Three-dimensional simulation of exhaust particle dispersion and concentration fields in the near-wake region of the studied ground vehicle

In the present study, the interaction effects of different vehicle speeds and exhaust tailpipe exit velocity and temperature conditions on the three-dimensional flow structure, exhaust particle dynamic behavior, formation and evolution processes (i.e., nucleation, coagulation, condensation, and dispersion), number and volume concentration, and nucleation rate fields in the near-wake region behind the studied ground vehicle in urban road microenvironment were comprehensively simulated using large-eddy simulation (LES) with the aerosol dynamics and dispersion model based on our recently established direct quadrature method of moments (DQMOM) approach. The numerical results show that the particles are drawn up into the recirculation region and then moved toward the upper vortex by its lower vortex accordingly. The particle nucleation rate is much more associated with the distribution of the root mean square (RMS) temperature than with that of the temperature itself. Some of the sulfuric acid concentration emitted from the vehicular exhaust tailpipe are trapped by the vortex and then recirculated back to the rear end of the studied ground vehicle. As a result, the exhaust particle concentration in this region is much higher than a farther distance downstream and the nucleation rate is also enhanced in this region. The exhaust particle volume concentration is the highest in the recirculation region behind the studied ground vehicle and is diluted gradually beyond the recirculation region in the downstream.