Numerical Analysis of Diffusion around a Suspended Crossroad by a Multi-scale CFD Model

The Ikegami-Shinmachi crossroad in Kawasaki is a one of the most polluted areas in Japan due to heavy traffic and industrial activity. The crossroad is an intersection of the metropolitan expressway between Tokyo and Yokohama, which consists of a suspended four-lane road, a six-lane ground-level road under the expressway, and another ground-level road that intersects with the two roads. Lining the road are two- and three-story buildings and residential houses. To mitigate the high concentrations of pollutants, a multi-scale CFD model is being used to investigate the diffusion in the vicinity of the crossroad. The CFD model is based on FUJI/RIC α-FLOW and is two-way-nested for three scales. The largest scale is 500mX500mX300m, and the boundary conditions are given from the output of the mesoscale model. The intermediate scale is 340mX160mX100m, and the smallest scale is 100mX45mX30m. The wind driven by the traffic itself was not considered.

The concentration depends on the wind velocity and small obstacles, such as trees planted along the ground-level road or sound-arresting walls under the suspended road. The exhaust heat from automobiles raises the temperature at the roadside by 1^o C. The level of traffic that causes the concentration of NO₂ to exceed the limits is being investigated.