Effect of fetch on street canyon flow field and pollutant dispersion

Many building blocks in microdistricts consist of finite rows of similar street canyons, exhibiting flow fields and pollutant dispersion patterns that vary with fetch—the distance from the first building row to the target canyon. The intermediate state of street canyon flows, situated between the isolated state (with no fetch) and the fully developed state (with infinite fetch), is considered more representative of real-world scenarios, especially for deep street canyons. This study examines the effect of fetch on flow dynamics and pollutant dispersion in 50 consecutive street canyons with aspect ratios (H/W, building height to street width ratio) ranging from 1 to 5, using Reynolds-averaged Navier–Stokes (RANS) simulations. Results show that fetch significantly alters the velocity field, elevating and decelerating primary recirculation in the first canyons. Higher aspect ratios amplify the effect of fetch. In deep street canyons with H/W = 3–5, the vortex merge occurs: multiple vortices form in the first canyon and merge into a single primary vortex as the flow approaches fully developed with fetch increase. This phenomenon makes fully developed flows demonstrate more efficient pollutant mixing and uniform distribution, reducing air pollution within deep street canyons with H/W = 3–5. A new index, the marginal rate of change, quantifies the proximity of street canyons to the fully developed state, reached at approximately canyon 30 (fetch = 60W) for H/W = 1–5. Since uniform finite street canyon flows mostly remain in intermediate transitional states, fetch is critical for urban design, operation, and management.