Length diameter ratio of laminar pods of variable length and wing-body intersection optimization

The length-diameter ratio 3.33 was found ideal for laminar pods which are intended to the fuselage where the length Reynolds number tends to get high. The laminar flow can not sustained for very high length Reynolds number, therefore the need of relatively short pod when compared to a wing airfoil shape. That sounds like a rule of thumb, in other words, a generalization that applies to one example, but is not necessarily applicable to everything.

However, in case of engine pods, it would require some investigation to determine the optimum length/diameter ratio. On the wings, the length Reynolds number for a laminar engine pod would be similar than that of the wing. Logic says that if the wing can sustain 60% laminar flow with its chord length, then the pod with similar length diameter ratio should be able to do that as well.

Therefore, what is the ideal length diameter ratio for a engine pod if the engine pod comprises of NACA 66-series laminar symmetrical airfoil (which provides zero lift at zero degrees angle of attack)? Is it still 3.33 or something else?

I was yesterday evening also reading some documents I have got links from a Internet friend of mine (a aerodynamics-guru) and was comparing that to what was told in Bruce H. Carmichael’s Personal Aircraft Drag Reduction Book. The fuselage-wing intersection optimization is described as a rule of thumb in the book, with the premise that the designer does not have access to CFD software, optimizing the streamlines of the fuselage to be similar than the streamlines of the wing, to avoid adverse pressure gradient.

However, today the CFD software does not need very expensive, in fact, OpenFoam is free software, and the situation might prove nowadays different than it used to be (still haven’t had enough time to learn how to use the OpenFoam, but I will find out sooner or later, because I must). It would be enlightening to try out the wing-body intersection optimization. One thing I also learned is that the fairing between the wing and body has to be turbulent airfoil which has very late separation, because the flow at the wing intersection on the fuselage is turbulent anyway, the laminar flow can not be sustained that far without active boundary layer control. I am not planning active boundary layer control for step 1, to get things done.

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    • dodlithr
    • January 14th, 2009

    > Logic says that if the wing can
    > sustain 60% laminar flow with
    > its chord length, then the pod
    > with similar length diameter
    > ratio should be able to do that
    > as well.

    You have to check the Reynolds number. It he reynolds number of the compared shape is similar to the tested one, then the tests are valid also for the compared shape.

    The lenghts and speeds need not to be similar. It is only required that the Reynolds numbers are similar.

    If for example the length is smaller then the speed have to be increased to get same Reynolds number.

    http://en.wikipedia.org/wiki/Reynolds_number

    Generally speaking: Larger shapes can be used if the speed is lowered. And vice versa.

    • dodlithr
    • January 14th, 2009

    > Logic says that if the wing can
    > sustain 60% laminar flow with
    > its chord length, then the pod
    > with similar length diameter
    > ratio should be able to do that
    > as well.

    You have to check the Reynolds number. It he reynolds number of the compared shape is similar to the tested one, then the tests are valid also for the compared shape.

    The lenghts and speeds need not to be similar. It is only required that the Reynolds numbers are similar.

    If for example the length is smaller then the speed have to be increased to get same Reynolds number.

    http://en.wikipedia.org/wiki/Reynolds_number

    Generally speaking: Larger shapes can be used if the speed is lowered. And vice versa.

    • Karoliina Salminen
    • January 15th, 2009

    Yes, I know that.

    • Karoliina Salminen
    • January 15th, 2009

    Yes, I know that.

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