The easier it is for a car to glide through an airstream, the lower its consumption, and the higher its clearing speed. At the Wind Tunnel Center, experts from Audi are working on the best aerodynamics for production passenger cars and racing cars.
Big benefits for the customer: the air flow
Audi has far-reaching competence in aerodynamics development. Way back in 1982, the Audi 100 posted a sensational best drag coefficient of cD 0.30, and today’s A8 has a drag coefficient of cD 0.26. That’s good news for the customer: When driving at highway speeds, aerodynamic drag accounts for almost half of the fuel energy used. The effect of even the smallest factor is felt in aerodynamics – one hundredth of a drag coefficient value corresponds to about one gram of CO2 exhaust gas per kilometer. If the drag coefficient of the Audi A8 were only 0.020 higher, it could be driven 24 kilometers (14.9 mi) less far on a full tank of fuel.
Numerous body details serve to reduce the drag coefficient of all Audi models – from the exterior mirror to the wheels to the small separation edges on the tail lights. The defined stall of the air flow at the rear end has a positive effect on a vehicle’s directional stability, particularly at higher speeds. The aerodynamic forces at work also include the air flow at the underfloor and the air flow through the engine compartment, which together can account for half of the aerodynamic drag. Each factor is exactly balanced with the other.
Downforce: the Audi R8 LMS racing car
The motorsport engineers work closely on aerodynamics together with the series production developers from the Wind Tunnel. The new
Audi R8 LMS GT3 racing car was improved with tailor-made solutions in the Wind Tunnel: an entirely redesigned CFRP body engineered for motorsport, a new air flow concept for the cooling systems and passenger compartment, a fully lined underfloor, an integrated rear diffuser and an optimized rear wing.
The underfloor and diffuser create a lot of downforce – the force that helps the car to grip the road and thus makes high cornering speeds possible. The more efficiently this works, the less downforce the rear wing needs to produce. So the wing can be made smaller – which in turn reduces aerodynamic drag. Thanks to this high aerodynamic efficiency – the ratio of downforce to aerodynamic drag – the clearing speed increases. Overall, Audi has reduced the drag coefficient of the R8 LMS by 20 percent to cD 0.4 – the new racing car superbly combines high-speed driving in curves and top speed.
Conceived for Audi Sport customer racing worldwide, the R8 LMS is equipped with a freely aspirated 5.2-liter V10 engine that can deliver up to 430 kW (585 hp). In March 2015 the car will be in action for the first time. The new racing car is continuing an impressive legacy – beginning in 2009, its predecessor model took home 26 international overall titles and seven victories at 24‑hour races.
In many cases, the series developers have also benefited from the knowledge of their motorsport colleagues. The new Audi TT and the big RS models, for example, feature vertical dividing bars in the air inlets that serve as spoilers up front. They direct air so that it flows cleanly along the flanks. The production of series cars and racing cars is so closely interlinked that both vehicle types are produced together up to the point that a certain stage is reached in the process.
Up to 300 km/h (186.4 mph): the Wind Tunnel Center at Audi
Audi’s Wind Tunnel Center in Ingolstadt comprises three wind tunnels that cover over 10,000 m2 (107,639 sq ft), all under one roof. The tunnels’ control rooms, a separate climatic chamber, three smaller testing labs for vehicle components and a workshop area round out the Wind Tunnel Center. Each of the three wind tunnels have been designed for different purposes, so they ideally complement one another.
The biggest facility is the Aerodynamics and Aeroacoustics Wind Tunnel (AAWT). Almost half of a car’s aerodynamic drag is generated at the underfloor and wheels, and in the wheel arches, which is why the AAWT is equipped with a moving belt that simulates the road surface – and can move at speeds of up to 235 km/h (146.0 mph). Four small moving belts set the wheels in motion, and rods inserted into openings in the body sills secure the car in place.
A rotor in the AAWT blows wind at the test vehicles – 1:1 scale cars as well as models in 1:4 or 1:2.5 scale – with tremendous force. The rotor measures 5.01 meters (16.4 ft) in diameter and is equipped with 20 blades and 27 vanes. It is driven by a three-phase motor that generates up to 2,720 kW of power. That enables it to produce wind speeds as high as 300 km/h (186.4 mph) – enough to conduct tests of the Le Mans motorsport prototypes.
Aeroacoustics are also a focus at the AAWT – a technical field in which Audi outperforms its competitors. The powerful facility is very quiet: At a wind speed of 100 km/h (62.1 mph), the sound pressure level is only 50 db (A). The air is routed in a closed circuit, and large damping profiles are positioned in the elbows that direct air flow. A noise reduction system cancels out low-frequency pressure waves that would cause false measurement results. Resonators filled with a noise-damping foam cover the walls of the measurement chamber.
The Thermal Wind Tunnel (TWT) is the competence center for heat conditions. This is where Audi tests all systems for cooling engines, brakes and passenger compartments. A heat exchanger in the air circuit makes it possible to heat the space up to 55 degrees Celsius (131 degrees Fahrenheit), and the heated floor simulates a road surface warmed by the sun. The turbines in the TWT can blow air up to a maximum speed of 275 km/h (170.9 mph); the measuring distance is somewhat shorter than in the AAWT because as a rule only the front of the car and its underside need to be subjected to an air flow with high flow quality.
The third facility is the Climatic Wind Tunnel (CWT), whose turbines can generate wind speeds as high as 300 km/h (186.4 mph). Three engines with a power output of over 3 MW produce cold, and steam produced by a heat exchanger serves to humidify the air. There are 40 burners to simulate sunlight, with beam power of up to 1.2 kW per m2. A water sprinkler system can dispense up to 2,500 liters (660.4 US gal) of water per hour, to realistically recreate heavy rainfall conditions.
The Climatic Wind Tunnel can reproduce nearly all conditions – from Siberian permafrost to the tropics of southern China, and its temperature range is from -25 to 55 degrees Celsius (-13 to 131 degrees Fahrenheit). The CWT eliminates the need for many test drives, thus also reducing CO2 emissions as part of the development process.
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