Simulating the urban canopy’s impact on wind-driven natural ventilation

The urban canopy affects wind in complex ways, making it challenging to predict wind-driven natural ventilation and cooling in buildings. Using large eddy simulations of coupled outdoor and indoor airflow, we study how the surrounding urban canopy and wind angle influence ventilation rates through four ventilation configurations: cross, corner, dual-room and single-sided. Flow visualisations demonstrate how both large-scale flow patterns and local interference effects can influence ventilation rates by 50 %–85 %. In general, lower density canopies give higher ventilation rates, and wind angles that align with a direct path between two openings also lead to higher ventilation rates. However, interference effects from surrounding buildings can significantly change the local wind speed and direction, thus also changing ventilation rates. The magnitude of these interference effects depends on both the wind angle and surrounding building geometries. The effect of wind angle is less pronounced in a higher density canopy, where the urban canopy geometry more strongly guides the flow. The results demonstrate that the canopy’s effect on ventilation rates is much more complex than those suggested by existing natural ventilation parametrisations.