Urban-canopy airflow dynamics: A numerical investigation of drag forces and distribution for generic neighborhoods, and their relationships with breathability.

Thorough investigations of urban-canopy drag primarily stemming from pressure drag on building surfaces are necessary given the turbulent flows within complex urban areas. Moreover, a gap persists regarding the relationships between canopy drag and breathability. Therefore, this work delves into the canopy-layer airflow dynamics for generic urban neighborhoods by performing three-dimensional Reynolds-Averaged Navier-Stokes simulations. A total of 32 subcases are examined, encompassing uniform- and varying-height and diverse plan area densities (λ, categorized into groups of sparse: 0.0625/0.067, medium: 0.23/0.25, and dense: 0.53/0.56). Results for the drag distribution highlight the windward-row shelter effect for the medium and the dense, local shelter by taller buildings, and distinct shapes of sectional drag forces (F). Local velocity and mean age of air are found strongly positively and negatively correlated to F, respectively, with distinct slopes in relation to λ. For the uniform-height, the normalized bulk drag (F, referred to as drag coefficient in literature) peaks for the medium with wake-interference regime; F demonstrates a maximum increase of over two times with height variation; moreover, F for varying-height groups exhibits a marked increase from the sparse to the medium, while remaining comparable values for the dense. The frontal area averaged drag (F) exhibits a decreasing trend against λ across all cases. Further, F exhibits strong correlations with λ and porosity, and with bulk ventilation indices such as spatially averaged velocity, air change rate, and normalized net escape velocity. Throughout the 'suburban-urban-suburban' canopy, medium neighborhoods exerting larger drag cause greater streamwise outdoor pressure drops and flow reductions compared to the sparse. However, dense neighborhoods with lower drag exhibit even larger pressure losses, which should be carefully scrutinized. The findings can inform urban planners in designing more aerodynamically efficient neighborhoods and guide strategies for improving air quality within urban environments.