Evaluation of polyhedral mesh performance for large-eddy simulations of flow around an isolated building within an unstable boundary layer

The urban heat island effect has been a noticeable issue in recent years. Much research has gone into understanding the thermal field around buildings to help improve human comfort in urban areas. However, the challenges about accuracy and cost of transient simulations of urban thermal environment still remain. This study aimed to evaluate the performance of polyhedral meshes on wind and thermal fields within an unstable boundary layer using large-eddy simulation. A precursor simulation was conducted to determine the wind and temperature fluctuations for the inflow boundary conditions. Through the grid sensitivity test of hexahedral meshes, the setting of 20 grids along the shortest building edge was recommended. The polyhedral cases were conducted following the grid sensitivity test for hexahedral meshes and shortest building edges were discretized into 20 grids. The accuracy of the predicted results obtained by polyhedral meshes was close to that of hexahedral meshes. Polyhedral cases with uniform boundary layer meshes exhibited 20% smaller relative error for standard deviation of temperature near the ground than the polyhedral case without boundary layer meshes. Moreover, the mesh number of the polyhedral mesh was 50% smaller than that of the hexahedral mesh, and its computational time was nearly 50% of that of the hexahedral mesh. This study expands the guidelines of simulations under non-isothermal condition and lays the foundation for further research of thermal environment in urban street and realistic city models to improve residence comfort.