Coke-bottle fairing

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

  1. revintage
    Joined: Nov 2016
    Posts: 211
    Likes: 41, Points: 28, Legacy Rep: 10
    Location: Sweden

    revintage Senior Member

    In an older thread started by Doug Halsey, Tom Speer recommended using "coke-bottle" fairings at the joints between two V foil members. Suppose the one below is an example of this, even if this one is a T-foil. I get an impression that the fairing below has flat triangular sides in the front. How critical are they in shape?

    EDIT: Picture back!

    coke.PNG
     
    Last edited: Nov 14, 2019
  2. Yellowjacket
    Joined: May 2009
    Posts: 619
    Likes: 91, Points: 28, Legacy Rep: 447
    Location: Landlocked...

    Yellowjacket Senior Member

    What you're seeing is the intersection of the two wing shaped surfaces with a constant diameter section. You'd actually be better off with a fillet between the two surfaces as that would provide smooth flow and lower drag. Obviously at both ends the section is tapered, but a fillet would actually be the lowest drag system.
     
  3. revintage
    Joined: Nov 2016
    Posts: 211
    Likes: 41, Points: 28, Legacy Rep: 10
    Location: Sweden

    revintage Senior Member

    Yellowjacket: The picture above was wrong , to me the fillet looked like it had a waist, but I was probably mistaken, so I removed it.

    The coke bottle fairing was recommended by tspeer to avoid cavitation where two lifting foils with small angles meet.

    I just found a render by tspeer that better shows what I ment and it is far more radical.

    bz1p0r0Grid.gif
     
  4. BlueBell
    Joined: May 2017
    Posts: 852
    Likes: 93, Points: 28
    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Is this the post?

    "You have a very acute angle between your foils which produces a locally accelerated flow near the junction. That will be cavitation city, no matter what section you choose.

    You can think of the velocities from each foil being superimposed on each other. Where the local velocity is high, you're going to get a double dose from each panel. There are several ways to attack this problem.

    One way is by making the junction wider. A rounded junction will help, as will a wider apex angle. This moves the panels away from each other, so their interference is less.

    Another way is to tailor the section shapes toward the junction. The maximum velocity on the suction side of the section needs to be lowered, as though you were designing for a higher cavitation speed. There's a limited amount that you can do this way, but it helps.

    Another thing that's been used in the past is a Coke-bottle shaped fairing. The idea is the fairing has a dumbbell shaped pressure distribution that rises to a peak near the fat parts and then has a higher pressure/lower velocity valley in between. By arranging the thick parts of the fairing to be near the leading and trailing edges of the foil, you're superimposing the fast parts of the fairing on the slow parts of the foil, and the slow parts of the fairing on the fast parts of the foil. The goal is to smooth out the pressure variation from nose to tail through the junction, and to reduce the superposition of the fast velocities.

    Something else you can try is to stagger the panels. This falls under the heading of "everything shouldn't get fat (or fast) at the same place." If the stagnation pressure at the leading edge of one panel were aligned with the peak pressure of the mating panel, or the increased pressure toward the trailing edge were aligned with the fastest parts of the mating panel, then you'd be reducing the superposition of fast on fast.

    As to which panel to move forward and which to move aft, I'm thinking the leeward foil is the most critical one. Given that the foil is loaded in side force as well as vertically, the outboard panel is more heavily loaded than the inboard panel, and in many ways the inboard panel is acting as a winglet for the outboard panel. My intuition says it would be better to move the inboard panel aft compared to the outboard panel. But I don't have anything to back that up."

    -tspeer
     
  5. revintage
    Joined: Nov 2016
    Posts: 211
    Likes: 41, Points: 28, Legacy Rep: 10
    Location: Sweden

    revintage Senior Member

    Yep, that is the third one by tspeer.

    The first:
    "If you're concerned about the strength of the joint between strut and foil, you might consider adding a fairing to the junction. The boundary layer tends to separate at the junction because the velocity gradients are higher - you're essentially adding the pressure field of strut and foil there.
    A bullet fairing might be designed to help counter this. It would have a "Coke-bottle" shape that by itself would have a dumb-bell-shaped pressure distribution with two low pressure zones and a high pressure zone in between. The fairing would be added to the foil junction so as to partially cancel out the interference effects, with the fairing's high pressure region aligned with the low pressure area in the junction, and the fairing's low pressure peaks aligned with the foil leading and trailing edge high-pressure areas. The goal would be to smooth out the changes in pressure along the junction so as to reduce the stress on the boundary layer and prevent separation.

