Volumetric drag coefficients for generic urban configurations: Insights from canopy flow analysis

Alternative drag approaches for representing unresolved buildings were proposed in literature for computational fluid dynamics (CFD) simulation of macroscopic urban airflow. As a contribution, the present work derives the volumetric drag coefficient ($C_d^{*}$) through canopy drag and velocity analysis and provides appropriate correlations for $C_d^{*}$against urban morphological parameters. A total of 72 cases across various urban configurations are investigated, categorized by building typology, horizontal layout, height variability, and plan area density (λ_p, from 0.0625 to 0.57). Reynolds-Averaged Navier-Stokes (RANS) simulations with periodic boundary conditions are performed to model fully developed flows. Results for the normalized drag force and superficial velocity and their relations with λ_p are evaluated. Subsequent evaluation of the profiles for the sectional coefficients ($C_d^{*}\left(Z\right)$) reveals four distinct types with variations in uniform-height cases and combinations in varying-height cases. A throughout correlations analysis, facilitated by data transformation, identifies the straightforward relations between $C_d^{*}$ and frontal area density (λ_f) and tortuosity (τ). The followed stepwise regression provides a recommended formula for $C_d^{*}$, demonstrating a proper fit with the simulated values. These findings facilitate the understanding and appropriate estimation of $C_d^{*}$ and $C_d^{*}\left(Z\right)$, promoting the application of macroscopic turbulence models, for neighborhood-scale wind and air quality studies.