This image shows a smoke flow visualization of a two-dimensional airfoil.
This image is great because it shows 3 phases of a stall/ flow separation as the airfoil pitches back further and further. Additionally the well defined, high contrast smoke lines do a good job of showing relative compression and expansion (pressure regions) within different areas of the flow. Lastly this demonstration does an excellent job show the turbulence that is occurring in the separated flow region.
Schreck, S. & Robinson, Madison. (2007). Wind Turbine Blade Flow Fields and Prospects for Active Aerodynamic Control: Preprint. 10.1115/FEDSM2007-37651.
