Mixing in the three-dimensional wake of an experimental modelled vehicle

Abstract

Experimental results on tracer gas diffusion within the near wake of a simplified model car (Ahmed model with a rear slant angle of 25 • ) are presented. Pollutant emission is simulated using heated air injected through a small pipe at one side of the model base. Fine cold wire thermometry is used to measure instantaneous temperature excess and variance of temperature gradient in the near wake. Measurements of the three mean velocity components were made using a laser Doppler anemometers system. Characteristics of the mean and fluctuating temperature fields, time-averaged flow streamlines and scalar dissipation measurements are presented and discussed. The local mixing time scale is determined from the measured mean dissipation rate of temperature variance. Its value shows that micromixing is not a limiting phenomenon for chemical reactions in the near wake.

Temporal Evolution of Vehicle Exhaust Plumes in a Congested Street Canyon Environment

“…Our estimates mean that after 10 s at 30 km h −1 , these vehicle exhaust plumes would have a width of just 4.5 m. The low values of parameter a estimated here may also have been biased because we chose very clear and well-defined plume segments for analysis. Nevertheless, the observations of [21] suggest exhaust plumes maintain their position behind vehicles over several metres, and σ y seems about ¼ the vehicle width at that point. They noticed that heated gases from the exhaust pipe were trapped in the recirculation zone just downstream of the vehicle and that the plume is quite narrow over a number of vehicle lengths.…”

“…CFD is a successful and popular approach to describing vehicle exhaust plumes [10,21,32,69]. However, it is computationally expensive and remains sensitive to dispersion, so it is best suited to stable atmospheric conditions [70].…”

“…Vehicles induce turbulence and canyon winds along the canyon axis [19,20]. Automotive exhaust plumes, especially during congestion and queuing [21,22], lead to high concentrations and chemical reactions that cause rapid variation in street-level air pollutants in this dynamic environment, with the potential for rapid ozonolysis of nitric oxide (NO) to nitrogen dioxide (NO 2 ) [19,23]. Furthermore, nucleation, condensation, secondary particle production, and adsorption of pollutants onto the aerosol are likely to be important [24].…”