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12.11.2009
Turbulent times
Without the success achieved by wind tunnel research the development of dynamic and fuel-saving vehicles would not be possible.
The information acquired is significant to such an extent that, in addition to vehicle developers, perfectionists from the field of sport also make use of wind tunnels to prepare for competitions. With the opening of the Aerodynamisches Versuchszentrum (AVZ), the Munich-based aerodynamics testing facility, in June of this year, the preconditions were created for reducing the air resistance of BMW Group vehicles by up to 30 percent.
The scene is almost surreal: Four men wearing helmets sit huddled together, one behind the other in a straightest possible line, their heads slightly lowered, their muscles tensed to the limit. They retain this position for around ten minutes whilst another person repeatedly points a droplet spray lance at them, and a turbine, eight metres in width, puts their stamina to the test with increasingly strong winds of 60, 80, 100, and 140 km/h.
The men seek the ideal body posture in order to provide the gale-force winds with as little contact surface as possible. However, their main objective is to test the design and aerodynamics of their vehicle – a bobsled with which they are out to win a medal at the next Winter Olympics in Vancouver, Canada. For up to eight hours the team headed by world-class bobsledder Karl Angerer and the bob manufacturer’s team of aerodynamics engineers collaborate with their BMW colleagues to achieve a reduction in air resistance. Time and time again they measure and analyse the most minute air turbulences utilising sensors and computer technology or make them visible by means of a cloud lance. “In bobsledding even a hundredth of a second is sometimes crucial and the smallest detail can be worth its weight in gold,” pusher Andreas Udvari emphasises. He considers these tests most certainly worthwhile – even if the athletes do have to sit motionless in their bobsled and feel somewhat like “crash test dummies”.
A lot of turbulence because of a hundredth of a second
Like the bobsled team captained by Karl Angerer or tobogganists such as Alexander Resch and Patric Leitner other Olympic champions, world champions or world cup winners, their coaches and mechanics constantly “train” or rather “tinker” in the BMW wind tunnel in order to improve their sports equipment and body posture millimetre by millimetre. “Tests like these are a corner stone of our success,” world champion Alexander Resch emphasises. He believes that wind tunnel tests are as a rule the first “moment of truth” for state-of-the-art bobsled or toboggan design. Whilst Resch and Leitner were testing an entirely newly conceived toboggan prior to the Olympic Games in Salt Lake City, it was not the anticipated advantages of the new model that became evident, but the almost ideal design of the old one. “It is not enough to be just modern,” the athletes conclude. The world champions left the modern but less streamlined toboggan in the garage, reverted to the old model and became Olympic champions. Next year in Vancouver – also thanks to further enhancements – they are seeking to repeat their success.
 
Aerodynamics Testing Centre
For this purpose use could also be made of the AVZ, the aerodynamics testing centre in Munich which opened in June. There, as opposed to the previous wind tunnel permitting a maximum of 180 km/h, it is possible for the first time to simulate speeds of up to 300 km/h, whilst measuring accuracy has been increased significantly at the same time. Ideal conditions for preparing for a high speed track like the one in Vancouver on which bobs can reach speeds of up to 160 km/h.
The considerably increased performance of the new testing facility is indeed important but, naturally, its primary task even more so. And this is the further enhancement of the aerodynamics of all of BMW Group models – also and chiefly due to the fact that this tasks requires even more precise adjustments. It is a fact that although the front surface of a bobsled is considerably narrower and, consequently, should be compared with a motorcycle rather than with a car, the air drag coefficient (Cd value) is even slightly higher than that of current automobiles. The latest BMW 3 Series, for example, has a drag coefficient of just 0.26. The Cd value of a bobsled is between 0.32 and 0.35.
There is always room for improvement
“Although today’s BMW Group vehicles are currently some of the best in their class, there is always room for improvement in the field of aerodynamics,” explains Hans Kerschbaum, head of BMW Group Aerodynamics. “We assume that thanks to the new testing facility we will be able to reduce the aerodynamic drag of our vehicles by up to 30 percent during the years to come.” If this is achieved, a fuel saving of up to eight percent would be possible. However, the engineers are not only occupied with improving aerodynamics. They are also working, for instance, on precision technology for the supply of cooling air to the engine, the transmission and to the braking unit, the reduction of wind noise and above all of lifting forces. “Vehicles travelling at 200 km/h can, as a result of the lifting force, be 120 lighter on the rear axle alone,” Kerschbaum emphasises. Therefore, vehicles must be designed in such a way as to ensure road stability even at higher speeds.
Rolling roads
25,000 square metres in size and costing 170 million Euros, the AVZ is the world’s most modern aerodynamics testing centre. The heart of the facility are the two wind tunnels. The larger of the two is 22 metres long, 16 metres wide and 13 metres high. Maximum power input is 4.4 megawatt. Like the smaller model wind tunnel “AEROLAB”, it has a “rolling road” with which road travel can be simulated using a conveyor. Whilst a total of five conveyors (one for each wheel and one for the area between the axles) carry out this task in the first wind tunnel, a single wide conveyor does this in the second. The advantages of the systems: Five smaller conveyors take up less space and are, therefore, less vulnerable to damage and the vehicle is more easily accessible while it is “travelling”. But the job of the large three metres wide and nine metres long conveyor in the AEROLAB is to simulate undercarriage airflow that is virtually identical to the air turbulence produced later on the road.
Moreover, in both wind tunnels the models can, for example, be pointed diagonally into the wind direction in order to simulate cornering. Thus a vehicle’s behaviour can be precisely analysed in a multitude of situations at a very early stage in their development.
Adapting the “spoiler lips”
During the development of a vehicle tests are initially carried out on 1:2.5 or 1:2 small-scale models and later on original-size vehicles modelled from foam or plasticine. The design of the vehicle is constantly modified slightly in accordance with the test reading. Millimetre by millimetre individual surfaces are scraped down or new layers added to other sections. This includes the shaping of the so-called “trailing edges” at the rear which are designed to counteract disturbing turbulence.
Aerodynamics experts, mechanical engineers, acousticians and designers at the AVZ constantly work on the models in close cooperation with other BMW Group departments. During the course of its development a vehicle completes around 20 measuring cycles before it attains its optimum aerodynamic shape. This includes the further enhancement of the “spoiler lip” developed by BMW during the late eighties. Due to the fact that wheels and wheel arches are responsible for more than 30 percent of a vehicle’s air resistance, small rubber lips are mounted in front of each wheel arch, optimising airflow around the wheel arch and the wheel. Thanks to this effective innovation, air drag and lifting force are reduced to such an extent as to prompt other manufacturers to follow suit and also make use of this invention.
However, it seems highly unlikely that tinkers at the AVZ will develop a similar groundbreaking idea for bobsledders and tobogganists in the near future. Firstly, a “skid-compatible” mounting has to be designed for the conveyor system. But secondly, the engineers are fully booked months ahead and exclusively for the development of models that provide best possible aerodynamics and safety.
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