Understanding Dispersion of Nanoparticles in Vehicle Wakes Combining Field Measurements and Wind Tunnel Simulations

This work presents selected results of an EPSRC–funded project investigating the dispersion of nanoparticles in the wake of moving vehicles. The aims were to study the changes in particle number distribution (PND) due to the competing effects of dilution and transformation processes (e.g. coagulation, nucleation, condensation) over the travel time from tailpipe to roadside, and to model the fate of these particles in the near and the far wake regions of a moving vehicle. To achieve these objectives, firstly ground–fixed and on–board measurements of PNDs were performed using a fast response (sampling frequency up to 10Hz) differential mobility spectrometer (Cambustion DMS50) in the wake of a diesel engined car moving at a range of speeds from 20 to 50 km h. Secondly, wind tunnel simulations were carried out on reduced scale (1:5 and 1:20) models of the car used for the field experiments. The flow and turbulence fields were characterised both in the near and far wake of the modelled vehicle by using a two component laser Doppler anemometer. Concentration measurements were obtained by using a fast response (frequency >350 Hz) flame ionisation detector and a hydrocarbon tracer gas released from the modelled tailpipe. A high resolution experimental data base was obtained from both the field and wind tunnel measurements for formulating the basis of fast mathematical parameterisations that can be used with operational nanoparticle dispersion models.