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Vehicles on the road operate in the turbulent flow field resulting from the combined effects of the natural wind and the wakes of other vehicles. While substantial data exists on the properties of the natural wind, much less information is available for the wake properties of road vehicles. The wake information available for road vehicle shapes is mainly restricted to the near wake region, but to understand the vehicle operating environment it is the wake downstream of this region which is of interest. To determine the range of this area of interest requires some knowledge of the decay of the wake properties. From wind tunnel studies using small simple bluff bodies in free stream and in ground proximity the principle wake properties, velocity deficit and peak turbulence intensity have been measured. The maximum velocity deficit is shown to approximately decay with x-2/3, where x is the distance downstream, while turbulence intensity decays at a slightly slower rate.

The vortex structure in the wake of a car creates drag. In the case of a simple wing this drag component is well defined as a function of lift, but for road vehicles the relationship is more complex. The backlight surface has been shown to be a significant source of vortex drag and in this paper the influence of backlight aspect ratio on both vortex and base drag is investigated. The vortex drag factor is found to be independent of aspect ratio, while the base drag component is shown to be dependent on the ratio of base to frontal area.

It has been recognised that the ideal flow conditions that exist in the modern automotive wind tunnel do not accurately simulate the environment experienced by vehicles on the road. This paper investigates the effect of varying one flow parameter, freestream turbulence, and a single shape parameter, leading edge radius, on aerodynamic drag. The tests were carried out at model scale in the Loughborough University Wind Tunnel, using a very simple 2-box shape, and in the MIRA Full Scale Wind Tunnel using the MIRA squareback Reference Car. Turbulence intensities up to 5% were generated by grids and had a strong effect on transcritical Reynolds number and Reynolds sensitivity at both model scale and full scale. There was a good correlation between the results in both tunnels.

The aerodynamic development of cars is conventionally carried out in the wind tunnel with the working section floor representing the ground surface. While recognising that the boundary conditions are false it has generally been assumed that this basic ground simulation is adequate for road cars. This situation is currently being reappraised. In this investigation a simple 1/4 scale car model has been tested in the MIRA Model Wind Tunnel, using both moving and stationary belt to represent the ground surface. The body and wheel drag contributions were measured independently. The influence of rear body shape, front spoilers, cooling airflow, and underfloor roughness on the aerodynamic characteristics, for both ground conditions has been assessed.