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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.

As part of the general program for comparing leading European automotive wind tunnels, three reference cars derived by Volkswagen (VW), Pininfarina (PF) and FIAT Research Center (CRT) from production models, were tested in different configurations. This report contains the result of tests carried out on a CRF reference car in CRF, VW, DB and PF wind tunnels as well as on the road. The wind tunnel tests of the car in 7 different configurations show a fairly good agreement particularly for the drag coefficients measured in the various tunnels. Road tests were carried out with the car in three configurations at constant speed (measuring front and rear axle lift, body pressure and visualising the wake) and in coast-down (measuring aerodynamic resistance). The results obtained proved that road driving conditions with no side wind or turbulence are correctly simulated in wind tunnel tests.

Comparative aerodynamic force measurements on a full scale notchback type vehicle have been performed between six European companies operating prominent full scale automotive wind tunnel facilities. The results obtained with 8 different vehicle configurations show a remarkably good correlation between the drag coefficients measured in the various wind tunnels. Pressure distributions also show an acceptable agreement. The comparison of front axle lift measurements revealed differences between the various wind tunnels; these differences are partly explainable. Six component measurements also show a satisfactory correlation between the various wind tunnels except where lift does contribute to the forces or moments. Suggestions to extend these correlation tests are presented.

A correlation test program between four European full-scale automotive wind tunnels has been performed on a passenger car modified in five different rear-end configurations which cover most of the present passenger cars. The aim of this program was mainly to evaluate to what extent the aerodynamic drag coefficients, measured in these four different wind tunnels, are reliable and therefore can be used for technical or legislative purposes. The correlation program was extended also to the other force coefficients, that is, lift and side forces, and to the pressure distributions. Tests were carried out at different speeds from 90 to 150 Km/h and at different yaw angles β from − 50° to + 50°. The comparison shows a generally good agreement between the results of these four tunnels, the differences for tests at β = 0° being, in terms of the standard deviations, in the order of ± 2% as regards the drag coefficient, ± 0.025 and ± 0.011 as regards the front and rear lifts.

After a short review of the work done in the past to reduce the aerodynamic drag of a body moving in the vicinity of the ground, a new theoretical method is developed in order to determine the shape of the body when a certain lift distribution is imposed. Considerations on the induced drag suggest that the total lift is zero as also should be zero the pitching moment for stability reasons. These conditions together with that of gradual variation of the area and shape of the cross sections of the body lead to the determination of the basic shape of the body. A model was realized and tested on the Pininfarina wind tunnel. The results show a good substantiation of the theory and a very low drag coefficient. The dimensions of the model where such as to achieve the actual Reynolds numbers of motor cars. Energy implications of a reduction on the aerodynamic drag are also indicated.