Investigation of the Flow Structure in Step-Up Street Canyons—Mean Flow and Turbulence Statistics

Abstract

A step-up street canyon is a characteristic urban element composed of two buildings in which the height of the upwind building ( $$H_\mathrm{u}$$ ) is less than the height of the downwind building ( $$H_\mathrm{d}$$ ). Here, the effect of canyon geometry on the flow structure in isolated step-up street canyons is investigated through isothermal wind-tunnel measurements. The measurements were acquired along the vertical symmetry plane of model buildings using two-dimensional particle image velocimetry (PIV) for normal approach flow. The building-height ratios considered were: $$H_\mathrm{d}/ H_\mathrm{u} \approx 3$$ , and $$H_\mathrm{d}/ H_\mathrm{u} \approx 1.67$$ . For each building-height ratio, the along-wind lengths (L) of the upwind and downwind buildings, and the street-canyon width (S) were kept constant, with $$L \approx S$$ . The cross-wind widths (W) of the upwind and downwind buildings were varied uniformly from $$W/S \approx 1$$ through $$W/S \approx 4$$ , in increments of $$W/S \approx 1$$ . The objective of the work was to characterize the changes in the flow structure in step-up canyons as a function of W/S, for fixed L, S, and $$H_\mathrm{d}/H_\mathrm{u}$$ values. The results indicate that the in-canyon flow structure does not vary significantly for $$H_\mathrm{d}/H_\mathrm{u} \approx 3$$ for the W/S values considered. Qualitatively, for $$H_\mathrm{d}/H_\mathrm{u} \approx 3$$ , the upwind building behaves as an obstacle in the upwind cavity of the downwind building. In contrast, the flow patterns observed for the $$H_\mathrm{d}/H_\mathrm{u} \approx 1.67$$ configurations are unique and counter-intuitive, and depend strongly on building width (W/S). For $$W/S \approx 1$$ and $$W/S \approx 2$$ , the effect of lateral flow into the canyon is so prominent that even the mean flow patterns are highly ambiguous. For $$W/S \approx 3$$ and 4, the flow along the vertical symmetry plane is more shielded from the lateral flow, and hence a stable counter-rotating vortex pair is observed in the canyon. In addition to these qualitative features, a quantitative analysis of the mean flow field and turbulence stress field is presented.