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Technical Paper

Update on the Pininfarina “Turbulence Generation System” and its effects on the Car Aerodynamics and Aeroacoustics

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.
Technical Paper

The Body Shape of Minimum Drag

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.
Technical Paper

Simulation of Transient Phenomena with the Turbulence Generation System 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.
Technical Paper

FIAT Research Center Reference Car: Correlation Tests Between Four Full Scale European Wind Tunnels and Road

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.
Technical Paper

Engine Compartment Airflow Investigations Using a Laser-Doppler-Velocimeter

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.
Technical Paper

Comparison Tests Between Some Full-Scale European Automotive Wind Tunnels — Pininfarina Reference Car

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.
Technical Paper

Comparison Tests Between Major European Automotive Wind Tunnels

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.
Technical Paper

Aerodynamics and Aeroacoustics of Passenger Cars in a Controlled High Turbulence Flow: Some New Results

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.
Technical Paper

A New Technique to Measure the Aerodynamic Response of Passenger Cars by a Continuous Flow Yawing

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.
Technical Paper

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.