Urban Flow and Dispersion Simulations with a Coupled CFD-MM5 Model

Flow and pollutant dispersion in a densely built business/commercial area of Seoul are numerically investigated using a computational fluid dynamics (CFD) model coupled to a mesoscale model (MM5). The CFD model we developed is a Reynolds-averaged Navier-Stokes equations (RANS) model with the k-ε turbulence closure scheme based upon the renormalization group theory. Inflow boundary conditions for the CFD model are provided by the MM5.

In the MM5 simulation, four domains with one-way interaction are considered and the horizontal grid size in the innermost domain is 1 km. The Mellor-Yamada scheme is chosen as a planetary boundary layer scheme because the scheme prognostically calculates turbulent kinetic energy and this information is required for the CFD model integration. As initial and boundary conditions for the MM5, the NCEP final analysis data in 6-h intervals and on 1°x 1° horizontal resolution are used. The MM5 is integrated for two days starting from 18 UTC 31 May 2004. During this period, high-pressure system was dominant over the Korean peninsula and synoptic winds were weak with clear skies. Under these weather conditions, the MM5 simulates relatively strong local circulations characterized by land-sea breezes and mountain-valley winds over the central western part of the Korean peninsula where the innermost domain covers. Comparisons of the MM5 simulation with observations (automatic weather station and synoptic data) show that although there are some systematic biases in temperature and wind speed near the surface in densely developed areas of Seoul, the overall performance of the MM5 is quite good. Those biases are to some extent attributed to the lack of a proper parameterization of urban effects in the MM5.

Results from CFD model simulations indicate that flow and pollutant dispersion are strongly influenced by the building configuration and ambient wind direction. Various flow patterns in this area of Seoul are analyzed in detail, including recirculation zones, horse-shoe vortices, channeling flows, etc. Also, analyses are made on a reduction in mean flow and an increase in turbulent kinetic energy in the presence of buildings, especially their dependency on ambient wind direction. The CFD model domain covers an area of Cheonggae stream, which was recently restored. The coupled CFD-MM5 model is expected to give valuable information on changes in wind field and thermal environment after the restoration of Cheonggae stream.