Quote:
Originally Posted by Vman455
I was just thinking about this more--over the stationary floor, the velocity at the floor/boundary layer interface is 0, so all the kinetic energy of the airstream there is being converted to pressure. That could make the pressure under the front of the car overly positive if not corrected, and suctioning it off could address that.
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The air immediately adjacent to the floor boundary is always at zero velocity,whether in a calm,or moving air mass.The difference would be whether or not a boundary layer was present,either laminar,or turbulent.For the size of a full-scale model,it would always be a turbulent boundary layer.
With a stationary-floor tunnel,without suction,you'd not only have a turbulent boundary layer forming and growing on the underside of the model,you'd also have one present and growing under the model on the floor.This creates 'Wall-Effect',choking off the inviscid free flow,accelerating it to an artificial high velocity and lower static pressure.This wouldn't happen in the real world.It is for this reason that you'd want to vacuum that layer away.Or provide a curved wall/ceiling to compensate for the blockage effects of the model in the test section.DARKO and A2 use this.