FEFLO CFD model study of flow and dispersion as influenced by tall buildings in New York City

This paper studies flow and dispersion patterns in Manhattan using the multipurpose finite element code FEFLO-URBAN. The computational domain is 3.2 km in the East-West direction, and 2.6 km in the South-North direction. This study was conducted as part of a collaborative effort with several agencies to support a field experiment carried out in the Madison Square Garden area in March, 2005. Three other CFD models also participated in this collaboration - Fluent (EPA), CFD-Urban (CFD Research), and FLACS (GexCon). With FEFLO-URBAN, a very large eddy simulation (VLES) model was used to simulate wind and dispersion conditions. First, as part of the planning exercise, five continuous releases from points at street level around Madison Square Garden with winds from the South-West were simulated. Then, a wind from the West-North-West was simulated to represent the actual conditions found during the field experiment. For this last wind condition, five continuous and five puff releases were simulated. A tetrahedral mesh of 24 million elements was used for this study. A resolution of approximately 1 meter was set at the street level and close to walls. A logarithmic wind profile was assumed as the inflow boundary condition, assuming the observed wind speed of about 5 m/s at a height of about 200 m. The simple Smagorinsky turbulence model was used as closure to the filtered flow equations. An explicit integration in time was used to capture the unsteady patterns of the flow. A time period of 1,000 seconds was integrated for the SW wind, and 1,500 seconds for the WNW wind. Profiles of turbulent kinetic energy (TKE), as well as concentration levels, were extracted at selected locations. A qualitative study of flow and dispersion patterns around the Madison Square Garden is presented in this paper. The study includes the enhancement of the plume's width related to the wind direction with respect to the street direction, and the resulting chimney effects behind the surrounding tall buildings.