The new aeroacoustic wind tunnel: New tool in the battle against wind resistance and wind noise
With the more than 70 year-old wind tunnel in Stuttgart-Untertürkheim, Mercedes-Benz was the first automobile manufacturer to possess a wind tunnel. With the new aeroacoustic wind tunnel at the development centre in Sindelfingen, the company has once again placed itself at the forefront of aerodynamic testing.
As a measure of the company’s confidence in the future, the go-ahead for the construction of the new Mercedes-Benz aeroacoustic wind tunnel was given in 2008 – in the middle of the most severe economic crisis for decades. This means that from mid-2013, Mercedes-Benz developers will for the first time have the right facilities directly at the passenger car development centre in Sindelfingen. Joining the climate tunnel taken into operation two years ago and the new driving simulator centre, this rounds off the new test facilities at the Mercedes-Benz Technology Centre and strengthens the development potential of Mercedes-Benz in Germany.
Blower: wind speed of 265 km/h, 5 MW output
The new wind tunnel follows the Göttingen design, i.e. the air is redirected to the blower after the measuring stretch and re-accelerated, which saves a great deal of energy. The blower has a diameter of nine metres and has 18 vanes which set the air into motion. At 202,150 Nm the maximum torque of the electric drive motor is approx. 1000 times that of a powerful automobile. At a wind speed of 250 km/h the power consumption is five megawatts. The blower is then rotating at 238 rpm, and the volume of air passing through it is 2000 cu. m. or around three family houses per second. The maximum wind speed is 265 km/h.
The air temperature in the wind tunnel is maintained at 23 to 24°C. To ensure that measurements are precise even in wintry outside temperatures, the concrete shell of the tunnel is surrounded by an outside wall and therefore insulated. During operation, the drive motor of the blower heats up the air circulating within the wind tunnel. In warm outside temperatures it is therefore cooled by a heat exchanger located downstream of the blower.
Before the air accelerated by the blower reaches the measuring stretch via a nozzle system covering 28 sq. m., it must be directed and smoothed to eliminate unwanted turbulence and eddies. This done using rectifiers and sieves.
Extensive noise insulation measures are integrated to allow use as an acoustic tunnel where interior and exterior wind noise can be measured for the relevant test vehicle. Even at 140 km/h the air flowing through the measuring stretch is therefore as quiet as a whisper
Measuring stretch: five conveyor belts up to 265 km/h
The centrepiece of the 19-metre long measuring stretch in the wind tunnel is the roughly 90-tonne conveyor belt/balance system with a turntable. The new wind tunnel has a 5-belt system to simulate the road: a small conveyor belt runs under each wheel, and a central belt with a length of nine metres and a width of over one metre runs between the wheels. All five belts are synchronised with the wind, so that up to 265 km/h they exactly simulate the road conditions. The 24-tonne balance on which the vehicles are fixed in place is extremely sensitive, with a weighing precision of a few grams. Even the measuring cables must be routed so that they do not introduce extraneous forces into the system. The values obtained with the help of the aerodynamic balance are used as the basis for calculating the coefficients of wind resistance, lateral forces and lift at each axle, and pitching, rolling and yawing movements.
The conveyor belt/balance system is integrated into a turntable with a diameter of twelve metres. This also enables the test vehicles to be subjected to an angled airflow to simulate cross-winds. The turntable also allows vehicles in the test stretch to be exchanged rapidly. The test vehicles are prepared in workshops within the wind tunnel building, immediately adjacent to the measuring stretch. Great attention was paid to a rapid exchange of test vehicles, so that the wind tunnel can be used as effectively as possible.
Traversing system: precise measurements even at 265 km/h
The traversing system enables the engineers to position a variety of aerodynamic sensors and microphones around the test object with a high degree of precision, so that pressure, acoustic and speed measurements are exact. The system in the new Sindelfingen wind tunnel has seven axes (three translation [parallel movement] and four rotation axes), enabling it to cover a measuring volume of 19 x 14 x 5 metres. The weight of this system is 26 tonnes, as the measuring sensors must remain exactly in place with no vibrations even at the maximum wind speed.
In addition to advanced computer simulation programmes, Mercedes-Benz now has all the facilities required to further consolidate its leading position in aerodynamic efficiency and further improve the acoustic comfort of its vehicles. Dr. Teddy Woll, Head of the Aerodynamics/Wind Tunnels department at Daimler AG, explains: “Computers and wind tunnels are outstanding tools that complement each other perfectly: using numerical flow calculations we are able to examine very complex airflow phenomena and follow even the smallest turbulence back to its source. And in the wind tunnel we are able to test a large number of variations very rapidly – which means that one day in the wind tunnel can often lead to major improvements both outside and inside the vehicle.”
