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Why is a thicker boundary layer more turbulent? I’m mainly referring to the effect of the varying speed over the boundary layer, and why it makes it more turbulent. I know it has something to do with the Reynolds number, but what happens physically to make a turbulent boundary layer?

Wyatt
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A boundary layer starts at thickness zero and grows from there. Since the transition from laminar to turbulent flow can be anywhere between the start of the boundary layer and a local Reynolds number of several million, turbulent layers tend to occur when the boundary layer has already reached some thickness. Turbulent boundary layers will not go back to a laminar state, so the thickest part is almost always turbulent (except for small Reynolds numbers which on the external surfaces can only be found on small model airplanes - there, a boundary layer can stay laminar until its separation).

Next, turbulence lets a boundary layer grow more quickly than laminar flow. Actually, the question should be reversed: A turbulent boundary layer is thicker.

Peter Kämpf
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  • Oh okay, I see, thanks. One small question : why do boundary layers turn turbulent as they travel over a wing/surface? – Wyatt Jan 28 '24 at 16:25
  • I edited my question to capture more of what I really was asking, so that might help to see where I am at. – Wyatt Jan 28 '24 at 18:39
  • @Wyatt You should really ask a new question instead of re-writing one that has already an existing answer. Now the answer is off-topic. – Peter Kämpf Jan 28 '24 at 21:33
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    I just realized that, thanks for pointing that out. I'll edit it back and ask a separate question for that. – Wyatt Jan 28 '24 at 21:39
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A short answer to the question is: Viscosity.

Boundary layer is absolutely a viscous phenomenon, it arises due to "friction" between fluid (air) and surface of the wing, this might be thought of as the moving air loses some of its energy while contacting the surface, so some of the velocity is gradually lost..

When you say "varying speed over the boundary layer", if you mean the gradual vertical decrease in velocity, this is because the rate of change of velocity with respect to height over the surface is linearly proportional to the shear stress that appears due to friction, according to this law: $\tau = \mu \frac{\partial u}{\partial y}$

Where $\tau$, the shear stress, is typically a positive value (in direction of positive x-axis), so the velocity $u$ increases gradually as height above the surface increase, until the velocity equals the freestream velocity $u=u_{\infty}$.

This is also explained in the image: Image from NASA

أحمد صلاح
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    Oh okay. But why does the pressure gradient make turbulence? – Wyatt Jan 28 '24 at 20:24
  • Sorry for the late comment, I believe the main answer is still: viscosity, and hence, friction, I think it's intuitive to think of it this way: if an airfoil is extremely smooth, streamlines of air would move on a path exactly on the surface of the airfoil, taking exactly its shape, this is called a laminar flow. – أحمد صلاح Jan 29 '24 at 23:12
  • Friction on the other side, causes changes in the air flow, streamlines exchange velocity with each other, both velocity and direction of streamlines changes in a chaotic manner. That's why there's no theory that actually models turbulence as far as I know until today, because it's chaos. This is how I think of it, it might not be very accurate physical explanation, but it's just my intuition and I hope it helps... – أحمد صلاح Jan 29 '24 at 23:13
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    Thanks for your intuition! There's also another question I asked here that got some good answers. – Wyatt Jan 29 '24 at 23:29
  • Thank you, by the way when I said: streamlines exchange velocity, this is not accurate, I meant layers of air above the airfoil change velocity, but anyway you got the point – أحمد صلاح Jan 30 '24 at 01:04
  • Ah I see, thanks for pointing that out! – Wyatt Jan 30 '24 at 01:08