A study of flow fields in step-down street canyons

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 approximate to 0.08H(u), to span the range: 0.08 <= H-d/H-u <= 1. The configuration H-d/H-u approximate to 1 corresponds to a deep canyon. The footprints of the buildings were square, with the widths (W) and lengths (L) being, W(= L) approximate to 32 mm. Four different street-canyon widths (S) were considered, with S/W approximate to 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 approximate to 2.5, 2, and 1.5, increasing the downwind building height from H-d/H-u approximate to 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 approximate to 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.