Numerical Analysis of Diffusion around a Suspended Express Way by a Multi-Scale CFD Model

The Ikegami-Shinmachi crossroad in Kawasaki City ranks at one of the top of locations with the worst concentration of NO2 in Japan. The crossroad is the intersection of the metropolitan expressway between Tokyo and Yokohama. It is a suspended four-lane express road with a six-lane ground-level road under the expressway and another four-lane ground-level road intersecting the two roads. Two- or three-story residential houses and buildings line the roadside. At this site, the concentration of NO2 exceeded the national standard (daily mean) for more than 10% of days in a year. Some mitigation techniques, one known as a “green wall,” which makes use of plants that adsorb pollutants, and the other, which uses photo-catalysts in the pavement etc., have already been applied, however, those effects are not clear. A green wall was established under the suspended expressway, which completely divides the lanes in each direction on the ground-level road. It is probable that the wall obstructs the crosswinds and diffusion near the roads. A multi-scale CFD model is used to investigate the complicated situation in the vicinity of the crossroads. The CFD model used in the present calculation is FUJI/RIC α-flow (Fuji Research Institute Corporation, 1992). The model consists of momentum equations, turbulent equations for k and ε , a temperature equation, a chemical transport equation without reaction, and an equation of continuity. The model is three-times a two-way nested model. The largest domain is 500mX500mX300m, and only several big buildings are given as obstacles. The second domain is 340mX160mX100m, and blocks of houses that are surrounded by small roads are given as obstacles. The smallest domain is 100mX45mX30m, and the suspended expressway is the only obstacle. At this time, no attention has been given to the airflow and diffusion created by traffic. The boundary conditions of the largest area of the CFD model are given by a mesoscale meteorological model both for wind and turbulence. The calculation was performed for Dec. 8, 2003 and the performance is compared with measured NOx concentration at the roadside monitoring post. The performance is good as a whole, however, it is not good when wind direction calculated with mesoscale meteorological model was not well calculated. The effects of green wall (positive or negative) are discussed.