Are humming foils slow?

Discussion in 'Sailboats' started by Nigel Hale, Nov 9, 2007.

  1. granite
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    granite Junior Member

    The rudder on my UK Cherub does a similar thing the humming will cut in at about 13Kts and get louder and higher in pitch as speed increases.

    Putting a small bevel on the trailing edge has worked for other foils that have done the same. Also going over the surface with wet and dry to get rid of any small defects seems to help.

    This particular rudder is not quite right in general shape and trailing edge is too thick. I have not tried beveling the trailing edge yet as I am probably just going to build a new one.
     
  2. jehardiman
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    jehardiman Senior Member

    300Hz @ at 12knts indicates a vortex width of 3.9mm and 800 @ 20 indicates a width of 2.5mm. If I assume that the trailing edge could be as wide as 4mm, this indicates to me that the trailing edge might be a little too wide (it should be 2% of thickness but manufacturing processes may get in the way). If the trailing edge is much less than 4mm, the data indicates that the aft section is either too fat or not fat enough (we would need to see it) because seperation is occuring before the TE.

    Additionally, does the 1/4 cord line sweep forward or back? The rules don't define a foil size or shape so we are fairly free here. You might want to reconsider tip sweep as it could be causing early seperation.
     
  3. clanning
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    clanning Junior Member

    Nigel/all:

    www.philsfoils.com

    There is an interesting technical section that talks about thickness/chord and other real-world issues for CBs.

    Phil (and Dave Bradly) are skiff sailors, so maybe a few questions put directly to them could elicit interesting opinions regarding design, construction, stiffness and hum!

    Try GoldWave for a sound editing program -- if you can get a sample of a tone, I believe GW will let you adjust frequency until you reproduce what you are hearing.

    Jehardiman, Regarding VGs, I always looked at the leading edge strakes on F-15/16/18 and wondered if there were gains to be had by placing essentially a strake (or skeg) immediately in front of the keel to help glue flow to a higher aspect ratio keel when AoA is either large or unstable -- how often does a keel actually stall, anyhow? Is this a solution where no problem really exists? Extending that idea, are strakes for the rudder a good idea or a bad idea (hydrodynamically speaking, that is)? Empirically (looking around the boat yard) I'd have to say no, but does anyone have any insight into the concept?

    Thanks!

    Chuck
     
  4. jehardiman
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    jehardiman Senior Member

    There have been several boats that played with the idea of a LE flap along with a TE flap on a high aspect foil (notably Blackhaller's "Geek" AC 12M). Nowadays, most class rules generally do not permit this many moving pieces. Additionally, due to the seaway and motions, flow into the keel is not as steady as aero. AOA variation of +/- 20 degrees is common. In average conditions, a foil design that is "middle of the road" makes more sense.

    Now fillets and cavitation strakes are an entire other matter. They are fitted to help with flow problems that decrease lift, they do not increase lift of the foil itself.
     
  5. Petros
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    Petros Senior Member



    There are actually many items that must generate vortexes in water to operate properly. For example most of the modern swim fins operate by generating two large trapped vortexes to generate thrust. Not real efficient, but it works well considering it would be rather difficult to have swim fins with high aspect ratio blades.

    If you are getting unstable vortexes on the surface of the centerboard, than vortex generators will stabilize them. It may not be a good permanent solution, but it is perfectly good as a diagnostic tool. I would use only a few very small ones to start, small triangular wedges about 2 inches long and only 1/4" deep or so tapped near the tip of the center board at a small angle (10 degrees) to the flow direction. Place one or two of these a few inches apart on each side of the board. The tip may be where the problem is since it sounds like the trailing edge will close at a much steeper angle than higher up on the board. If this is where the vibrations are coming from, they would be much exaggerated being at the tip.

    You could also reshape the tip profile by adding material (such as plastic filler, or layers of tape). If it does not work you peel it off and you have not permanently changed the centerboard.

    Good luck with your tests. Let us know what you find out.
     
  6. Nigel Hale
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    Nigel Hale Junior Member

    What do you mean by this? (I understand what a chord is..)

    I think that what you are asking can be answered by saying that the front edge of the cb sweeps back much more than the back edge sweeps forward. The back edge is nearly straight. If you think of an 18ft skiff mainsail circa 1996, with a very bent back top mast and only a very small amount of roach in the leech, you would have a similar shape to our board (but of course upside down).

    Nigel.
     
