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A rough surface takes energy from the boundary layer, so doesn't a slow, "tired" and thick boundary layer have too little momentum to fight against an adverse pressure gradient?

If the wing surface is very rough or let's say dirty, will this shift flow separation upstream due to the reasons above?

If yes, how can we distinguish "bad" vs "good" turbulent flow, because vortex generators produce turbulent flow to delay separation?

Peter Kämpf
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user707264
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1 Answers1

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There is no universally "bad" or "good" turbulent flow. Outside of Hollywood movies or clumsy propaganda, it's always a mixture of good and bad.

Take those vortex generators: They increase the thickness of the boundary layer and, therefore, friction-induced losses, so their added turbulence is mostly bad. But if you have trouble with oscillating shocks or early flow separation on the outer wing, they can remove the last obstacle to certification. For a few minutes in a hours-long flight this added turbulence becomes good and those vortex generators become indispensable.

A rough surface takes energy from the boundary layer, so doesn't a slow, "tired" and thick boundary layer have too little momentum to fight against an adverse pressure gradient?

Yes, it does, but not always. Large, fast airplanes have wing Reynolds numbers where laminar flow is insignificant. Gliders, on the other hand, can be very sensitive to this phenomenon. Modern glider airfoils, on the other hand, take this into consideration and are rather insensitive to added roughness or bugs. Make no mistake, this early transition costs performance, but handling remains largely unchanged with those modern airfoils.

Peter Kämpf
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  • example;if you polish windsurfing fin and make it glossy,you often get problems with spinout because fin stall,so every fin is matte with sandpaper 400-600. Question is how much roughness is good and how much is too much?https://www.youtube.com/watch?v=71OEyklxqsk&ab_channel=JeepGarage – user707264 Dec 04 '21 at 17:30
  • @JurgenM That depends entirely on the Reynolds number. The fin is similar to a golf ball and profits from enforced transition, but you don't see dimples on airplane wings. – Peter Kämpf Dec 04 '21 at 21:34
  • Sanding submerged parts with 400 sandpaper “catches” water in between the tiny peaks & troughs, and friction during forward motion is then water-on-water instead of plastic on water. So sanding of the fin reduces friction. – Koyovis Dec 04 '21 at 23:25
  • @Koyovis That sounds like voodoo physics. Is there any serious proof of this or just surfer hearsay? – Peter Kämpf Dec 05 '21 at 07:26
  • @PeterKämpf slalom board bottom:https://lh3.googleusercontent.com/proxy/5GIgzLeepzQ8pCGE208v_RrGfFgCzwCNFK-VF1WUsiH-tvEgbSpYTjz7kwsZeg4bHIAmAKhDWxS735VjIwGzJU2nYP1eJPhzDb2kLxoM5ubVbdTf8eCDgnnQttJkShKF-JYTEfK6Lo0WIurqw7nqUwO3hEJNSJj4x5Z7C4PRkMfn8pT-K648 – user707264 Dec 05 '21 at 08:02
  • @PeterKämpf I dont know if Koyovis explantion is correct,but this is true that all race windsurfing boards and fins are matte with sandpaper 400-1000,they are faster than glossy surface,this has been proven last 30 years in slalom/speed racing.Also polished gloss fins has problem with stall(spinout).Because of sanding, bottom of board looks like it was repaired even when buy it new. Does sanding surface works similar as riblets shark skin? – user707264 Dec 05 '21 at 08:04
  • @JurgenM "Does sanding surface works similar as riblets shark skin?" Yes, if sanding direction is in flow direction. Is it? – Peter Kämpf Dec 05 '21 at 08:13
  • @PeterKämpf Yes,in flow direction.Before every race riders re-sand board and fin to clear any imperfection of dirty/salt.And race fins has trailing edge sharp as knife!You can cut yourself easy if you are not careful. – user707264 Dec 05 '21 at 08:21