Numerical evaluation of the effect of twist wind on pedestrian comfort surrounding semi-open structures and influential factors under solar heating

Cities near mountainous terrain, such as Hong Kong, frequently encounter topography-induced twist wind. Twist wind profile (TWP) refers to the vertical variation of wind direction, distinguishing it from conventional wind profile (CWP), and exerting a significant influence on pedestrian wind and thermal comfort. The arcade design has been extensively employed in urban areas due to its capacity for enhancing pedestrian comfort; however, a comprehensive understanding of its performance under realistic solar radiation and the twist wind effect remains elusive. The paper aims to investigate the impact of twist winds on arcade design and analyze wind and thermal comfort by varying parameters such as elevation ratios, arcade width ratios, canyon width, street length and depth ratios (L/D), and asymmetric building heights under CWP and TWP. The steady Reynold-averaged Navier-Stokes model with standard k-ε models with discrete ordinate (DO) radiation were applied. The results demonstrate that the weakened downwash flow caused by the twist wind within the street canyon was insufficient to counteract the buoyancy-driven flow generated by solar radiation. This condition facilitated the escape of heated air from the lower levels of the street canyon, especially in step-down configurations, resulting in a physiological equivalent temperature (PET) value that was on average 1–2 °C lower under TWP compared to CWP. Increasing the arcade width may negatively impact thermal comfort under TWP. The cooling effect of twist winds was more pronounced in shorter street canyons, with the PET increasing by 2 °C under CWP and by 4 °C under TWP as the L/D increased from 1 to 10. The wind velocity in the downstream area decreased fivefold under TWP, whereas it remained relatively stable as the L/D increased from 1 to 10 under CWP.