I want to simulate a wing in a wind tunnel test section. I am using the coupled pressure based solver in Fluent with a pseudo transient time step. The wind tunnel walls are set as free slip walls by specifying a zero shear force at the wall. I know from wind tunnel experiments that the flow does not separate over the flap. However, this is not what happens at the free slip walls in the CFD simulations. The flow stays attached over the flap away from the wall, but at the wall it separates, which makes no sense to me.
Does someone have an idea why this happens?
I checked the y+ value at the free slip walls, which I think should be zero. However, the second image shows that it is not zero, although the shear force at the wall really is zero.