A comparison of on-road aerodynamic drag measurements with wind tunnel data from Pininfarina and MIRA

The principal development tool for the vehicle aerodynamicist continues to be the full-scale wind tunnel. It is expected that this will continue for many years in the absence of a reliable alternative. As a true simulation of conditions on the road, the conventional full-scale wind tunnel has limitations. For example, the ground is fixed relative to the vehicle, allowing an unrepresentative boundary layer to develop, and the wheels of the test vehicle do not rotate. These limitations are known to influence measured aerodynamic data. In order to improve the representation of road conditions in the wind tunnel, most of the techniques used have attempted to control the ground plane boundary layer. Only at model scale has the introduction of a moving ground plane and rotating wheels been widely adopted. The Pininfarina full-scale wind tunnel now incorporates the Ground Effect Simulation System which allows testing with a moving belt and rotating wheels. A major feature of this facility is that test vehicles can be easily installed with only minor modifications. This paper compares aerodynamic drag measurements for a large saloon, in various configurations, obtained both in the wind tunnel and on the road. The wind tunnel results are presented for various ground simulations. These are: moving belt with rotating wheels and stationary belt with fixed wheels at Pininfarina, and the conventional fixed ground in the MIRA full-scale wind tunnel. The on-road data is derived from coastdown tests.