recovering thrust with a tfoil

Discussion in 'Hydrodynamics and Aerodynamics' started by philSweet, Jan 7, 2013.

  1. DCockey
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    DCockey Senior Member

    No, the wake drag of an aircraft is not described by thin ship theory. Aircraft do not generate free surface waves.

    See related thread for discussion of how shock waves are fundamentally different from free surface waves.
  2. markdrela
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    markdrela Senior Member

    In detail they are different. But in a more abstract sense they are very similar.

    The vortex drag of a subsonic aircraft, the vortex+wave drag of a slender supersonic aircraft, and the wave drag of a slender hull are all associated with or "due to" the kinetic energy in the flow left behind by the vehicle. In all cases the deposited motion, when still close to the body, is a non-dissipative potential velocity field. Hence, in all cases the "lost" energy can in principle be recovered to varying degrees by adding appropriately loaded surfaces. To reduce vortex drag you can add a span extension or winglets. To reduce supersonic fuselage wave drag you can add a volume behind it (effectively increasing its length). To reduce ship wave drag you can add the loaded t-foil. All these devices manipulate the potential flow so as to reduce the outgoing kinetic energy and thus reduce the associated drag.

    All vehicles also have viscous or "profile" drag, whose power loss which shows up as heat in the boundary layers rather than as outgoing kinetic energy in the potential flow. In the case of an airfoil the viscous dissipation is 90% complete at the trailing edge, so at most you can recover 10% of the profile drag by eliminating the viscous wake. The messy flow behind the transom probably offers more than 10% possible recovery, but it's definitely less than 100%. There is no fundamental limit to reducing induced or wave drag -- you could remove all of it with infinite span, infinite effective waterline, etc.
  3. DCockey
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    DCockey Senior Member

    Mark, see posts 193 and subsequent of this thread for the context of Phil's claim above and my comment on it. Phil appears to be equating thin ship theory and slender body aero theory.

    I agree with your comments about the differences between inviscid drag and viscous drag. I'd add that one frequent obstacle to effective use of devices which recover/reduce inviscid drag is that such devices sometimes cause an increase in viscous drag which exceeds the reduction of inviscid drag.
    Last edited: Jan 9, 2013

  4. philSweet
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    philSweet Senior Member

    I'm trying to sort out dissipative and dispersive.-

    For a stern wake, if it is near the ship and is not dispersive, a foil that is stuck in the wake and flattens it will have the effect of flattening the entire wake directly behind the foil. Eventually, energy will creep in from elsewhere, but the original pattern should be lost.

    In contrast, If the wake was somewhat dispersive in nature, a foil could not obliterate the pattern because the energy velocity is different than the velocity of the foil. You could create an node, but not a flat wake.

    If the field is nondissipative, I take it that it is isentropic.

    If it is the decay product of a dissipative event, the potential for recovery is reduced.

    A foil for recovering wake energy would want to operate much like a flap on a wing. The slot between the transom and the foil would be designed specifically to reduce dissipation.

    Please consider the above statements as questions/propositions.

    And yes DC, I buggered up the model references. Being neither an Aero guy nor a Hydro guy, I have decided to try to compare them by analogy. I have a decent bit of the math ability to support it, but not the cannonical training. My ducks didn't arrive all in a row.
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