  7. Nigel Hale
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    Nigel Hale Junior Member

    The centreboard shown at phil's foils here:

    http://www.philsfoils.com/Designs/images/phantomSet1.jpg

    would be a similar profile shape to the board in question, except that ours is much longer, and high aspect. The top half of the board has straight, parrallel leading and trailing edges. However the tip is similar to the one shown here.

    Thanks again for the replies, I'll be certainly trying a few things and posting the results.

    Nigel.
     
  8. jehardiman
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    jehardiman Senior Member

    Ok, here is where I climb on my soapbox and go where water addict did not want to go early. First, a vortex does not generate thrust, lift, or drag. It is a physics response to the pressure differential in the fluid. Because of it's ubiquity, and it's easily measurable strength, it was seized upon by early fluid dynamicists as a measurement tool for what I will call "drag" ( "lift" being nothing but drag in the direction you want to go). It is not a cause, but an effect. BTW, modern materials have made extreme aspect ratio (both in cord and span) dive fins possible. It is not that they are hard to get correct or use, but rather that they are a pain to get into and out of and carry around. Do a good patent search and you will see a plethora of work in the last two decades and a half decades on this.

    This is what water addict wanted to avoid; where a poster begins using terms as they understand them in their mental image of the world.
    First, the Rn of a narrow CB (say 220mm cord) at 12kts is ~1.1x10^6 so the flow is all turbulent. The whole surface is totally covered with irregular micro vortices that grow to a thickness of 5mm at the trailing edge. All you would do by adding turbulence stimulators to the board is to increase the drag, and possibly move the separation point further up the cord, as they are useless to affect the tip or trailing edge vortices while buried in the boundary layer. In aero dynamics, VGs work because of the desire to stabilize the separation point and prevent flutter by inducing the turbulent transition on the foil. They are not needed in "real boat/ship sized/speed" hydrodynamics and are actually detrimental to getting the separation point as far aft as possible.

    The tip and trailing edge vortices of a foil generating lift are formed by the rollover of energy in the fluid from the high pressure side to the low pressure side of the foil. Any thing that effects these vortices needs to be able to effect the pressure distribution over the foil; such as tip shape, sweep, and root fillets. This is why I asked about 1/4 cord tip sweep. The swept back LE/straight TE tip shape which causes the flow to sweep up the span is detrimental to performance and causes the generation of a larger tip vortex. Ellipitical tip shapes are not a good shape to use unless the 1/4 cord line is made to sweep forward. Hoerner goes so far as to say in FDL on page 3-6 in the section on tapered wings
    Reshaping the tip, I believe, is the way forward as I pointed out above.
     
    Last edited: Nov 16, 2007
  9. Petros
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    Petros Senior Member


    The vortex is self does not generate lift, but it can be used to "bend" the flow of the free stream fluid and cause lift. This effect is used to generate lift on a number of aircraft and other machines that operate in a fluid. I am well aware of what is going on with swim fins, I have worked for one of the larger dive equipment manufacturers. In fact I happen to have a patent in my name on one of the type fins they have been producing since about 1986. What I mean by high aspect ratio swim fins is putting wide wings on each foot would make it hard to kick since they would strike each other as they passed each other. I am not aware of anyone that has made that viable. Someone is making a large single "tail" like fin that you put both feet into and you kick like a dolphin, but most people do not find that natural or comfortable.





    This has nothing to do with the turbulent transition point. You can have turbulent but attached flow, or turbulent and detached flow. Detached flow will cause vibrations because it is unstable. If you have turbulent unstable vortexes than a VG will stabilize it. This works in water as well as in air, both act as a fluid. We were suspecting that it was UNSTABLE vortexes causing the vibration. Sure adding VGs will increase the drag, but if those unstable vortexes are trapped in one place with the VG, it will not wander around on the surface and it could stop the vibration. It is not the presence of vortexes that are at issue, it is that they are free to cause rapid changes in pressures over the surface in a rhythmic fashion. If you stabilize them with VGs you will stop the vibration. True you may increase the drag, but you have quickly confirmed that unstable vortexes at the tip are causing it. Now you can formulate a plan to change the tip profile.
     
