A study of flow fields in step-down street canyons

Abstract

A step-down street canyon is a street canyon in which the upwind building height ($H_{u}$) is greater than the downwind building height ($H_{d}$) ($H_{u}>H_{d}$). Here, the effect of downwind building height and canyon width on the flow structure in isolated step-down canyons is investigated through wind-tunnel measurements. The measurements were acquired along the vertical symmetry plane of the model buildings using two-dimensional particle image velocimetry for normal approach flow. For the present study, $H_{u}$ was kept constant at $120$ mm, and $H_{d}$ was increased in increments of $\approx$0.08$H_{u}$, to span the range: $0.08\le H_{d}/H_{u}\le 1$. The configuration $H_{d}/H_{u} \approx 1$ corresponds to a deep canyon. The footprints of the buildings were square, with the widths ($W$) and lengths ($L$) being, $W (= L) \approx 32$ mm. Four different street-canyon widths ($S$) were considered, with $S/W \approx 2.5, 2, 1.5, 1$. This resulted in a total of 48 test cases, with 12 cases for every street-canyon width. The flow topology in the near-wake of an isolated tall building ($H_{d}=0$) is characterised by a bow-shaped structure comprising the vortex core, saddle point, and ground originating shear layer. For $S/W \approx 2.5, 2$, and $1.5$, increasing the downwind building height from $H_{d}/H_{u} \approx 0.08$ to $1$ resulted in the in-canyon flow structure transitioning from wake dominated to deep canyon wake interference regimes. Similar increase of the downwind building height for $S/W \approx 1$ resulted in the flow structure transitioning from wake dominated to deep canyon skimming flow regime. The results indicate that in step-down canyons formed by tall and slender buildings, momentum transport into and out of the canyon around the building sidewalls plays a crucial role in the determining the overall flow patterns in the canyon.