I have a little confusion between turbulent flow and separated flow. Consider a flow over a typical airfoil: Does turbulent flow region have boundary layer ? Does separated flow region have boundary layer ?

  • $\begingroup$ The boundary layer is the volume between null airspeed surface and ambient airspeed surface. The null airspeed at some point stops being bound to the wing surface, at the surface the flow is reversed. The boundary layer still follows the null airspeed surface (source). So the boundary layer looks a bit like this (there is a mistake in the exact location, but this is the idea) $\endgroup$
    – mins
    Sep 30, 2017 at 18:39

1 Answer 1


Yes. There are boundary layers (BL) in laminar flow, turbulent flow, and the transition between the two.

"Flow separation occurs when the boundary layer travels far enough against an adverse pressure gradient that the speed of the boundary layer relative to the object falls almost to zero. The fluid flow becomes detached from the surface of the object, and instead takes the forms of eddies and vortices."

From: "Flow Separation" https://en.wikipedia.org/wiki/Flow_separation

Note that the shear stress at the wing surface drops to zero (or very close to zero) after the BL separates.

It is also possible for BL to re-attach to the wing surface after an initial separation. In that case the shear stress returns to a non-zero value.

  • $\begingroup$ what are the values of shear stresses at the wing surface in the flow separation region ? Do they have values that opposite the drag !? $\endgroup$
    – Dat
    Oct 21, 2018 at 16:31
  • $\begingroup$ Most often in models I have seen, the shear stress is assumed to be zero, or very close to zero at the wing surface after separation. $\endgroup$
    – Lysistrata
    Nov 10, 2018 at 9:11

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