Is this flow turbulent or laminar?

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Discussion Overview

The discussion centers around the characterization of flow over a wing, specifically whether it is turbulent or laminar based on Reynolds numbers ranging from 350,000 to 1,500,000. The scope includes theoretical considerations of fluid dynamics and the application of Reynolds number in different flow contexts.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants suggest that flow is considered turbulent for Reynolds numbers greater than 4000.
  • One participant questions whether the criterion of Re > 4000 for turbulence is applicable only to pipe flow.
  • Another participant argues that the calculation of Reynolds number varies by flow type, indicating that for open systems like wings, hydraulic radius should be used instead of characteristic length from pipe flow.
  • It is noted that the critical Reynolds number for external flow is not fixed at 4000 and can vary significantly, potentially reaching up to 1 million.
  • One participant estimates that for Reynolds numbers between 350,000 and 1,500,000, the flow is likely turbulent, but acknowledges that laminar flow could still occur on very smooth surfaces.
  • There is a suggestion that the transition from laminar to turbulent flow may depend on the surface roughness of the wing.

Areas of Agreement / Disagreement

Participants express differing views on the applicability of the Reynolds number threshold for turbulence, with no consensus reached on the exact conditions under which flow transitions from laminar to turbulent over a wing.

Contextual Notes

The discussion highlights the complexity of determining flow characteristics based on Reynolds number, with limitations noted regarding the dependency on surface conditions and the need for experimental validation to ascertain flow behavior accurately.

teoman
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Hello,

is it a case of turbulent flow over a wing if the relevant Reynoldsnumbers are 350.000 to 1.500.000?

Teoman
 
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Isn't Re > 4.000 = turbulent only applicable to pipe flow?

Hello,

I thought Re > 4.000 = turbulent is only applicable to pipe flow?

Am I wrong?

Teoman
 
It's turbulent for any Re > 4000. The calculation of Re number depends on what type of flow it is. For open systems (such the wing is in this case) you have to use hydraulic radius instead of charasterictic length used in pipe flows.

There is a wikipedia article how to calculate certain flows hydraulic radius:
http://en.wikipedia.org/wiki/Hydraulic_radius#Hydraulic_radius
 
random54 said:
It's turbulent for any Re > 4000. The calculation of Re number depends on what type of flow it is. For open systems (such the wing is in this case) you have to use hydraulic radius instead of charasterictic length used in pipe flows.

There is a wikipedia article how to calculate certain flows hydraulic radius:
http://en.wikipedia.org/wiki/Hydraulic_radius#Hydraulic_radius
Not really. Hydraulic radius is for channel flow. For external (non-channel) flow, it's true that Re = 4000 is not necessarily the critical reynolds number. Unfortunately, it can be difficult to get the true critical reynolds number without experimentally testing it, but it can be anywhere from a couple thousand up to roughly 1 million. In general, for a wing at Re = 3.5*105 to 1.5*106, I would guess that the flow is turbulent, as it is near the upper end of this range. At the very upper end of this range (Re just under 106), you would probably only get laminar flow on a very smooth plate. Of course, just because turbulent flow exists doesn't mean that it's turbulent over the whole wing. If the wing surface is rough at all, you'd probably get a transition to turbulent flow pretty early on, but if the wing is smooth, a significant portion of the wing would have laminar flow. Again, I'm not sure how to determine exactly how much would be laminar aside from experimentally.
 

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