The Shear Layer above and in Urban Canopies

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Abstract
The nature and role of the shear layer, which occurs at the level of the average building height in urban canopies, are poorly understood. Velocity data are analyzed to determine the characteristics of the shear layer of the urban canopy, defined as the broad, linear segment of the mean velocity profile in a region of high shear. Particle image velocimetry measurements in a water tunnel were undertaken to resolve velocity profiles for urban canopies of two geometries typical of Los Angeles, California, and New York City, New York, for which the aspect ratios (average building height-to-width ratio) H/wb are 1 and 3, respectively. The shear layers evolve with distance differently: For H/wb = 1 the urban canopy shear layer extends quickly from above the building height to ground level, whereas for H/wb = 3 the urban canopy shear layer remains elevated at the vicinity of the building height, only reaching to a depth of z/H ∼ 0.5 far downstream. Profiles of the mean velocity gradient also differ from each other for urban canopies associated with H/wb of 1 or 3. Values of shear dU/dz increase toward ground level for an urban canopy associated with H/wb = 1. For an urban canopy associated with H/wb = 3, localized peaks of shear dU/dz exist at the building height and at ground level, with values of shear decreasing to zero at building midheight and far above the building height. A consequence of the different forms of the shear layers of the two urban canopies is that the ground-level dispersion coefficient is likely to be greater for urban canopies associated with H/wb = 1 than for those associated with H/wb = 3 because of an increased ventilation and exchange mechanism for cities such as Los Angeles relative to cities such as New York City that possess urban canyons.

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