Effect of the Model-Sidewall Connection for a Static Airfoil Experiment

A study was conducted to show that the gap between an airfoil and the wind tunnel wall can have a significant effect on the three-dimensional near-wall flow topology for a static angle of attack, without being detectable at the midline of an airfoil model. The gap between wall and model is sealed using wall inserts specially machined for each airfoil, and the angle of attack is changed by rotating a round insert in the sidewall. A gap of 1.5 mm remains between the model ends and the wind-tunnel walls, and this is not further sealed. The model was fitted with 48 Kulite unsteady pressure sensors on the centerline, from which the pressure lift, pressure drag, and pitching moment were computed during static measurements and dynamic stall. The measurements show that care must be exercised, when comparing full three-dimensional modeling of the wind tunnel and model with experiments, not to draw the wrong conclusion from a good fit between computation and experiment.