Flow visualisation and tracer concentration measurements were made in rectangular and square (plan view) street canyon models in which the ratio of street width to the height of the upwind building was held constant at 0.83. The ratio of the heights of the upwind and downwind buildings was varied among 2.0, 1.0, and 0.67 for the rectangular block and was held constant at 1.0 for the square block. Tracer measurements were obtained across the faces of both the leeward and windward buildings for emissions in the local street canyon. Orientation of the canyon axis with respect to the free stream flow was varied from 0 to plus or minus 90 degrees in 10-degree increments. The structure of the dispersion patterns is strongly dependent on the canyon's asymmetry and is less dependent on the canyon's orientation to the prevailing flow. The patterns are also sensitive to the shape of the block: the rectangular block has a concentration maximum in midblock and the square block has maxima near the ends of the block. Concentrations are also significantly greater for the rectangular block than for the square block. Comparisons among vertical concentration profiles observed on the two canyon faces and among estimates from several commonly applied models for the rectangular block configuration are presented.
Abstract