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.