[AAS15-P08] Development process of convective boundary layer and turbulence characteristic above urban canopy with heterogeneous heating at building and street surface
Keywords:Atmospheric boundary layer, Buoyancy, Coherence structure, LES, Unstable stratification
An accurate description of turbulence thermal transport processes in convective boundary layer developed above an urban canopy can be used for applications such as development of numerical weather prediction models. However, a detailed analysis of the effects of heterogeneity in the thermal conditions of building and street surfaces has not been reported, while many studies have investigated turbulence characteristics in urban canopies.
In this study, we performed large-eddy simulations (LESs) for a thermally convective flow developed above an urban canopy with buildings and streets under a heterogeneous surface heating condition, which mimics heat fluxes with insolation. The conditions of convective boundary layer and the configurations of urban canopy were set according to previous studies by Moeng and Sullivan (1994), Kanda (2006). An open source CFD code, OpenFOAM, was used. The present simulation was based on a LES version with PIMPLEfoam of OpenFOAM, in which the option of RANS 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.
We discussed the vertical profiles of turbulence fluxes in thermal and flow fields of the convective boundary layer. The urban canopy yielded the increased in thermal turbulence fluxes, while the effects of urban canopy on the flow fields were quite small, suggesting that the urban canopy enhanced a dissimilarity between thermal and flow fields. Such a dissimilarity was 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 instantaneous temperature and velocity fluctuations in horizontal planes.
In this study, we performed large-eddy simulations (LESs) for a thermally convective flow developed above an urban canopy with buildings and streets under a heterogeneous surface heating condition, which mimics heat fluxes with insolation. The conditions of convective boundary layer and the configurations of urban canopy were set according to previous studies by Moeng and Sullivan (1994), Kanda (2006). An open source CFD code, OpenFOAM, was used. The present simulation was based on a LES version with PIMPLEfoam of OpenFOAM, in which the option of RANS 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.
We discussed the vertical profiles of turbulence fluxes in thermal and flow fields of the convective boundary layer. The urban canopy yielded the increased in thermal turbulence fluxes, while the effects of urban canopy on the flow fields were quite small, suggesting that the urban canopy enhanced a dissimilarity between thermal and flow fields. Such a dissimilarity was 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 instantaneous temperature and velocity fluctuations in horizontal planes.