I've a question regarding the definition of the velocity boundary layer. The boundary layer is defined (correct if I'm wrong) as the region close to the body where viscous effects are important and cause gradient of velocity from 0 (non-slip) at the surface to the free stream. Moreover it can be divided in several zones according to the Reynolds number (ratio of inertia to viscous forces).

Now my question: why the turbulent boundary layer it is still called "boundary layer" even though, at high Re, the effects of inertia forces became predominant? I know that there is still a laminar sub-layer to full fill the non-slip condition as well as that in the turbulent boundary layer there are significant gradient of velocity.

I hope the question is clear

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    $\begingroup$ I think they define boundary layer as that region where flow velocity rises from zero to bulk flow velocity. In case of turbulent flow you look at mean velocity profile and define b.l. for it. $\endgroup$ – Deep Sep 20 '16 at 9:00
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    $\begingroup$ You seem to have answered your own question. Even at high $Re$ numbers, $Re$ close to the wall/object is very low and flow is predominantly laminar. Velocity gradient in that layer is high, then levels off quickly (at high turbulence) towards the 'bulk' of the flow. This transient area is the boundary layer. $\endgroup$ – Gert Sep 20 '16 at 13:18
  • $\begingroup$ I agree with your point of view but as you can see from the images I added to the original post (taken from Heat and Mass transfer Cengel and Ghajar) the entire turbulent region is part of the boundary layer and not only the viscous sublayer. $\endgroup$ – horowitz Sep 20 '16 at 20:12
  • $\begingroup$ Is it your understanding that a boundary layer can only be viscous? $\endgroup$ – nluigi Sep 21 '16 at 13:29
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    $\begingroup$ The velocity boundary layer is region near the wall where the velocity varies rapidly with distance from the wall, going from a value of zero at the wall to the free stream velocity at the outer edge of the boundary layer. $\endgroup$ – Chet Miller Sep 22 '16 at 0:11

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