Coke-bottle fairing

Discussion in 'Hydrodynamics and Aerodynamics' started by revintage, Nov 11, 2019.

  1. fallguy
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    fallguy Senior Member

    It seems like foils are in an infancy developmentally, but that isn't really true.

    Things like the angles, dimensions, and even fairings seem like they would computer model rather well. Obviously, the original vessel is a part of all that..

    I am building a Wood's Skoota semi-displacement powercat and I have this silly notion that even it could have foils a/o that someday foils could be sold like outboards or props or sails, etc.

    Again, just a quiet lurker, I have little to add here.
  2. rnlock
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    rnlock Senior Member

    Something tells me that if the guys at Boeing got it right with the 707, then a fancy CFD code isn't required to do it right.
  3. rxcomposite
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    rxcomposite Senior Member

  4. tspeer
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    tspeer Senior Member

    You could use an area rule to design the fairing. The problem is, "What cross sectional area to include?" The transonic area rule is based on farfield supersonic drag, so the entire cross section of the body and wing is included. But a fairing is a near field problem, and the effects are more localized. So you could take 100% of the fairing cross sectional area, but how much of the wing and strut do you include?

    One way might be to apply a 1/(1+radial_distance^n) kind of weighting to the wing & strut, with the exponent chosen by how quickly you want the influence of the wing to drop off.

    The supersonic area rule aims to make the total cross sectional area that of a Sears-Haack body, which is the body of revolution that has minimum supersonic drag. It's not clear what the right target shape is for the 3D fairing. Perhaps an airfoil thickness distribution (say, a NACA 6-series or 1-series) taken to the 3/2 power might be a place to start.
  5. ziper1221
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    ziper1221 Junior Member

    What about staggering the foils fore-and-aft, assuming it is structurally acceptable? The bulb would have to be more stout to take the load, so more form drag. Ought the fairing still follow a double coke bottle shape, narrowing once for each foil, or is that extraneous?
  6. DCockey
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    DCockey Senior Member

    A good aerodynamic panel code should be sufficient to evaluate a series of design iterations if free surface interaction is not significant.
  7. tspeer
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    tspeer Senior Member

    That's a good strategy. It keeps everything from getting fat at the same place. It's usually employed without a pod fairing. If you did use a pod, you'd use the same principles as the Coke-bottle fairing and let the computations tell you what shape it ought to be.
  8. CocoonCruisers
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    CocoonCruisers Junior Member

    Regarding methods, my impression is that Stanford's open-source adjoint optimization code SU2 | Multiphysics Simulation and Design Software and SU2 Foundation could be a great tool for this (if one has some fluid dynamics knowledge, a bit of time and access to a hefty workstation or a small cluster).

    Rationale would be that:
    - Free surface effects are negligible at the fairing location (fortunately, because current SU2 is only single-phase)
    - Flow can become highly 3-dimensional at the fairing and turbulent boundary layers will probably matter - not easy for panel codes
    - Shape is very complex AND objective function is complex: Cavitation avoidance and L/D optimisation, plus avoidance of spanwise flow down the strut, to reduce the risk of ventilation etc. Parametric approaches (tweak geometry, analyse, repeat 200 times) aren't very efficient for such problems

    The promise of adjoint optimization is to go one step further than the usual RANS cfd analysis, which only shows flow, resistance etc around a given geometry; SU2 will also calculate a sensitivity map telling which sections of the geometry should be pushed inwards or pulled out according to the objective function. It can even deform the geometry and propose optimized shapes. Computational overhead is low compared to RANS.

    (TUHH in Hamburg is even working on an adjoint code for multiphase (simulating air,water and the interface) cases: FDS > Research > Current Projects > DynAForm . But this is closed-source academic code for the moment, and multiphase is not needed for this fairing).

    I don't have hands-on experience with this and am busy enough struggling with OpenFOAM, but the tutorials on the SU2 page come close to the problem at hand.

    If someone wants to experiment, but doesn't have access to the required computing power, i'd be happy to contribute core hours on my 768-core Xeon Phi KNL cluster, access to the cfMesh pro Mesher plus modest meshing and admin skills. And i'd be pleased to learn from you. (Can all be done remote).
    Last edited: Dec 7, 2019
  9. DCockey
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    DCockey Senior Member

    I was not aware of the SU2 project. Thanks for the links.

    A good higher order panel code with a turbulent boundary layer analysis and displacement allowance such as VSAERO should be sufficient for determining if the flow will stay attached and if cavitation will occur. Its been close to three decades since I used VSAERO and today a PC with a fast CPU and GPU should have plenty of horsepower. (An order of magnitude or more horsepower than the Cray XMP I ran VSAERO on.)
    CocoonCruisers likes this.

  10. Chase Kenyon
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    Chase Kenyon New Member

    seems to me that the shape should follow air density changes along the root cord length rather than an arbitrary Coke Bottle shape. Dad oversaw some research into that shaping at the Langley Wind Tunnel when he worked at NACA. This picture of a navy fighter show what I am trying to say. (hey, dad was the air and water fluid dynamics Navy and helicopter engineer sent with Flettner to Kaman, I'm the robotics engineer)

    BlueBell and revintage like this.
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