Large eddy simulation of flow structures and pollutant dispersion in the near-wake region of a light-duty diesel vehicle

A three-dimensional large eddy simulation (LES) of flow structures and pollutant dispersion in the near-wake region of a light-duty diesel vehicle for both high and low idling conditions within the urban road microenvironments was performed. Four different grid resolutions with and without coordinate transformation were used for evaluating the grid independence on computational results of the LES approach. Time-averaged velocities, temperatures and pollutant concentrations along the centreline of the vehicular exhaust plume have been calculated for both high and low idling conditions. The computational results show that the finer grid cell provides a better agreement with the experimental results for both idling conditions. Furthermore, the computational work has been extended to investigate the effect of ambient wind conditions (i.e., wind speed and direction) on the flow dynamics and pollutant dispersion in the near-wake region of a light-duty diesel vehicle for high idling condition. For the low ambient wind speed case, the vehicular exhaust jet plume dominates the flow structures and the pollutant dispersion patterns. On the other hand, for the higher ambient wind speed case, the vortices and turbulence mixing induced by the interaction between the vehicular exhaust jet and the ambient wind dilute the pollutant concentration, while the oblique ambient wind may alter the spreading direction of the vehicular exhaust jet plume.