CFD analysis of natural cross ventilation in high-rise buildings: Impact of surrounding roof shapes and urban density on airflow performance

Abstract This study investigates the natural cross ventilation performance of a high-rise building surrounded by lower-level building blocks with varying roof angles and urban densities, using Computational Fluid Dynamics (CFD). The high-rise building model comprises three stacked building blocks, each designed with inlet and outlet openings, and is evaluated with surrounding buildings having roof angles of 0°, 15°, and −15°. Each building block is 16m in full-scale. The surrounding buildings are arranged according to three planar area ratios (PAR = 0.25, 0.4, and 0.6) to simulate low, medium, and high urban densities, respectively. Ventilation performance is assessed through Dimensionless Flow Rate (DFR), Air-Exchange Efficiency (AEE), pressure loss (ΔCp), and flow resistance (R) at each block of the high-rise building. Results reveal that roof angle and urban density significantly impact ventilation performance, particularly at the lower-level block (L1). For PAR = 0.25, the 0° roof angle configuration (case 1) yields the highest DFR of 0.1915 and AEE of 48.63% in L1, while angled roof configurations reduce these metrics by up to 38%. Similarly, at PAR = 0.4, the −15° angle (Case 6) improves DFR by 11.5% and AEE by 8.63%, while the 15° angle (case 5) reduces DFR by 37% and AEE by 15%, indicating how roof angle can either enhance or impede ventilation. At PAR = 0.6, angled roofs generally enhance ventilation in L1, with the −15° configuration (case 9) increasing DFR by 31.9% and AEE by 13.8% compared to the 0° roof angle. Analysis of pressure loss (ΔCp) and flow resistance (R) shows that R is highest in L1 across all densities due to the sheltering effect of surrounding buildings, while ΔCp and R in upper levels (L2 and L3) remain largely consistent across cases. These findings underscore the critical role of surrounding roof shapes and urban density in optimizing natural ventilation. The results provide valuable insights for designing high-rise buildings in dense urban environments, enhancing ventilation performance, and reducing reliance on mechanical systems, thereby contributing to sustainable urban design.