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How thick is the layer of air that is not moving(zero velocity) at the surface of wing, and can this be taller than the relative roughness height?

Because everything that is inside this layer (V=0) will not contribute to skin friction drag, intuitively speaking.. Am I right?

Or maybe layer (V=0) is infinitely small?

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  • $\begingroup$ Yes, it is infinitely small. Otherwise there would not be a speed gradient at the wall, and it is there where it is largest. There is, however, a laminar sublayer directly at the wall because any disturbance orthogonal to the main flow speed is mechanical damped by the wall's presence. Also, black on dark blue gives a poor contrast. $\endgroup$
    – Peter Kämpf
    Commented Jan 27, 2021 at 18:49
  • $\begingroup$ Clear cut boundaries and gradients may exactly what is not happening here, and "roughness" is known to affect the boundary layer. V=0 in a dynamic system (anywhere): unlikely $\endgroup$
    – Robert DiGiovanni
    Commented Jan 29, 2021 at 16:21
  • $\begingroup$ "everything that is inside this layer (V=0) will not contribute to skin friction drag" just the contrary. Any effect that slows air down contributes to drag by reaction. Air at rest slows down moving air by effect of viscosity, the base principle of the boundary layer. Air at rest and at negative velocity exists in separation bubbles $\endgroup$
    – mins
    Commented Feb 11, 2021 at 16:48

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