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To accurately measure the drag of a car, it has to be on the ground. So, raising the car is not what you want to do.
I am talking about the effect the moving belt (and its surface texture) has in the wind tunnel. It would accelerate the air in the boundary layer, in the direction of the air in the "middle" of the tunnel. And this would lower the pressure of the boundary air at the front of the belt.
By vacuuming air out of a slot at the front of the belt - this would further lower the pressure there. And it would reduce the amount of air that would be accelerated by the belt. Which is "artificially" affecting the air pressure under the front of the car.
My take is that air would need to be ADDED at the front of the belt; not removed - to offset the effect of the moving belt.
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