The Flow Environment of Road Vehicles in Winds and Traffic

The aerodynamic development of a new vehicle is usually performed in smooth flow EFD or CFD domains with the vehicle in isolation. However the flow environment on-road is complex due to the presence of atmospheric winds, the wakes of nearby stationary objects and, depending upon driving conditions, the wakes of other vehicles. Winds and traffic generate a turbulent flow environment and can augment or reduce the mean velocity experienced by the moving vehicle. Recent work on turbulence arising from vehicles traversing the atmospheric boundary layer is reviewed and the consequence of upstream vehicle wakes is considered. The influence of distance and rear slant angle is examined, via wind-tunnel measurements of wakes of Ahmed bodies using dynamically calibrated multi-hole probes. The effect on very closely coupled vehicles (such as may occur in future platoons) is investigated via force and surface pressure measurements on two and three vehicle Ahmed bodies of varying rear slant angle. It is argued that the typical turbulence intensity for current highway driving is about 5%, but this can be significantly augmented when in the close proximity of other vehicles and/or during high winds. Further it is shown that for some vehicle forms, close coupling can increase the total platoon drag.