Vehicle queue effect on the characteristics of air flow, and exhaust scalar dispersion and distribution fields in the vehicle wake

The characteristics of air flow, and vehicular exhaust scalar (i.e.; pollutant) dispersion and distribution fields in the near-wake region of a scale-model vehicle which was placed alone or behind the preceding one(s) in a closed-circuit wind tunnel facility were experimentally investigated for typical urban driving conditions. The wake structure behind a queue of studied vehicles is mainly dominated by the last one, while the preceding vehicle(s) will lead to a stronger downwash flow in the wake. For the vehicle with rear slant angle (α < 30°) which has a pair of trailing vortices in the wake flow, the vehicular exhaust jet plume will be mainly trapped inside these two trailing vortices and fills an "m-shaped" scalar distribution region behind the vehicle. Half of the m-shaped region which is on the vehicular tailpipe exit side shares a larger portion of scalar distribution than the other half. This unbalanced scalar distribution is enhanced by the preceding vehicle(s). For the vehicle with rear slant angle (α > 30°) which has a two-dimensional wake flow, the vehicular exhaust jet plume will be carried by such a wake flow to form an "n-shaped" scalar distribution region behind the vehicle with a peak scalar region at its center. The preceding vehicle(s) will further enlarge the n-shaped scalar distribution region and push the peak scalar region closer to the ground. It is clearly shown that the two- or three-dimensional flow behind the studied vehicle can provide different shapes of exhaust scalar dispersion and distribution fields in the vehicle wake.