Wind Tunnel Testing

December 14, 1999

New Audi Wind Tunnel Centre in Ingolstadt
Text and photos courtesy of Audi AG

AUDI AG has just put its new Wind Tunnel Centre into operation, comprising two innovative test rigs, an aeroacoustic wind tunnel and a thermal wind tunnel. The aeroacoustic wind tunnel serves primarily to investigate airflow round the vehicle and sources of noise. However, the test rig programme also involves tasks normally associated with a conventional wind tunnel such as reducing aerodynamic drag of the body or analysing lift and downforce. In this way Audi aims to lower the level of wind noise for its passenger cars still further and extend its technical lead in the aerodynamic body design area. The thermal wind tunnel's central task is to optimise airflow through the vehicle's engine compartment and the car's cooling system.

Audi's aeroacoustic test rig with its 47 metre long tubular section is very large, but also very quiet: it currently ranks as the fastest and, at the same time, quietest passenger-car wind tunnel in the world. It is capable of generating wind speeds of up to 300 km/h; at such speeds up to 916 cubic metres of air flow from the jet every second. The performance specification for this system called for state-of-the-art technical know-how to be applied even during the planning phase.

The difficulty lies in keeping the operating noise level extremely low in spite of the enormous performance potential. This is essential if one is to determine the wind noise caused by the test vehicle reliably. At the same time, the test rig was required to generate more authentic ground simulation than was formerly possible in wind tunnels.

This represents an important advance, because previously vehicles could only be investigated with the wheels at a standstill, a factor that greatly compromised the accuracy of the final results. 50 percent of aerodynamic drag on a passenger car is created by its underbody. For this reason, the new Audi test rig with its ground simulation effect represents a particularly effective means of finding ways to reduce the cD value.

Together with universities and research institutes, the specialists at Audi's wind tunnel have been working on solutions to this problem. In order to achieve the targeted low operating noise level, they had to delve deep into the process of how noise is created. Air moves around the wind tunnel in a closed circuit, driven by a powerful 2600 Kilowatt fan, and makes its way through a tube into the pre-chamber, where turbulence can be effectively filtered out. The jet which follows then accelerates the airflow into the measuring zone where the test vehicle is standing. To the rear of the vehicle there is a huge movable collector which traps the air and feeds it back in the direction of the fan.

Audi's engineers looked closely at the areas where noise is generated - the fan, pre-chamber and measuring area. The objective was to ensure that the airflow was subject to as little resistance as possible in order to minimise energy costs and reduce airflow noise. The fan is a component of particularly ingenious design in the aeroacoustic wind tunnel, but also the major single source of noise in the system. Here, the Audi technicians put their faith in specially designed rotating and stationary blades, which help to ensure that many of the noise components generated in the wind tunnel are prevented from dispersing any further.

Compact silencers made of non-fibrous material also serve this purpose: they are installed in the tubes' diversion angles in front of and at the rear of the fan.

The measuring area is lined with the same material using an even more refined technical principle. This helps to prevent any acoustic tunnel effect with a powerful echo occurring on the test rig, something that would falsify the test results.

One of the test rig's most important technical highlights is the anti-noise system. How does this innovation work? Low frequency vibrations generated in the wind tunnel tube may compromise the accuracy of the measurements. Until now, such pressure resonance could only be attenuated by taking steps which also increased the noise level - something that naturally was out of the question for Audi's aeroacoustic wind tunnel. The new method exerts an active influence on the acoustic field: microphones are installed which record pressure vibrations in the air column. After a precisely defined delay, loudspeakers then emit counter-vibrations which neutralise the original resonance.

Another special feature of Audi's aeroacoustic test rig is the low air flow cross section. Air emerges from the jet over a surface area of only 11 m2. The advantages are obvious: low operating noise, reduced system and energy costs and - of particular significance - an opportunity for test personnel to approach the air flow to within a distance of 1 metre, even at top wind velocity. Because the human ear is one of the most sensitive of noise sensors, this advantage in everyday testing capability is indeed a decisive one.

Also new and unique in the area of aeroacoustic wind tunnels for passenger-car development is the technology which enables relative motion between the road and the vehicle to be simulated. By making the floor surface below the test vehicle move at the same speed as the wind - a moving belt runs at speeds up to 235 km/h - airflow conditions which match those encountered on an actual road are created. The car's wheels are driven by four miniature moving belts - this in turn enables the airflow in the area of the wheel to be accurately simulated.

In the area of aeroacoustics not only the general airflow noise is important, but also the interior noise level within the car. Audi's technicians investigate wind noise, in order to be able to influence it in line with psychoacoustic criteria.

In other words, the emphasis is on what people actually perceive - and not, for example, on purely physical data. In this way, the acoustic experts have been able to confirm that as far as the perception of wind noise is concerned it is quite often the timbre of the sound rather than the noise level that is important.

For this reason, the aeroacoustic investigators have placed a number of dummies into the test vehicles, with microphones fitted into their heads. These are matched precisely to the physiology of the human ear. In this way, wind noise can be analysed and modified in accordance with the subjective perception of the vehicle's occupants.

In the thermal wind tunnel the Audi technicians investigate airflow through the vehicle's engine compartment and its effect on the cooling systems. Possible questions to be looked at during the test are: how big must a radiator be for absolute reliability in all operating conditions? Or: how do different engines react with existing cooling systems?

Another test situation is caused by the fact that early prototypes have no engine or one that does not comply to the final specification, so that coolant, engine and gearbox oil temperature increases have to be simulated on the test rig. This enables developers at this early stage to optimise the functions of the various radiators and coolers. A particular feature is that the ground below the test rig can be heated to up to 60 C, so that extreme conditions can be simulated: how does a test vehicle react on a journey in summer temperatures across hot asphalt or with high loads caused by a trailer and operation of the air-conditioning system?

Through the test rig's heated glass floor the aerodynamic specialists can detect from below ground level how the airflow behaves under the vehicle. In this way, critical flow conditions - a major source of thermal problems - are easy to identify and eliminate. Additional technical features in the thermal wind tunnel are the "quattro dynamometer" and a rapid-action closure flap. The quattro roller dynamometer is capable of investigating specific thermodynamic conditions under load on vehicles equipped with permanent four-wheel drive.

Conclusive tests are also made possible by the rapid-action closure flap located directly behind the jet. This sliding metal plate cuts off the flow of air immediately, in a manner similar to the action of a guillotine, so that Audi's engineers are able to simulate situations in which the wind speed drops dramatically - for example, if a sudden brake application is needed when confronted with a traffic tailback or in stop-and-go traffic.

Copyright (c) 1999 AudiWorld