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Urbanization has intensified the urban heat island phenomenon, a critical concern for human habitation in recent decades. Analyzing flow fields around building arrays is vital for mitigating issues such as heatstroke and air pollution, thereby enhancing human comfort in urban environments. However, challenges in accuracy and computational efficiency persist in simulating urban thermal environments. This study evaluates how boundary layer meshes in polyhedral meshes affect wind and temperature distributions in neutral and unstable boundary layers, utilizing large-eddy simulation (LES). A total of ten cases were examined, including eight with local mesh refinement and two without. The study demonstrates that local mesh refinement is an effective strategy to reduce mesh count by approximately 60 %, thereby reducing computational time by 60 % using the same computational configuration. The performance of the polyhedral meshes was found to be comparable to that of the hexahedral meshes. For the mean velocity and temperature, the discrepancies between the hexahedral and polyhedral cases were within 10 %, while for the second-order statistics, the discrepancies were within 20 %. Within the neutral boundary layer, boundary layer meshes on the ground and building surfaces had minimal impact on the wind field. However, these meshes enhanced the accuracy of low percentile wind speed estimations by approximately 20 % within the unstable boundary layer. Moreover, this study lays the foundation for research on the thermal environment in actual urban areas and contributes to the guidelines for LES with non-isothermal inflow conditions. • Local mesh refinement reduced mesh count while maintaining acceptable accuracy. • Boundary layer meshes had minimal impact on velocity fields in neutral layers. • Boundary layer meshes yield poor results near building corners in neutral layers. • Boundary layer meshes generally improved the result accuracy in unstable layers. [ABSTRACT FROM AUTHOR]

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. • LES well reproduced the wind flow around a building under unstable condition. • LES with 20 grids per building width achieved comparable accuracy to experiment. • Boundary layer meshes reduced 20% relative error on temperature results. • Polyhedral meshes saved 50% computational resources compared to hexahedral meshes. [ABSTRACT FROM AUTHOR]