euler scheme for time integration were chosen. Comparison with the LES for ideal case (without roughness) showed that the roughness enhanced the dissimilarity between thermal (temperature) and flow (velocity) fields, i.e., the vertical profiles of turbulence flux in the thermal fields significantly differed from those in the flow fields. The urban canopy yielded the increases in thermal turbulence fluxes, while the effects of urban canopy on the flow fields were quite small. Such differences were caused by differences of coherence structures developed above the urban canopy between thermal and flow fields, i.e., the thermal and flow fields were controlled by roll and streaky structures, respectively, which were visualized by contours of eiinstantaneous temperature and velocity fluctuations in horizontal planes.
