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Aerospace Do all airfoils (infinite wing) have a flow separation?

  1. Oct 23, 2016 #1
    Do all airfoils(infinite wing) have a flow separation at a finite or zero AOA? I wonder if there exists such an airfoil that despite the presence of adverse pressure gradients, exhibits no flow separation phenomena, perhaps turbulent intensity could be high enough to sustain enough speed near the surface of a wing so that a back flow does not occur. Do most commercial aircraft have a flow separation?

    Thank you!
     
  2. jcsd
  3. Oct 23, 2016 #2

    FactChecker

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    Flow separation results in increased drag, a decrease of lift, and possibly a stall. Most airplane wings do not have flow separation in normal flight. The flow is laminar.
    CORRECTION: Should not say "The flow is laminar." (see @boneh3ad post #3 )
     
    Last edited: Oct 23, 2016
  4. Oct 23, 2016 #3

    boneh3ad

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    I'd be careful saying the flow is laminar. Most airplane wings have turbulent boundary layers over the majority of their surfaces.
     
  5. Jan 25, 2017 #4
    Indeed the boundary layer and its turbulent flow increase as laminar flow breaks down, particularly after the point of peak thickness. The extent of this breakdown is affected by surface friction, aerofoil shape, velocity and Reynolds Number. Spanwise flow, such as exists with finite wings (and spanwise changes of geometry) may trouble the trailing edge and hasten the breakdown of chordwise flow, but the effect is negligible at low angles of attack. An infinite wing is therefore not immune to flow separation, regardless of aerofoil choice. All aircraft exhibit flow separation, although suction holes are being considered for Flying Wings (or BWBs as they are now termed) to minimize this.
    Your question is a little confused. Flow separation results in turbulence but the boundary layer can remain within acceptable limits or, in extreme cases, be controlled by vortices.
     
  6. Jan 25, 2017 #5

    boneh3ad

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    This isn't true. In fact, spanwise flow can (and does) fundamentally alter the laminar-turbulent transition mechanism in a boundary layer. This is why swept and unswept wings have fundamentally different transition behavior.
     
  7. Feb 1, 2017 #6
    You're right boneh3ad. But since the topic was about aerofoils and infinite wings, I took the comparison to be limited to straight wings. I suppose an infinite swept wing is no less likely than an infinite straight one but that would be a whole new topic.
     
  8. Feb 2, 2017 #7

    boneh3ad

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    Regardless of wing geometry, spanwise flow signals that there is a spanwise pressure gradient that exists. This is fundamentally different than the behavior of a 2D boundary layer on a 2D wing geometry. In that sense, a 2D wing with spanwise flow has a lot of fundamental similarities (from a stability and transition point of view) to a swept wing, which has spanwise flow by default.

    In a truly 2D boundary layer on an infinite swept wing, the transition process is dominated by 2D waves called Tollmien-Schlichting waves. Adding in a spanwise component means, at the very least, that you now have another competing mechanism called the crossflow instability. How much spanwise flow dictates whether or not that becomes important. My point was simply that spanwise flow cannot be assumed to have a negligible effect, even at low ##\alpha##. You can get a wing to transition due to crossflow at ##\alpha = 0^{\circ}##, which obviously has a great effect on whether or not separation occurs and where.
     
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