    So the fairing would serve a dual hydrodynamic and structural purpose."

    The second:
    "As for foil joints, one might consider some sort of fairing to act as a structural fillet. Dave Keiper found bullet fairings to be very effective at eliminating ventilation on Williwaw's bow foil, which probably indicates they were eliminating flow separation there.
    The local velocities at the junction are amplified, so cavitation will begin there as well. In effect, the velocities from the strut and foil are added together at the junction. So not only is the maximum velocity higher, the velocity gradient coming down from the peak is also higher, leading to earlier flow separation.

    One approach I've looked at for designing a fairing is to try to achieve some cancellation of the exaggerated velocity gradients in the junction area. If you consider a torpedo-shaped axisymmetric body, there will be a low pressure peak at the junction between the nose section and cylindrical center body, and another low pressure peak at the junction between the body and tail cone. If you pinch in the center body to make it more dumb-bell shaped, the pressure peaks are even bigger and there's a high pressure region in the center.

    So now combine the dumb-bell body's velocity distribution with that of the foil junction. The foil is going to have a high pressure area at the leading edge stagnation line, a comparatively high pressure (a little higher than ambient) at the trailing edge, and a low pressure region in the middle near maximum thickness. By making the dumb-bell longer than the foil chord, you can position the low pressure peaks of the body on top of the high pressure regions of the foil, and put the high pressure region in the middle over the low pressure area of the junction. The result should smooth out the pressure gradients in the junction area and reduce the maximum velocity.

    Finally, instead of keeping the axisymmetric tail cone, pinch it in to form sharp trailing edges that will help carry the lift through the junction region so there's not a big increase in induced drag at the junction due to a notch in the spanwise lift distribution there. In order to maintain the same cross sectional area, the pinched in parts of the fairing will have to extend farther out on the strut and foil.

    This early design for a Williwaw-like bow foil shows such fairings used where the foils join at acute angles. This was just my first cut at a fairing design - I've not done any design refinement or tried to estimate the drag of the fairing compared to the bare junction. But it illustrates the concept. It may not look like it from the panel shapes, but the noses are smoothly rounded (but not circular) in cross section.

    I think it's evident such a fairing would help to reinforce the joint structurally, as well as provide a place to hide fasteners and control linkages."
     
  6. revintage
    Joined: Nov 2016
    Posts: 211
    Likes: 41, Points: 28, Legacy Rep: 10
    Location: Sweden

    revintage Senior Member

    Whether one wants to make a regular or coke-bottle fairing, the first thing should be to add the cone.

    Question is where to place it over the joint and what diameter it should have.

    Next question is what counts as an acute angle?

    This is what tspeer said about Doug Halsey´s 60 degree angled foils with NACA0012 sections:

    "You have a very acute angle between your foils which produces a locally accelerated flow near the junction. That will be cavitation city, no matter what section you choose."

    Did this simple sketch and guesstimated placement and diameter of the cone alone.

    cone2.png
     
  7. tspeer
    Joined: Feb 2002
    Posts: 2,228
    Likes: 185, Points: 63, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    Paul Bieker applied the same principles in fairing the rudders on OTUSA's AC72:
    [​IMG] [​IMG]
    Boeing also did something similar in the design of the Model 929 Jetfoil
    upload_2019-11-13_17-55-15.png
     

    Attached Files:

    revintage likes this.
  8. rnlock
    Joined: Aug 2016
    Posts: 72
    Likes: 4, Points: 8, Legacy Rep: 10
    Location: Massachusetts

    rnlock Junior Member

    Just how fast do we expect these foils to go? I suspect that a foiling Moth would never cavitate. OTOH, a good fairing might cut intersection drag anyway. I suspect the optimal fairing shape will be quite different depending on whether the speed is high enough to worry about cavitation.

    I wonder if anyone has explored using a supercritical airfoil as a hydrofoil section? Seems to me that such a foil could raise the speed required to cause cavitation. It might get by with smaller fairings at intersections too.
     
  9. tspeer
    Joined: Feb 2002
    Posts: 2,228
    Likes: 185, Points: 63, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    I don't think the cross sectional shape of the fairing is critical. The cross sectional area is much more important.

