Neighbourhood-scale wind characteristics in a Nanjing heterogeneous residential area: An aerodynamic drag perspective

Characterizing wind drag amidst heterogeneous urban textures and directional effects is a fundamental necessity that provides the basis for urban porous media models. This work investigates a realistic residential district (170 ha; 252 slab-shaped or irregular buildings) in Hexi New Town, Nanjing, China. Computational fluid dynamics simulations across 16 directions are performed utilizing the 3D steady RANS equations. Canopy-averaged velocity, aerodynamic drag, and volumetric drag coefficients (Cd) roses are analyzed for each neighborhood. Equations governing the drag-velocity relationship are adopted to reconstruct velocities, defined here as drag-VRM method, and are also applied to estimate velocity time series. Results indicate that, considering diverse shelter effects at district and neighborhood levels, the examined high-rise (up to 57 m) low-to-mid density neighborhoods exhibit normalized Cd* values ranging from 0.05 to 1. The Cd roses show significant anisotropic features, yielding an average coefficient of variation of up to 52%. The drag-VRM reproduces slightly lower average velocities, though larger discrepancies arise for irregular layouts and low ambient wind conditions. Further discussion reveals that the 300 m urban tile zoning method offers morphological and aerodynamic characterizations comparable to the neighborhood approach. Notably, the relationship between Cd* and frontal area density (λf) for these realistic zones deviates from patterns in idealized urban configurations, underscoring the necessity of verifying drag models when applied to complex, local urban contexts.