INSIDE VIEW
Q&A
Hans Kerschbaum Head of Aerodynamics BMW Group
What were your main goals when establishing BMW’s new Aerodynamic Test Centre? During the time I’ve been with BMW and responsible for aerodynamics, we have always made modifications to our original aerodynamic wind tunnel in Aschheim. For example, we had previously made upgrades to the balance, and to the aerodynamic performance,. We even modified the concrete test section plenum to increase the height. So, with the new Aerodynamic Test Center (ATC), first we wanted a very large facility, with plenty of opportunity for future modifications. With the new ATC, we have planned and built large test section plenums. They are very large and we can make changes if we need to, we have flexibility for the future. For example, we could incorporate some new measurement technology in 20 years, that does not currently exist. The second goal was to have two separate wind tunnels. The ATC features a full-scale wind tunnel with acoustic treatment in the circuit. With some minor additions, we could make this into one of the quietest aero-acoustic wind tunnels in the world, if we wish. The ATC also features a Model Wind Tunnel (the AEROLAB), that is not only for 50-percent scale models but also for full-scale vehicles. The AEROLAB could be used for fundamental aerodynamic research very early in the model development process that you never normally have the possibility to do. With the AEROLAB, we are now able to simulate what happens on the road in reality, in a repeatable and controllable wind tunnel test environment. What is the focus of that research, the areas that you really want to dig deep into? We have to reduce the vehicle aerodynamic drag. We need to find more possibilities to reduce the drag while not making too many restrictions on the styling department. For us, it wouldn’t be difficult to develop a car with a drag coefficient below 0.20. That’s not a problem, but this car would not sell very well, since it would not represent the current BMW styling. But in the future we must find a way to develop a car with such a low drag coefficient, that does reflect the BMW styling. As an example, today you see BMWs with very short overhangs ahead of the front wheels, that is a very typical BMW design feature. We don’t design vehicles that have a long nose like Porsches or the Jaguar E-Type. For aerodynamic efficiency, this is a very, very difficult design constraint. For these design themes, we must find solutions to achieve a low aerodynamic drag. When will we see the first benefits of the aerodynamic work from the ATC reach production vehicles? In one or two years, approximately. It takes a little time to understand
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The sheer scale of the AEROLAB can be gauged from this view. Note that the huge rolling road is 9 m (29.5 ft) in length and 3.20 m(10.5 ft) in width. The airfoil collector system ahead of the wind tunnel return can be clearly seen at right
the wind tunnel facility and to use the facility and the integrated test systems correctly. We have integrated the latest rolling road, or moving floor belt, technology in the ATC wind tunnels to be able to better simulate the aerodynamics under the vehicle. However, this is not the first time for BMW to be working with moving belt technology. Because we had no moving belt at the original BMW aerodynamic wind tunnel (at Aschheim), we tested a lot at other wind tunnels. For example, we tested at the Swift wind tunnel near San Diego, Cal., in Pininfarina in Turin, Italy, at St. Cyr in France, at the FKFS (the university wind tunnel in Stuttgart), and at RUAG Aerospace in Emmen, Switzerland. Through the many years of working in these wind tunnels, we gained knowledge that we used to make sure we made no mistakes when planning our ATC wind tunnels. Through this work you must have gained a lot of knowledge about the correlation between wind tunnels. What can you say about your correlation work on the ATC tunnels? At the moment we are performing our validation program of the wind tunnels. We are determining what we measure in the new wind tunnels with our different cars that have already been tested in other wind tunnels. That’s the work we are doing now and we can’t say anything about the results at the moment. But in one year, we will be able to publicize results from the correlation tests. I was at a recent SATA (Subsonic Aerodynamic Test Association) conference in the United States and everyone was asking me the same question. I gave the same answer – that I’ll have the information about a year from now. What are the most important areas of a car for the aerodynamicist? The areas that produce the aerodynamic drag are most critical. About 40 percent of the drag comes from the shape and upper surface of the vehicle body, 30 percent from the wheels and wheelhouses, 20 percent from the underfloor and 10 percent from the cooling and flow through the engine compartment. Many people look at the shape and surface of the vehicle and think that the shape is responsible for the aerodynamic drag. But, the shape and upper surface are only responsible for 40 percent of the total drag. 60 percent comes from other areas. For lift force, the shape and upper surface of the vehicle has a larger effect. Wind tunnels were originally developed for the aviation and aerospace fields. What lessons can you take from these industries and apply to automotive aerodynamics? To be honest, not very much. WIND TUNNEL INTERNATIONAL | 2009