An Intercomparison of Four Computational Fluid Dynamics Models: Transport and Dispersion Around Madison Square Garden

As part of the New York City Urban Dispersion Project (NYC UDP), the mean wind, turbulence, and concentration fields computed by four different computational fluid dynamics (CFD) models have been compared in order to assess similarities and differences between the codes. The four models, FLACS, CFD-URBAN, FLUENT, and FEFLO, share much in common, but each have distinct features. The codes have, for example, different numerical schemes (e.g., finite difference, finite volume, and finite element), different mesh types (e.g., rectilinear vs. tetrahedral), turbulence schemes (e.g., k-epsilon and large eddy simulation), and pressure solver schemes. The simulations were performed for a region around Madison Square Garden in Manhattan, used identical mean wind inflow conditions, and utilized the same 3D building database. However, the domain (e.g., extent, grid size), turbulence inflow profile, and surface characteristics were specified differently for each model. To our initial surprise, each of the four models showed similar results for the mean flow in the horizontal plane, producing very complex flow patterns around the Madison Square Garden area with mean winds at street level blowing in most all directions over a several block area. For the presentation, we will show comparisons of the mean horizontal flow fields, the mean vertical velocity fields, and the turbulent kinetic energy fields. The concentration fields for several near-surface point-source releases will also be compared. One of the goals of this intercomparison study is to learn more about CFD model strengths and weaknesses when applied to urban dispersion problems. Hence, we will discuss the reasons that many features of the flow were computed similarly by the four models, and propose explanations for why there were some differences. In companion presentations, some of the models evaluated here will be compared to experimental data from the NYC UDP Madison Square Garden tracer experiment and sensitivities to inflow conditions will be explored.