page_title: Urban canyon
section_title: Wind perpendicular to canyon
hierarchical_section_title: Urban canyon / Effects / Wind / Wind perpendicular to canyon
caption_attribution_description: English: The effect of a street canyon on winds. The formation of a vortex inside the canyon when mean wind is perpendicular to the street.
context_page_description: An urban canyon is a place where the street is flanked by buildings on both sides creating a canyon-like environment, evolved etymologically from the Canyon of Heroes in Manhattan. Such human-built canyons are made when streets separate dense blocks of structures, especially skyscrapers. Other examples include the Magnificent Mile in Chicago, Toronto's Financial District, and Hong Kong's Kowloon and Central districts.
  Urban canyons affect various local conditions, including temperature, wind, light, air quality, and radio reception, including satellite navigation signals.
context_section_description: When the roof level/background wind direction is perpendicular to the street, a vertically rotating wind flow is created with a centered primary vortex inside street canyons. Based on the aspect ratio, different flow regimes are defined in street canyons. In the increasing order of aspect ratio these flow regimes are: isolated roughness flow, wake interference flow and skimming flow. The total number of vortices made and their intensities depends on many factors. Numerical model studies done for isolated street canyons have shown that the number of vortexes made increases with increasing aspect ratio of the canyon.  But there is a critical value of the ambient wind speed, above which the number and pattern of vortexes become independent of the aspect ratio.
  Numerical and wind tunnel studies have shown that for symmetric canyons with an aspect ratio = 0.5, a ground level secondary vortex can be seen near the leeward side building wall. For symmetric canyons with aspect ratio ≥ 1.4, a weaker ground level secondary vortex can be seen near the windward side building wall and for aspect ratio ≥ 2 secondary vortexes are seen right below the primary vortex. In asymmetric and step-up canyons the formation of secondary vortexes can be more common. Wind tunnel studies have shown that, in a step-up canyon where the upwind building is shorter, a stagnation point can be identified on the windward face of the taller building. The region below this stagnation point is called the interaction region, since all streamlines in this region are deflected downward into the street canyon. The characteristics of the vortex flow patterns inside the canyon strongly depend the height ratio of buildings on either side of the canyon. For a downwind building height Hd to upwind building height Hᵤ ratio of 3, a single primary vortex has been observed. But for Hd/Hᵤ=1.67, counter-rotating vortexes can occupy the whole depth of the canyon.
  Other factors that have an effect on the strength of this re-circulation flow are traffic induced turbulence and roof shapes of the buildings. Physical model studies have shown that two way traffics increases the turbulence in the lower half of the canyon and that pitched roof on either side of the canyon shifts the main area of turbulent production downstream and reduced the intensity of the re-circulation flow inside the canyon.
  Under these perpendicular wind conditions, mainly at the street level, at each end of the canyon, horizontally rotating corner/end vortexes are made. The horizontal extent of these corner vortexes are different at each end of a canyon and this leads complex surface level wind patterns at intersections. Field experiments have further shown that corner vortexes can extend in the whole depth of canyon but with changing horizontal extent with height.
  The structure of the neighboring area of a street canyon; for instance a series of street canyons, add more complexity to flow field.
  All the above-mentioned results are for situations with no heating effects. A numerical model study have shown that, when a surface in a street canyons heats up, it changes the characteristics of the vortex flow. And heating of different surfaces; upwind wall, downwind wall, canyon floor, changes the vortex flow in different manners.
caption: In the skimming flow regime, the wind vortex made inside a street canyon, when mean wind direction is perpendicular to the street (after Oke, 1988)
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