Influence of blockage ratios in shaping wind dynamics in urban environments

Abstract

Analytical urban canopy models (UCMs) based on Prandtl’s mixing length theory usually ignore the blockage effects caused by building structures, which greatly reduces their accuracy in representing wind flow and turbulence variations within urban boundary layers. This study employs large-eddy simulations under neutral atmospheric stratification to investigate the effects of various blockage ratios on wind dynamics in urban environments. Detailed analyses are conducted on variations in instantaneous flow fields, mean velocity, Reynolds shear stress, and vorticity around buildings. Results indicate that higher blockage ratios restrict airflow above buildings, leading to increased local wind speeds and intensified turbulence within the urban canopy layer. In contrast, lower blockage ratios allow smoother airflow over the canopy, minimizing interactions between the airflow and buildings. Vorticity analysis suggests that higher blockage ratios induce smaller, denser vortices in the wake region, while lower blockage ratios generate longer, more dispersed vortices near the rooftop. Furthermore, this study introduces a modified friction velocity that reduces the bias in velocity by about 17% at a low blockage ratio of 4.44%, resulting in a more accurate representation of the velocity distribution around buildings. As a result, for neutral stratification at a specific moment, known parameters such as atmospheric boundary layer height can be used to predict velocity without additional simulations, thus significantly reducing the computational costs.