News Archive

ECARA - Working Group "Unsteady Aerodynamics" - Nov 2000

Some Workgroups have recently been formed within ECARA (European Car Aerodynamic Research Association). These groups intend to deal with some specific aspects of particular interest in the aerodynamic development of new car models.

They deal with:

• the ' Ground Effect Simulation' in a Wind Tunnel
• the measurement errors caused by the interference between the Wind Tunnel and the test Vehicle
• the aerodynamic behaviour of vehicles in presence of 'Unsteady Flows'.

Pininfarina contributes to the work of all these groups. In particular it co-ordinates the work of the third group ' Unsteady Aerodynamics'.

Unsteady flows can be of different types and be caused, for instance, by:

• presence of atmospheric wind
• overtaking maneouvers
• the pitch or yaw motion of a vehicle.

At present, the measurement procedures used in automotive wind tunnels, do not take account for these flow conditions despite their common occurrence when a vehicle is running on the road.

This latter point gives rise to the interest to analyze more in detail these topics, in order to achieve a better understanding of their importance, as well as of the possibility of simulating these flow conditions in a wind tunnel.

Pininfarina hosted the first two meetings of this Workgroup, in May an November 2000.

Several experts, from Automotive Companies (Jaguar, Land Rover, Porsche, Volvo, VW), Research Centers (IVK, IAT), and Universities (Monash Un. , T.U. of Turin), have attended these meetings.

Next Meeting is scheduled for March 2001

SAE 2000 World Congress - Mar 2000.

The SAE Tech. paper n° 2000-01-0870 "Presentation of Flow Field Investigation by PIV on a Full-Scale Car in the Pininfarina Wind Tunnel " (by A.Cogotti and F. De Gregorio) has been presented at the 2000 International SAE Congress.

The paper reports a detailed description of the PIV measurements made in Pininfarina, last October '99.
The main advantages, the present limitations and the future potential of this measuring technique, are reported.
Results of comparisons made with two other well known measuring techniques (14-hole probe and LDV) are also presented.
Some of them are shown in the "PivNet Workshop" section of this site.

Workshop on PIV at Pininfarina - Oct 1999.

On October 7 - 8, 1999 Pininfarina, in co-operation with CIRA, has organized and hosted a Workshop on PIV (Particle Image Velocimetry). Subsequently, a demonstration on the use of this technique on a full-scale car has been carried out in the aerodynamic wind tunnel.

The Workshop and the Presentation of the PIV technique are part of the works foreseen in the frame of the European program "PivNet Thematic Network": it is a program aiming at transferring the PIV technology from Universities and Research Institutes (where this technique has been developed) to the Industrial Full-Scale Aerodynamic Facilities.
Particle Image Velocimetry is a recent measuring technique: its main charateristic is the capability of producing instantaneous images of the velocity flow field of a vehicle.

These instantaneous images may be used to visualize the behaviour of unsteady flows or to get a better understanding of the flow field velocity changes occurring in case of transients, as in the case of vehicle quick maneuvers.

A sufficiently long set of instantaneous images can then be used to obtain time-averaged velocity and turbulence fields.

As an example, the velocity flow fields measured downstream of a Peugeot 406 Coupé rear view mirror and of its front left wheel, are reported.

Further improvement of the ambient noise in the test section of the full-scale Wind Tunnel – Feb 99.

The quality of the aeroacoustic measurements in the wind tunnel is strongly dependent on the background noise present in the test section.

In order to further reduce this background noise, Pininfarina carried out a number of modifications, in the period Dec 98-Jan 99, in that part of the aerodynamic circuit, which includes the collector, and the areas immediately in front of and behind the fan.

In particular, 11 acoustic baffles were installed upstream of the fan, as well as 40 stator vanes behind it.

The 11 baffles reduce sound transmission from the fan to the test section, while the 40 stator vanes recover a part of the rotational kinetic energy behind the fan.

In that way, the pressure loss produced in the flow by the 11 baffles is offset.

Furthermore, the collector was replaced with one of a new design, acoustically treated in order to reduce the noise generated by the shear layer of the jet on the collector.

As a result of these modifications, the ambient noise in the test section is now reduced to only 72 dBA at a wind velocity of 100 Km/h. The maximum wind speed is unchanged at 202 Km/h At the moment the actual noise level has been reduced to 71dBA at 100 Km/h wind speed

A New System to alter car standing heights during the tests in progress in the Wind Tunnel - Jun 98.

Since June ’98, a new system, remotely controlled by computer, has been operational in the Pininfarina Wind Tunnel. This system makes it possible for us to alter car standing heights and to carry out aerodynamic measurements without stopping the wind, as it is already possible for yaw tests.

An accurate analysis of a car's optimum standing heights can be performed easily and in less time.

In the case of racing cars, this system can be used to check the change in lift and pitching moment which happens in braking and acceleration. Any roll or pitch angle can be simulated, in combination with any yaw angle.

In summary, the main advantages of this system are:

• time saved when testing a given number of standing heights, with a consequent economic savings;
• the possibility of testing a full envelope of standing heights, by simultaneously changing pitch, roll and yaw angles;
• a better development and set-up process of passenger and racing cars.
  

Noise investigation by acoustic mirrors - May 98.

Since May ’98, the investigation of acoustic sources on a vehicle’s surface has been made using two acoustic mirrors installed on the test section side walls and facing the right and left sides of the vehicle.

Since Nov. 1999 the acoustic mirror technique has been improved by adding a camera to the acoustic mirror. In this way the position of the acoustic sources can be seen by playing back a movie recorded on a CD.

This technique is used to detect acoustic sources in a mid-high frequency range, from 800 Hz to 10 KHz. The following map shows an example of aeroacoustic sources detected in the 5000 Hz third-octave band .