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.