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Vehicle aerodynamic development is normally undertaken in smooth flow wind tunnels. In contrast, the on-road environment is turbulent, with variations in the relative velocity experienced by the moving vehicle caused mainly by the effects of atmospheric turbulence. In this review the turbulence inherent in the atmosphere is considered, following the approach of wind engineers. The variations of atmospheric wind velocity with time, height, terrain and thermal stratification are summarised and discussed. Statistical parameters presented include mean velocity, turbulence intensities, spectra and probability density functions. The resulting fluctuating approach flow (relative velocity) of the moving vehicle is then considered. The effect of the fluctuating velocity field on parameters of interest to vehicle aerodynamicists (such as aerodynamic noise) are made.

This review paper presents an analytical framework for measuring the turbulent environment of ground vehicles, motor cars in particular, both on the road and in wind tunnels. It is suggested that this framework can be used to start to measure and assess the level of modeling required of the natural wind environment of motor cars and the turbulent environment needed in wind tunnels. This approach should also be adopted for vehicle aerodynamics and associated acoustics problems and computational fluid dynamics simulation. This approach should also supply the framework by which the importance of turbulence for vehicle aerodynamics can be rigorously assessed.

This work presents a second series of turbulence measurements made in a range of different on-road terrains and traffic conditions. Wind measurements were captured using a rake of four separate multi-hole pressure probes mounted to the front of a test vehicle traveling at a road speed of 100 km/h. Analysis of the data shows how the turbulence intensities and length scales are modified by terrain type, road side obstacles and the upstream wakes of other moving vehicles. A vertical ‘profile’ of turbulence near the ground is generated and spatial correlations between probes are examined. These on-road results are then compared to the turbulence levels generated by the Monash University wind tunnel. A new method and a series of targets are then proposed for improving the modeling of turbulence in automotive wind tunnels.