    Consider this axisymmetric body that has a cylindrical body with a rounded nose (from Journal of Marine Science and Engineering https://www.mdpi.com/2077-1312/7/9):
    [​IMG]
    There is a modest low pressure peak at the shoulder where the nose meets the body. If you were to add, say, a conical tail, you'd see another low pressure peak at the start of the tail, and the whole pressure distribution would have a dumbbell-like shape to it. For this cylindrical body, the pressure coefficient on the body is near zero, indicating the flow is essentially at the freestream velocity. If you were to narrow the body, creating the Coke-bottle shape, that would result in a more positive pressure coefficient in the middle and would also exaggerate the low pressure peaks.

    Unfortunately, to design it right, taking into account both the fairing and the foils, requires at least a panel code such as CMARC, along with the requisite gridding and post processing programs.
     
    revintage likes this.
  10. tspeer
    Joined: Feb 2002
    Posts: 2,228
    Likes: 185, Points: 63, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    Supercritical airfoils and subcavitating hydrofoil sections are much alike and are designed using similar philosophies. In each case there is a velocity threshold one is trying to stay under - Mach 1 for the supercritical section and cavitation for the hydrofoil. This leads to "rooftop" pressure distributions that rapidly come up to the threshold, then run flat for much of the chord before finally dropping back to near freestream pressure at the trailing edge. At high speed, the pressure due to thickness alone can be up against the threshold on both the upper and lower surface. This leads to using aft loading to produce the desired lift, which is why you see highly cambered trailing edges with hollows on the underside. The forward half of the section is almost symmetrical.

    A cambered leading edge can also be used to generate lift, as shown by this Boeing section design philosophy:
    upload_2019-11-13_21-34-46.png
     
    rnlock likes this.
  11. revintage
    Joined: Nov 2016
    Posts: 211
    Likes: 41, Points: 28, Legacy Rep: 10
    Location: Sweden

    revintage Senior Member

    tspeer, thanks for showing this picture. It was actually the same picture I removed from my first post. A little ashamed, but I didn´t look close enough at the foil curve and looked at it backwards. Have now added it again with explaining remarks;). To me rear looks almost triangular. It also shows that it even can be useful with shallow angles.
    Being without your knowledge and computer capacity, I will anyway try it the empirical way.
     
    Last edited: Nov 14, 2019
  12. tspeer
    Joined: Feb 2002
    Posts: 2,228
    Likes: 185, Points: 63, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    Paul Bieker was shaping those fillets by hand. The rudders had to be repainted each night because of erosion from cavitation, so he had the erosion to help guide him.
     
    BlueBell, fallguy and revintage like this.
  13. fallguy
    Joined: Dec 2016
    Posts: 2,140
    Likes: 142, Points: 63, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    Thanks for this factoid!

    I am lurking and sitting here trying to decipher how anyone would have modeled this thing well enough to get it right.

    And I never really see it explained mathematically, so in my brain, I am wondering how the he||?

    But now with the anecdotal; surely someone has ventured into the quadratics?

    The shape and graph you show are not it, right?
     
  14. revintage
    Joined: Nov 2016
    Posts: 211
    Likes: 41, Points: 28, Legacy Rep: 10
    Location: Sweden

    revintage Senior Member

    fallguy, read the jetfoil pdf supplied by tspeer above. Quite old but it maybe is what you are looking for?
     

  15. tspeer
    Joined: Feb 2002
    Posts: 2,228
    Likes: 185, Points: 63, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    Unfortunately, the fairing design is going to require some cut-and-try artistry. This is where the panel code comes in. What you're looking to achieve is to keep the local pressure within the cavitation limit for the design speed and lift. If there's a pressure peak that goes past the limit, then you'll want to flatten the contours in that area a bit, because the local curvature has the biggest influence on the pressure distribution. You'll also probably want to look at the pressures along surface streamlines (or just take streamwise cuts through the junction) to smooth out variations in the pressure as much as is practical.

    For changing the foil sections, you can take the difference between the pressure distribution of the 3D section cut and the 2D calculation of the same section shape, then subtract that difference from the design pressure distribution. This will give you a new design pressure distribution that you can use in Xfoil to generate a new section shape. When you put the revised section into the 3D context, it will come close to meeting the original design pressures. It only takes a few iterations like this to home in on a section shape that is appropriate for the 3D design.

    Aircraft designers do something similar when dealing with wing-fuselage interference. Here are the sections from the Boeing 707, moving from the root outboard on the wing. Notice how the root sections have negative camber in the middle to compensate for the increased velocity on top due to the fuselage:
    [​IMG]
    [​IMG]
    [​IMG]
    [​IMG]
     
    fallguy likes this.
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.