The influence of turbulence on the aerodynamic optimisation of bluff body road vehicles

In order to promote further understanding of the effects of the atmospheric environment encountered by road vehicles in the real world, a wind tunnel based investigation was conducted into the effect of small scale turbulence on the road vehicle optimisation process. An initial investigation was carried out using a I-box model with variable leading edge radii from 10mm to 100mm. Measurements of time averaged forces were made over a range of Reynolds numbers from 200,000 to 1,300,000 (based on the square root of frontal area) and free stream turbulence levels from 0.2% to 5.1%. The transcritical Reynolds number based on edge radius was established as a basis for comparison between turbulence levels. Centreline pressures and PlV vector fields are presented to provide information on separation and reattachment. The investigation was extended to a more representative 2-box model using the same radii as before and a reference model at full scale, where the edge radii varied from 25mm to 150mm and turbulence intensity from 1.8% to 4.3%. It was shown that there is a strong reduction of separation under increased turbulence, and a small increase in skin friction. A further experiment was carried out to investigate the influence of freestream turbulence on the characteristic effect of changing backIight angle on lift and drag. It is shown that there was a reduction in drag due to the action of turbulence on the separation over the backIight, which may be driven by an effect on vortex strength. Tests were also carried out on two full scale vehicles to investigate the effect of increasing turbulence intensity on front and rear spoilers, cooling drag, and A-pillar vortex flows. The observed changes were small but would often be cumulative in their effect, so that optimising a vehicle in a significantly different turbulence level could produce a difference in the total forces acting on the vehicle. These experiments have shown that the primary effect of the additional freestream turbulence introduced by grids is on the boundary layer, as was expected from the literature. The results showed that increasing the turbulence intensity made separated regions smaller, and suggested that vortices become weaker and less well defined. The work provides a basis for continuing to investigate the effect of freestream turbulence on the process of optimising the aerodynamics of road vehicles.