On the Flow Past an Array of Two-Dimensional Street Canyons Between Slender Buildings

The flow above an idealized, two-dimensional series of parallelepipedal buildings is examined with the aim of investigating how the building width ( W ) to height ( H ) aspect ratio affects the turbulence in the roughness sublayer and the ventilation of the underlying street canyons. We compare the case of buildings with a squared section ( AR B = W / H = 1.0) with a configuration with slender buildings ( AR B = 0.1) both in the case of unit canyon width ( D ) to height ( H ) aspect ratio ( AR C = D / H = 1) and in the case of AR C = 2. The former corresponds to skimming flow and the latter to wake-interference regime. Measurements are performed in a water channel, measuring velocity on a vertical mid-plane using a particle-image velocimetry technique. The mean flow, its second-order turbulence statistics, the exchange fluxes, and the integral time scales are investigated, with results showing that slender buildings enhance turbulence production and yield larger integral time scales in the region just above the building roof. Namely, in the skimming-flow and wake-interference regimes, the maximum vertical velocity variance is more than doubled and increased by 50%, respectively. The combined analysis of the turbulence production fields and the snapshots of the flow during sweep and ejection events demonstrate that the shear layer between the canyon and the external flow is significantly more unstable with slender buildings, mainly because the damping effect of the vertical velocity fluctuations from the flat roof of the upwind building is substantially missing. Consequently, a larger (downstream) portion of the interface is prone to the direct interaction of the external flow structures. The higher turbulence intensity promotes the ventilation at the canyon interface, which is increased by a factor of two in the skimming-flow regime and a factor of 1.26 in the wake-interference regime. In summary, the present experiments show that the effect of the reduced building aspect ratio is particularly significant when the urban canopy consists of narrow canyons. The result is of interest since narrow street canyons are typically bounded by slender buildings in the urban texture of the old European city centres.