Air Circulation in Street Canyon - A.W. Spirn

Excerpt from Spirn, Anne Whiston. Air Quality at the Street-Level: Strategies for Urban Design. Cambridge: Harvard Graduate School of Design. (1986)


Air Circulation in Street Canyon

Principle
The same principles that influence the patterns of air circulation around an isolated obstacle also apply to the street canyon. The form of a street canyon, its porosity, and its orientation relative to wind direction all influence the pattern of air flow within it.

Variables
The variables that influence patterns of air circulation within the street canyon are the same as those that influence air flow around an isolated obstacle:

• Orientation (in relation to wind direction).
  
  
• Shape (in plan and profile). In street canyons with a rectangular cross-section oriented perpendicular to wind direction, pollutants will tend to concentrate on the windward side of the street.
  
  
• Dimensions (height, width, length).
  
  
• Porosity (dimensions and locations of openings, including plazas and intersecting streets).
  
  
• Surface Roughness (heights and shapes of buildings in relation to each other). The more abrupt the changes in building heights and the more open areas around them, the rougher the surface posed to the wind and the greater the turbulence created. Street canyons lined with buildings the same height tend to have poorer air circulation than street canyons lined with buildings of different heights or that are interspersed with open areas.
  
  

These variables combine in multiple ways to produce the following effects:

• Channelization effect.  Winds may become channeled when street canyons are oriented parallel to the wind direction. The occurrence of this effect is a function of total length, width, openings, and average height of the canyon. Channelled winds may be uncomfortable to pedestrians, but will disperse air pollutants generated at street level.
  
  
• Venturi effect.  When winds are funneled through a relatively small opening, speeds are accelerated. When combined with a channelization effect, accelerated speeds may prevail along the entire length of the street canyon. Although this may serve to disperse air pollutants, it can also produce uncomfortable conditions for pedestrians. The Venturi effect is a function of the width, total length, average height of the sides, and the size of openings.
  
  
• Bar effect. When air flows over a street canyon oriented at a 45 degree angle to the wind, an area of accelerated wind speed may occur on the leeward side. The bar effect is a function of the width, total length, average height of the sides, and the size of the openings.