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Aerodynamic drag of a modern car is generated mainly by underbody flows. A better understanding of these flows and of their interactions with the car underbody, may contribute to the future improvement of the car drag characteristics. This paper reports the results of a parametric study carried out in the Pininfarina wind tunnel, on a full scale simplified car model, by using the Ground Effect Simulation System built in 1995. The main aim of this study was to investigate the effect on the aerodynamic coefficients produced by important geometric changes which affect the flows under the car, in proximity of the ground, and are often difficult or impossible to be modified when tests are made on real cars. The model chosen for this research program is that defined by the SAE “Open Jet Interference Committee” as a reference model to be used for investigating wind tunnel interference and for comparison between wind tunnels. In particular it has no wheels.

Modern vehicles require a low aerodynamic drag to minimize fuel consumption. A not negligible share of the overall CD-value of a vehicle is produced by the engine compartment air flow. Therefore this share has also to be optimized. Furthermore, customer wishes for higher powered engines as well as for more safety and comfort result in more tightly packed engine compartments. Even the reduction of pass-by-noise required by legal reasons is often achieved with the help of underbody covers which in turn affect the engine compartment flow. All these items may lead to rising underhood temperatures. To reduce the development time of new vehicles, numerical simulations of engine compartment air flow are more and more used to predict high temperature fields and to show ways to develop suitable remedies in the concept phase of the vehicle development. The experimental basis for such codes is provided by aerodynamic investigations in a wind tunnel.

The aerodynamic development of a convertible passenger car requires the optimization of additional characteristics, when compared to those which are considered for normal passenger cars.

In 1995, in order to bring the wind tunnel simulation as close as possible to the road condition, Pininfarina integrated the “rolling road” with the balance system and provided a means to rotate the car wheels. That was certainly an important first step to improve the airflow simulation of a vehicle moving on the road. However, to reproduce real on-road flow conditions in the wind tunnel, it is necessary to do a second equally important step. That is, it is necessary to be able to reproduce turbulent flow conditions similar to what a vehicle usually experiences on the road. In fact, it is known from previous works that for most of the time, a road vehicle is moving in the presence of turbulent flows, generated either by some natural low-speed wind or by other vehicles moving upstream. These flow conditions are very different from the almost perfect low-turbulence flow that is typical of modern automotive wind tunnels.

Aerodynamics of modern cars is usually investigated in condition of very low turbulence flow and zero yaw. Furthermore, the majority of the tests are often carried out in wind tunnels with fixed ground and static wheels. The effects of a more realistic flow simulation on the car underbody produced by the ground motion and the wheel rotation have been reported in the SAE paper 980031 presented at the 1998 Int’I SAE Congress. This parametric study was carried out in the Pininfarina wind tunnel, by using the Ground Effect Simulation System (“GESS”) built in 1995 and a full-scale simplified car model. This paper reports the follow up of this investigation. The same simplified car model and its underbody interchangeable underbody parts has been tested again, using the “GESS”.

Particle Image Velocimetry (PIV) is a recent measuring technique, which has been used up to now mainly by University Laboratories in small-scale wind tunnels and by Aeronautical Research Centers in small and large facilities. Its use in full-scale automotive testing is not common. It is not so easy, often rather difficult, due to a number of problems, sometimes of practical nature, sometime caused by technology limitations. This paper reports the results of some tests, carried out by CIRA (Centro Italiano Ricerche Aerospaziali) in the Pininfarina wind tunnel on a full-scale car, in the frame of the European Thematic Network “PIVNET”. A description of the test set up, of the instrumentation used for these tests, as well as an analysis of the advantages provided by this technique and of its present limitations, are reported. During the tests, in order to outline the potential of this measuring technique, some specific areas of the car flow field, have been investigated.