Heisenberg wrote:Just wondering, when I see pictures of cars being tested in the windtunnel, it seems like they are sitting on the floor of the windtunnel. Katz mentions its important to elevate the car on an elevated plane as not to have the wind tunnel's boundary layer interfere with the results. Are the cars actually sitting on an elevated plane despite what it looks like?
And generally, how wide should the elevated plane be compared to the car for appropriate results?
cheers
Generally it is done at about 2-3 times the width of the vehicle model but it depends on the application. One example is the sting operation. Often the wheels are held by posts outboard of the moving belt and the wider the belt the more difficult this is to do. The posts also effect the airflow. You have to make compromises in the design because unavoidably the transition from moving belt to stationary floor sheds a pair of vortices that can effect the car values, the closer this is to the car the bigger the problem. You try to compare all of these factors and look at how the tunnel flow is. The real question is which problems are worse than others. As you can imagine, it is never perfect (accuracy is relative) and the wider the belt the more expensive it is to build and operate. Manufacturers need higher yaw angles for low speed crosswind tests compared to racing teams testing relatively small yaw angles for performance reasons. Another serious problem is keeping its vibration from effecting measurement, the bigger the belt the bigger the problem.
Here is an example photo of the windshear tunnel which uses two posts to the front wheels and a narrow moving belt
http://www.windshearinc.com/newsletter/ ... 008_lg.jpg
You couldn't really test a 50kmh crosswind at 50kmh on that setup as the car would be 45 degrees and wider than the belt.
