Environmental Engineering Reference
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Figure 5-55. Smoke flow visualization of dynamic stall on a two-dimensional airfoil
pitching in a wind tunnel.
airfoil surface beneath the vortex. This surface pressure minimum is responsible for the
strongly amplified C n levels observed during dynamic stall.
The middle panel, for a 25 deg angle of attack, shows that the dynamic stall vortex has
grown substantially and has convected aft on the airfoil chord, residing approximately over
midchord. The smoke line defining the outer perimeter of the vortex almost completely en-
circles the vortex, indicating the strongly rotational and energetic nature of the vortex. Consis-
tent with vortex size, the surface pressure minimum below it has grown and C n has increased.
In the lower panel, the vortex has grown still larger, and continues to exhibit a strongly
rotational appearance, indicative of the energy contained in the vortex. However, the dynam-
ic stall vortex has now reached the airfoil trailing edge, and has begun to lift away from the
airfoil surface. As the vortex sheds from the airfoil surface, the surface pressure minimum
weakens and C n undergoes a catastrophic decrease, or stall.
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