Evidence of enhanced vertical dispersion in the wakes of tall buildings in wind tunnel simulations of lower Manhattan

Observations of flow and dispersion in urban areas with tall buildings have revealed a phenomenon whereby contaminants can be transported vertically up the lee sides of tall buildings due to the vertical flow in the wake of the building. This phenomenon, which contributes to what is sometimes called rapid vertical dispersion, has important consequences for the dispersion of pollutants in urban areas and its understanding may be crucial to improving urban dispersion models. This venting effect was observed in a wind-tunnel study of dispersion from the site of the destroyed World Trade Center (WTC) in New York City, using a scale model of lower Manhattan, including a scaled representation of the rubble pile.

Enhanced vertical dispersion was seen on the downwind side of several tall buildings in the highly urban area surrounding the WTC site using a smoke tracer. This vertical dispersion was quantified using velocity vectors measured with laser Doppler velocimetry, and its effect on the plume were demonstrated with concentration measurements of a tracer released from the rubble pile. Notably, the World Financial Center buildings, which stood upwind of the WTC site for westerly winds, caused an initial vertical dispersion of the plume before it began to move downwind. This vertical dispersion was caused by a vertical flow in the wake of these buildings and resulted in transport of contaminants to heights above the building tops. The enhancement of the dispersion of the WTC due to tall building wake effects is analyzed in comparisons with Gaussian plume model predictions.