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9 Answers

Please explain about boundary layer

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I've read through Pilot's handbook and wikipedia regarding boundary layers, but i'm unable to get it 'clicked' in my mind... can someone please break it down for me how does:

boundary layer, laminar, turbulent, speed play together under skin friction drag?

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9 Answers

  1. Bill Trussell on Mar 17, 2012

    Think of boundary layer as the small amount of air space between the moving air and the stationary surface of a wing.  As you might imagine this area has to contain air that is both moving quickly, farther away from the surface of the wing, and very slowly, near the surface of the wing.  How smoothly the transition between the quick moving and slow moving air is determines the type of “flow” and the affects on wing performance.
    Ideally, the transition is very smooth, with every partical of air cooperating and going in one direction.  This is laminar flow.  Think of this as playing tug of war with all participants on the same side of the rope.  It is pretty easy to win under this condition.  The prize is more lift for less power.  This is laminar flow.
    A rougher transition causes air particles to move in different directions, which means drag increases and the wing becomes less efficient in the production of lift.  This is sometimes called turbulent flow.  Some describe it by asking the reader to picture water moving around a rock in a stream.  The water is smooth around the rock to a point but often becomes very turbulent, or white water, at some point around the rock.  When this happens to a wing the air flow close to the wing surface becomes a hinderance to lift production and eventually contributes to a stall condition.
    The ideal situation is to have laminar flow under all speed, wing configurations, and flight conditions.  Unfortunately, wing design, icing, flight conditions and configuration all play a part to some degree or another to spoil the fun.  Next time you are on the ramp check out the wing of say a piper warrior and compare it to a Cirrus or a Mooney.  Guess which one is more likely to keep a laminar flow for most of the time.

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  2. Brian on Mar 17, 2012

    The boundary layer is a thin layer of air located just off the surface of an aircrafts entire airfoil; fuselage, wing, tail, etc. Everywhere.  However, for simplicity, I will only discuss it in relationship to the wing’s airfoil.
    The words turbulent and laminar refer to how this thin layer of air presents itself; It can be viewed as a spectrum. The extreme laminar end of the spectrum requires a rivitless, perfectly smooth, and even waxed airfoil. In practice, a scratch or piece of hair on the wing can shift the flow from laminar to turbulent. In other words, a true laminar flow is nearly impossible to achieve in practice. Do recall, though, that it’s a spectrum. A smooth glider or high performance aircraft wing may not present with a perfect laminar flow, but the flow is far more laminar than that of a 172.
    These types of flow play a large roll in the amount of skin friction drag experienced. A laminar airfoil, by definition is an airfoil designed to keep the boundary layer flow on the laminar side of the spectrum, will have far less skin friction drag than a turbulent airfoil. Such an airfoil will be capable of greater cruise speeds. However, stall characteristics of a laminar airfoil are typical more unfavorable. Think of the gradual nature of a stall in a 172 or other trainer. In an aircraft with a laminar flow, stalls tend to be more sudden. 

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  3. nine on Mar 17, 2012

    Understood. Now, as my airspeed increases, not talking about laminar and turblent, but just the height of the boundary layer, will it decrease in height as speed increases??

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  4. Brian on Mar 17, 2012

    Oh boy. The mathematics involved with boundary layer thickness calculations are mind boggling. From what I can see, it would seem that boundary layer thickness increases with velocity. (Really just guessing.) However, I am certain that type of flow and distance from the leading edge where thickness is measured play a more integral role in boundary layer thickness.
    Thickness is increased with distance over the airfoil. Thickness increases more rapidly  (x^4/5 — where x refers to distance) with a turbulent flow than a laminar (x^1/2) flow. Hopefully Nathan will read this shortly as he can provide you with a more clear and accurate answer.
    Do you mind my asking, are you studying to be a pilot or an engineer? I’ve noticed you nearly always ask technical, design related, questions. And that leaves me curious as to your background/aspirations within this field. This would greatly impact how I format my answers to your future questions.

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  5. Dan Bernard on Mar 17, 2012

    Boundary layer thickness decreases with Reynolds Number.  Since Reynolds number increases with velocity, a higher speed flow leads to a lower boundary layer thickness (assuming that all the other factors that determine Reynolds number remain constant).

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  6. Nathan Parker on Mar 18, 2012

    For laminar flow, on a flat plate at zero AoA, the boundary layer thickeness is
    thickness = sqrt(kinematic viscosity of air * chordwise position / TAS)
    (source: Boundary Layer Theory, Schlichting, p. 31)
    So in this case, the boundary layer gets thinner the faster we go.  This doesn’t take into account, however, the adverse pressure gradient on an airfoil, a gradient that increases with AoA.  This adverse pressure gradient tends to thicken the boundary layer.  Still, these two effects work in the same direction:  faster, thinner boundary layer.

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  7. Brian on Mar 18, 2012

    “For laminar flow, on a flat plate at zero AoA, the boundary layer thickeness is”

    I’m not even sure how to ask what I wish to ask here, but I’m going to try. I see a v, defined as velocity component, in the numerator, and sometimes in the denominator through the derivations of boundary layer thickness in my fluids book. Why does it keep jumping around?

    I think I see the formula you refer to: Thickness = 10.78 (Mu dx / rho U) Where Mu is viscosity and U is velocity.

    But then I also see a turbulent boundary layer formula: Thickness = 0.37 (v / U) ^ 1/5 where v is velocity component and U is velocity. See my confusion? Perhaps velocity effects a turbulent boundary layer differently? Or perhaps I’ve not focused on the correct formula for a boundary turbulent layer?

    Possibly your library has the book Fluid Mechanics Seventh Edition by Streeter & Wylie? If so, pages 210-220.

    Thank you in advance. And sorry to the rest of you for having to read this gibberish! 😐

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  8. Nathan Parker on Mar 19, 2012

    “v” is kinematic viscosity, not velocity, in this formula.  It’s actually the greek letter “Nu”.

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  9. Nathan Parker on Mar 19, 2012

    BTW, one way you can see immediately that v and U can’t both be velocity is that if they were, the units of measure would cancel out, leaving no units, whereas we know the result needs to be something like “meters”.  Just by inspection, v needs to be something like “m^2/s”, which, it turns out, it is.

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