Modulation of dispersion pathways in a unit-aspect-ratio vegetation–building canyon: Impacts of vegetation permeability

Replacing the upwind building of a generic street canyon with a vegetation barrier of identical dimensions, thus preserving the overall canyon geometry, provides an idealised yet insightful framework for examining the aerodynamic role of vegetation in pollutant dispersion. Despite its geometric simplicity, this configuration enables a systematic isolation of how an upwind vegetation barrier modulates ventilation efficiency and, in particular, constrains pollutant backflow or upstream diffusion. Using large-eddy simulations across a range of leaf area densities (LAD), this study reveals that the vegetation barrier substantially weakens the near-wall shear and suppresses the formation of secondary corner vortices. In particular, the small recirculating vortex typically observed in the bottom-left corner of canonical building–building (B–B) canyons is strongly weakened and undergoes a reversal of its mean rotation at low LAD (0.5m²m⁻³), indicating a fundamental reorganisation of the near-ground flow topology induced by the vegetation barrier. This deformation disrupts the conventional canyon recirculation and redirects dispersion pathways, weakening local pollutant trapping while enhancing overall lateral leakage across the porous barrier. The coherent eddy structures exhibit enlarged spatial correlation scales, roughly twice that in the canonical B–B canyon, suggesting a shift toward turbulence-dominated ventilation. Contrary to the assumption that vegetation with extremely high LAD behaves as an impermeable wall, the results show that the flow and dispersion characteristics of vegetation–building (V–B) canyons do not asymptotically converge to those of pure B–B canyons. The findings provide guidance for assessing vegetation placement to modulate near-building air quality and ventilation pathways. © 2026 Elsevier GmbH.