Directional effects on urban-canopy drag

Abstract

Understanding the influence of wind direction on building drag is essential for predicting urban climate and assessing wind loads in complex urban environments. This study investigates the wind-directional dependence of building drag over the University of Bristol campus, comprising 110 buildings of diverse shapes and heights, using 24 building-resolved large-eddy simulations under a constant imposed pressure gradient. The overall campus drag coefficient exhibits moderate directional fluctuations, with $20\%$ of buildings contributing approximately $80\%$ of the total drag. In contrast, drag on individual buildings shows substantial variability with wind direction, primarily due to shielding by upstream structures. To quantify this, two dimensionless parameters are introduced: the upstream fetch ratio $L_s/H_s$ and the relative height ratio $H_s/H$. Using thresholds of $L_s/H_s = 5$ and $H_s/H = 1$, buildings are classified into four regimes; those in the near-wake shielded regime experience negligible drag, while those in the far-wake non-shielded regime experience the highest drag. A modified drag coefficient, computed by partially or fully excluding shielded buildings, reduces directional anisotropy and yields an effective frontal area that is more consistent across wind directions.