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Flow Recirculation and Separation

I have already commented some about airfoils and the coefficient of lift. On this last post I said that flow separation is responsible for the sudden decrease on lift that airfoils experience past some relatively high angle of attack. The theory behind flow separation, how does it occurs, and where does it occur is some what complex and it is still being understood for some geometries and types of flow. My purpose with this post is not to explain this phenomenon but to show you some results from a numerical simulation of flow over an airfoil and see how flow recirculation could indicate that separation has occurred.

The picture shown above shows the velocity vectors associated with a symmetric airfoil at 20° angle of attack at a velocity of 100m/s. When flow separation occurs, a high local pressure zone appears at the separation zone and a recalculating flow is set up. If you look really close to the surface, especially toward the back of the airfoil (but also near the leading edge), it is possible to see the velocity vectors pointing in the opposite direction of the flow which indicates flow recirculation possibly due to flow separation (and/or turbulence). Here is a closer look at the end of the airfoil.

Flow separation is usually recognized because the surface shear force experienced by the airfoil is zero where flow separation occurs. Flow recirculation could be induced by flow separation and/or turbulence and thus the recirculation observed is not a 100% indication of separation. However, the high angle of attack of the airfoil could indicate that perhaps flow separation has occurred.