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EVIDENCE OF ENHANCED VERTICAL DISPERSION IN THE WAKES OF TALL BUILDINGS IN WIND TUNNEL SIMULATIONS OF LOWER MANHATTAN
Citation:
Heist, D K., S G. Perry, AND G Bowker. EVIDENCE OF ENHANCED VERTICAL DISPERSION IN THE WAKES OF TALL BUILDINGS IN WIND TUNNEL SIMULATIONS OF LOWER MANHATTAN. Presented at 5th Symposium on the Urban Environment, Vancouver, British Columbia, Canada, August 23-27, 2004.
Impact/Purpose:
The objective of this task is to develop and evaluate numerical and physical modeling tools for simulating ground-level concentrations of airborne substances in urban settings at spatial scales ranging from ~1-10 km. These tools will support client needs in the areas of air toxics and homeland security. The air toxics tools will benefit the National Air Toxics Assessment (NATA) program and human exposure modeling needs within EPA. The homeland security-related portion of this task will help in developing tools to assess the threat posed by the release of airborne agents. Both sets of tools will consider the effects induced by urban morphology on fine-scale concentration distributions.
Description:
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. The flow responsible for this vertical dispersion was measured with laser Doppler velocimetry, and its effects on the plume were demonstrated with concentration measurements of an ethane 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 downward. This vertical dispersion was caused by a vertical flow in the wake of these buildings and resulted in rapid transport of contaminants to heights above the building tops. The enhancement of the dispersion of the WTC plume due to tall building wake effects is analyzed and compared with Gaussian plume model predictions.
Although this work was reviewed by EPA and approved for publication it may not necessarily reflect Agency policy.