Directional effects on urban-canopy drag

arXiv:2606.07625v1 Announce Type: new 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.