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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.

A comprehensive treatment of a new technique to measure vehicle aerodynamic response under side wind conditions is presented. In opposition to the traditional measurement technique of yawing the vehicle by rotating the turntable, the flow itself is continuously deflected by the Pininfarina Turbulence Generation System (TGS), while keeping still the vehicle under investigation. By using this new technique it is possible to assess the dynamic behavior of vehicles, with particular regard to side forces and yawing moment, in more realistic unsteady conditions opposed to the static setup of traditional measurements. In the first part of this work, the unsteady flow, generated by the TGS, is characterized both by means of 4-hole Cobra probe and stereo PIV measurements in empty and non-empty wind tunnel configuration. Some data acquired on the road will also be shown and compared with wind tunnel measurements.

Aeroacoustics of road vehicles is becoming more and more important as it directly affects the comfort of the passengers. The tests made in the wind tunnel, in low-turbulence flow conditions, show results that are qualitatively different from those measured on the road. To get a better understanding of this, Audi and Pininfarina decided to carry out a test campaign on some cars, both on the road and in the wind tunnel, in various turbulent flow conditions. In the case of road measurements, some typical turbulent flow conditions, like those caused by atmospheric wind and those produced by the traffic, have been investigated. Wind tunnel measurements have been performed both in the base wind tunnel (in Audi and in Pininfarina) and in the presence of turbulent flows generated, in the Pininfarina wind tunnel, by the Turbulence Generator System, already described in previous SAE papers [1,3,5,7].

The Turbulence Generation System designed and built in Pininfarina in the years 1999-2002, has been in operation since the beginning of 2003. The purpose of this device is to reproduce in the wind tunnel a flow condition more similar to that found by the cars on the road, in terms of velocity profile and turbulence intensity/length scale. The paper reports an updated description of its mechanical and aerodynamic characteristics. Then it reports examples of results measured on some cars, in the condition of turbulent flow, regarding: Aerodynamics - time averaged and time-dependent aero coefficients. Aeroacoustics - auto spectra and psycho acoustic parameters. Deformation/vibration of some car body parts (bonnet, doors). The differences between these results and those measured in standard low-turbulence conditions are presented. In addition a description of some techniques that are used for these time-dependent measurements is reported.

Knowledge of the air flow through and within the engine compartments of passenger cars has become more and more important in recent times. Flow visualization has been conducted, but, on the whole, there is a lack of real flow data. Experimental investigations by conventional probes have been, up to now, impossible, due to the highly turbulent and often unsteady nature of the flows resulting from the complexity of the engine compartment geometry. The availability of reliable flow data, would help in solving some critical temperature problems. Furthermore, experimental data could be used as a basis for a numerical approach by CFD codes which could probably help in analysing engine compartment flow in the future. In order to better understand this flow, the Technical Development Center of General Motors Europe has recently carried out an experimental test program, aimed at collecting data on the engine compartment air flow of an Opel Vectra in the Pininfarina wind tunnel.

It is shown that it is possible to measure some aspects of transient phenomena of either passenger or sports cars, by the use of the Turbulence Generation System (TGS) installed in the Pininfarina Wind Tunnel. In order to better assess the aerodynamic performance of the vehicle and its consequences in terms of stability and safety, the behaviour of a generic car under the influence of an unstationary flow has to be carefully measured and evaluated. Both time averaged and time dependent analysis of the aerodynamic coefficients are shown in the case of: 1a. Simulation of continuous yawing of the oncoming flow; 1b. Simulation of an impulsive change of the yaw of the oncoming flow; 2. Simulation of the first phase of an overtaking manoeuvre.

The Pininfarina full-scale automotive wind tunnel was recently upgraded to reach a wind speed of 250-260 Km/h and at the same time to further reduce by some dB, the background noise level in the test section. The speed increase is aimed at testing high performance cars at speed greater than 200 Km/h. The decrease of the ambient noise level is aimed at improving the aeroacoustic measurements, both in low and high turbulence flow. The upgrade was made by adding an array of 13 new fans in the return circuit. Each fan drive system has a power of 160 KW for a total of 2 MW. The new fans are operating together with the old main fan (1.1 MW). However, each new fan has a dedicated control system, so that, if necessary, each fan can have a different rpm or a different blade pitch. In the meantime, some new noise measuring techniques have been set up to improve and speed up the car aeroacoustic development.

This paper reports recent developments made at Pininfarina in the aeroacoustic field with the aim of reducing the background noise in the test section of its full scale wind tunnel as well as for improving the measuring techniques which are used during the acoustic development of new cars. The first part of the paper describes the changes made to the wind tunnel drive-line, i.e. new motor, new low-noise fan and new acoustic treatments. The new background noise levels are reported and compared with the old values, already published in ref. (2). The second part describes the three acoustic techniques (acoustic intensimetry, acoustic holography and acoustic mirror) which have been developed or improved in the meantime, to measure the exterior noise of new cars from the first stage of their design. Examples of results of these measurements are reported as well as an indication of the capabilities and limitations of each technique.

The new Turbulence Generation System (TGS), presented in the SAE papers 2003-01-0430 and 2004-01-0807, is now systematically used to get a deeper insight in the aerodynamic and aeroacoustic behavior of passenger cars in a controlled high turbulence flow. The TGS itself is used to generate a flow with characteristics more similar to those encountered by a car when traveling on the road, either for the presence of natural wind or for the presence of upstream vehicles. A comparison is presented between tests performed in the low turbulence flow, i.e. base wind tunnel, and in the high turbulence flow, i.e. wind tunnel with TGS. In particular, results regarding the following aspects are presented: Car Lift in the presence of various TGS setups; Brake Disk Cooling; Interior Noise. For each one of these aspects, some time-averaged and some time-dependent values are presented.

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.

A technique to map the wake behind passenger cars with different rear end configurations has been developed in the full-scale automotive wind tunnel “Pininfarina”. It is based on the measurement of total pressure in the wake, using a probe which is driven by a large traversing gear. Results are presented as coloured isopressure maps. Tests have been carried out on a 1:2.5 scale car model with two different front ends and eight different rear ends. The attainable body configurations are likely to cover the majority of passenger car shapes. Wake surveys have been conducted at several distances behind each car model in order to see the wake development. The paper shows and analyzes the results obtained for these sixteen car model configurations; it also emphasizes what kind of information can be obtained by this wake survey technique.