|
aerohead,
Thanks for the info that the difference is small. Do you know if they were testing changes with the tires rotaing, or the floor boundary layer, or both?
Chris D.,
The velocity profile of the oncoming air is different in the two cases.
When a car moves thru still air on the road, The velocity of that still air is zero (obviously) no matter what the height from the road. The car moves at velocity x thur an air stream of uniform velocity zero, no matter what the height of the air above the ground is.
When air is blown at velocity x by a stationary car, the air velocity is as desired (velocity x) at some distance from the floor, but the air velocity must decrease as you measure closer and closer to the floor.. There is a boundary layer at the floor where the air velocity is not as fast as the rest of the tunnel. At the face of the floor itself, the air velocity is zero.
The question then is how thick that boundary layer is, and is it important when testing cars that have air dams and skirts which get very close to the floor and thus (presumably) into that boundary layer at the floor.
|