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In this answer, it says:

Normally, increasing speed reduces the impact of viscosity (expressed as the Reynolds number). Viscosity is the reason why airflow detaches, and less of it means that flow will stay attached to slightly higher angles of attack.

I'm a bit confused as to why the Reynolds number changing (increasing, I think) would increase the critical angle of attack (cAoA) of the wing. From what I've heard, the Reynolds number just represents the ratio of inertial to viscous forces, determining whether the flow will be turbulent or not.

Why would simply changing the Reynolds number affect the stall AoA?

One thing I just realized while writing this is that it could be due to the turbulence from the higher Reynolds number helping the flow stay attached (which would make sense to me). However, the way the quoted text above is written makes me think it's due to something else.

One of the links in that answer also led to this answer, in which it states:

The lift curve slope increases a bit with Reynolds number

This is also puzzling to me, but it might require a separate question.

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  • $\begingroup$ Increasing Re (e.g. by increasing velocity) moves the transition region aft, therefore an increased AoA is needed to move it forwards (the critical AoA is when the separation is too much forward). Basically this is this question: What is the effect of airflow speed on separation? (again). $\endgroup$
    – mins
    Commented Sep 21 at 13:08
  • $\begingroup$ @mins Thanks for your comment. So basically the increased speed will help accelerate the air that has been decelerated over the wing? (At higher AoA the air will decelerate more over the wing than at shallower AoA) Also, this now faster moving air will resist separation more, allowing higher AoA $\endgroup$
    – Wyatt
    Commented Sep 21 at 17:11
  • $\begingroup$ There are multiple effects at work. A higher velocity re-energizes the boundary layer by mixing slow and fast air in eddies, preventing low (or inverted) air velocity and separation. Viscosity is also a cause of air velocity reduction. A larger Re means less viscosity efficiency (relatively to inertial effect). Both effects make it difficult to trigger separation. $\endgroup$
    – mins
    Commented Sep 21 at 17:42
  • $\begingroup$ @mins I see. So the higher Re will prevent the slower moving boundary layer air from separating by 'evening out' the velocity profile along the boundary layer? (higher Re causes more mixing of flow, evening out the velocity profile) $\endgroup$
    – Wyatt
    Commented Sep 21 at 18:32
  • $\begingroup$ Correct (to prevent any ambiguity: "Re will prevent the slower moving boundary [...]" means "Re will prevent the now faster moving boundary [...]") $\endgroup$
    – mins
    Commented Sep 21 at 18:46

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Bottom line upfront is higher Reynolds number will result in a thinner boundary layer with a fuller velocity profile. With the no-slip condition for the fluid over a solid surface (airfoil), the velocity will be zero right on the surface, then gradually increasing with distance from the wall in the boundary layer. Separation is defined as when, at any point in the boundary layer, the local velocity is zero. Then, in a more separated condition, the velocity reverses in direction. Point being, all else the same (in a subsonic, 2D flow over an airfoil) at higher Re, the velocity profile is further away from these conditions to begin with than at lower Re.
What typically causes separation in this problem domain is an adverse pressure gradient and it’s important to note the circularity involved. That is, pressure gradient effects the boundary layer and details of the boundary layer effect the pressure gradient.

Some helpful references include:

  • Fundamental Mechanics of Fluids by I.G. Currie, 1974, section 9.11
  • Boundary Layer Theory, 7th Edition, by H. Schlichting, 1979, section 22.c
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