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For vertical axis wind turbines applications

I'm wondering how one would find the static stall angle of an airfoil at a given Reynold number?

Is it just the critical angle of attach (AoA) where highest lift is achieved or some value beyond that? Consider the given graph for NACA0012 (for the use of vertical axis wind turbines) at Re of $3\times 10^5$, what will be the static stall angle in this case?

I found some stuff here and there, yet, I couldn't figure out my issue. The graphed data were taken from this Ref[i].

From Wikipedia:

In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded.

Other related questions: 1 and 2.

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[i] Sheldahl, R.E. & Klimas, P.C., Aerodynamic characteristics of seven symmetrical airfoil sections through 180-degree angle of attack for use in aerodynamic analysis of vertical axis wind turbines , Albuquergue, NM: Sandia National Laboratories, Report No.: SAND80-2114, 1980.

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  • $\begingroup$ I'd be a little suspicious of that data... getting your best CL at around 45 degrees AoA??? That seems, um, improbable. What you probably need to look at is the spike around 10 degrees AoA. Plotting what happens at AoA of 90 degrees or more seems unproductive, when you consider what that would look like in flight. $\endgroup$
    – Ralph J ♦
    May 9, 2020 at 22:24
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    $\begingroup$ @RalphJ This is actually a common characteristics at extremely large AOA. It basically just follows a sine curve. But at 45deg L/D is 1:1 or less, so it's not very useful anyhow. $\endgroup$
    – JZYL
    May 10, 2020 at 0:34
  • $\begingroup$ Are you asking about the definition, or how to computationally/experimentally find it? $\endgroup$
    – JZYL
    May 10, 2020 at 1:20
  • $\begingroup$ @JZYL, I am asking how to find it, as shown in bold. $\endgroup$
    – AlFagera
    May 10, 2020 at 19:01
  • $\begingroup$ @AlFagera Then what are you trying to demonstrate with the two plots? $\endgroup$
    – JZYL
    May 10, 2020 at 19:07

1 Answer 1

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Being a symmetric airfoil, the NACA 0012 does not have a particularly high maximum lift coefficient, especially at low Reynolds numbers. And yes, 300,000 is considered a low value for the Reynolds number of airfoils. On the other hand, there is no other airfoil that has been tested so much. You will find data all over the place.

Generally, the angle of attack range of interest for aeronautics is where little to no separation of the flow along the surface occurs. This is restricted to a small range around 0° (when the flow is hitting the airfoil more or less head on). Larger separation reduces lift, so right before separation first lets lift drop a little, the stall angle of attack has been reached. 180° polar for several airfoils

180° polar for several airfoils

But stall is not the end of lift. See above for 2D wind tunnel data collected by S. Hörner in his book Fluid Dynamic Lift. These results have been collected at a higher Reynolds number when the stall angle of attack is shifted to higher values. To be honest, a lot of factors influence this. In your case, even though there is another maximum at about 45°, the stall angle of attack is between 9° and 10°. Not more.

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