  10. jehardiman
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    jehardiman Senior Member

    :rolleyes: Sorry, another one of my pet peeves, but the vortex cannot effect the "flow of the free stream fluid" in the case of a hydrofoil because the fluid is stationary and static in energy until the work is done on the fluid by the foil. I find that it's a common point of perception problem with many of the useful fluid dynamic mathematical contrivances. What the vortices are is the work (energy transfer) done on the fluid (and they have not only velocity around their core, but also movement in the direction of travel) and to that extent it is possible to recover some energy expended back out of them, as many fast swiming and maneuverable fishes do.

    Oh? which patent? Send me the number so I can look it up and see which theoritical basis you favor. I seem to remember that US Divers/Aqua-Lung did a set of asysmetric shark tail fins about 20 years ago

    Here we differ in opinion. Because Nigel stated that it was a persistant audible hum at sailing speed, that immeaditely pointed me to a stable separation vortex (high frequency), not a gross unstable separated flow which would manifest itself at these Rns as a irregular physical, not audible, vibration. In all events, VGs of the size you recommended will cause separated flow prior to the TE. This in turn may mask the audible expected separation at the TE (as you pointed out) but would tell you nothing you did not already know. Just seems a waste to me when applying a strip of duct tape to the TE will tell him much more at this point.
     
  11. tspeer
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    tspeer Senior Member

    High frequency foil singing can be flexing of the trailing edge itself. See:

    Matveev Konstantin I., and Ivan Ivanovich Matveev, "Hydrodynamic Sound Generation on a Hydrofoil," 1996, 12th Conference On High-Speed Ship Design, Nizhiy Novgorod, Russia. Abstract: Mechanism of sound generation on a hydrofoil is considered. Mathematical model of the hydrofoil 'singing' effect is proposed. The frequencies of 'singing' are obtained. The theoretical results are compared with experiments. Contact the author to discuss: Konstantin I Matveev, email: matveev@hydrofoils.org, website: www.hydrofoils.org

    Matveev Konstantin I., "Investigation of Hydrofoil 'Singing' in the Carman Approximation," Phystech Journal, Vol.2, No.2, 1996. Abstract: "The effect of hydrofoil 'singing' has been investigated. The mechanism of sound generation is considered and a mathematical model of the process is proposed. The frequency of sound generated by a hydrofoil and the sounf intensity distribution are estimated. Experimental data are compared to calculations. Countermeasures against the hydrofoil 'singing' are proposed."

    Beveling the trailing edge helps to stiffen it as well as modify the hydrodynamics.
     
  12. tspeer
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    tspeer Senior Member

    Here's a paper that discusses hydrofoil singing a bit. What's happening is the trailing edge is flexing back and forth, and each time, it varies the amount of lift and a starting vortex is shed into the flow. The vortex shedding frequency increases with velocity. When it approaches the vibration frequency of the trailing edge, the two reinforce each other and you get singing.

    Mateev's solution was to round off both the upper and lower surfaces so they met in a sharp trailing edge with a larger trailing edge angle than the original shape. He used a larger radius on the suction side than the pressure side, so the rounding on the pressure side started a bit farther aft. This helped to preserve the aft camber of the original section and minimize the change in hydrodynamic characteristics. The thicker trailing edge was stiffer so its natural frequency was above the operating speed of the hydrofoil.
     
  13. Nigel Hale
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    Nigel Hale Junior Member

    I sanded the trailing edge, to make it at an angle to the centreline. The noise definitely changed a bit. I think that it was quieter, but we only had fairly soft breeze.

    Not clear whether it was faster or not, it seemed about the same.

    I'll post when further results are discovered.

    Nigel.
     
  14. gggGuest
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    gggGuest ...

    So, he said, uneducatedly, if you were to mess with the composition of the trailing edge, which is hardly structural, so that the resonant frequency varied in different parts of the foil, would it be possible to get a foil that had no single resonant frequency? Maybe parts of the trailing edge with softer more flexible laminate? Say patches with just microbibres in the layup and no carbon and so on?

    I also note from the paper that he doesn't list increased drag as a result of singing, but rather than increased drag was the result of the most conventional way of preventing singing when used on titanium foils... That seems to suggest that in some circumstances stopping the foils singing by chopping the trailing edge might actually be slower. Of course I may well be extrapolating far too much from what he doesn't say...
     

  15. tspeer
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    tspeer Senior Member

    Possibly, or you might just spread out the velocity range of the singing as each portion resonated at its own speed.

    I think I'd be inclined to make the whole trailing edge as stiff as I could, in the hopes of driving the resonant velocity out of the the operating range of the foil.
     
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