Wind blowing across two tall buildings sets up a closed, rotating "canyon vortex": shear at roof level drags the top of the air mass along with the ambient wind, and by mass continuity the air near street level must return against the wind, sweeping traffic exhaust toward the upwind building's canyon-facing wall — its leeward wall — while clean air descends at the downwind building's windward wall. This asymmetry (leeward ≫ windward) is the classic, widely-measured street-canyon finding (e.g. DePaul & Sheih 1986).
The velocity field is a closed-form single-cell stream function that satisfies no-penetration at all four canyon boundaries automatically:
u(x,y) = U·sin(πx/W)·[ −f(AR)·cos(πy/H) + (1−f(AR))·0.9 ]
v(x,y) = U·AR·f(AR)·cos(πx/W)·sin(πy/H)
AR = H / W (aspect ratio)
f(AR) is the recirculation-closure fraction: below AR≈0.3 the flow can't close into a vortex at all (isolated roughness flow — pollutants mostly blow straight through and vent out), between 0.3–0.7 a weak, distorted cell forms (wake interference flow), and above AR≈0.7 a single strong vortex fills the canyon (skimming flow) — this is the regime where wall trapping and the leeward/windward asymmetry are strongest. Traffic pollutant is emitted as tracer particles at street level, advected by this field, mixed by a turbulent-diffusion random walk that scales with wind speed, and particles that rise above roof height are counted as vented into the free atmosphere.
- Heatmap — accumulated (decaying) tracer density; red = high concentration.
- Wall strip chart — leeward vs. windward wall concentration over time.
- Aspect ratio — set by Building height ÷ Street width; drag either slider to move between flow regimes.