Effect of air injection on lift and drag of submerged hydrofoil

Discussion in 'Hydrodynamics and Aerodynamics' started by hump101, Mar 20, 2015.

  1. hump101
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    hump101 Senior Member

    I'm currently working on a design of hydrofoil to retrofit to my F40 catamaran. The primary requirement is that the boat is not modified significantly from its existing form, or if it is then this is done in a reversible manner. I don't want to lose my Golden Oldies eligibility!

    Due to structural limitations of the existing hull/beam connection, I can't use an L or J foil, with inherent heave stability, and I don't want a foil over the side arrangement as this would require external hull modifications to mount them and is not, in my opinion, a very elegant structural solution (read: heavy and ugly).

    The boat lives on a mooring, so I am trying to avoid any complex equipment in the water (wands, flaps, etc).

    Consequently, I am looking at the potential to replace the daggerboards with assymetric T foils (longer outboard), with the horizontal element mounted forward of the vertical element. This will require internal structural modifications in the hulls to cope with the lifting forces, but these will be hidden and may be removable.

    I don't have the necessary power available on board to pivot the entire T foil in the case to provide control of AoA (foil control will be entirely autonomous with a CPU control system). Consequently I am looking at the possiblilty of controlling vertical lift by injecting air onto the upper surface of the foil through internal tubing inside the foil. A series of tubes across the span will allow the lift to be regulated in a piece-wise manner, with the added benefit of reducing structural loading and very low power requirement. Any growth in the tubes can be blasted out with compressed air periodically to keep them clean, but the location of the tube outlets in the low pressure region should allow them to operate normally without a compressed air source, just a series of valves.

    I've done a literature review but haven't come up with any data I can use for design. Lots of data for cavitation avoidance/noise reduction/friction reduction, but no lift/drag info. I've done some crude calcs based on reduced effective area, but does anyone have any data before I start experimenting? Any comments on the viability of the proposed solution would also be appreciated. My main concern is whether the resulting system would have too high a drag to give a significant performance improvement.

    Thanks, Guy
     
  2. Rastapop
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    Rastapop Naval Architect

    Please excuse my ignorance, but I'm not entirely sure I understand what's being attempted here.

    The air you pump out will be at a higher pressure than the water on top of the foil, so is it going to have the effect you desire?
     
    Last edited: Mar 20, 2015
  3. hump101
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    hump101 Senior Member

    The water on the top of the foil is at low pressure, below atmosphere, typically, so it will suck air down the tubes. With a sheet of air over the top of the foil instead of water, the local water flow is disrupted and slowed, hence lift is locally reduced. By controlling how much and where the air bleeds out, based on heave, pitch, roll, speed etc. measurement, I am trying to control foil lift in a manner that has a suitably rapid response time to act as a dynamic heave control.

    I should add that we did something similar to a supercavitating foil some years ago, as a means of limiting structural loads, and the introduction of air does have an effect in reducing lift. I am hoping that for a sub-cavitating foil the effect will be more marked, and controllable.
     
  4. cmckesson
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    cmckesson Naval Architect

  5. hump101
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    hump101 Senior Member

    Thanks for those references, I'd not previously seen them. As you say, a different solution to a different problem.

    As an aside, reading that patent I would be very surprised if it could sustain any challenge, since the prior art supercavitating foils already "base ventilate" when placed in a suitable position, and they've been around long before this patent. It doesn't seem specific enough about a novel concept or detail to be defensible.
     
  6. johnhazel
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    johnhazel Senior Member

  7. hump101
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    hump101 Senior Member

    Thanks for the link, I've had a look through these previously and not found anything relevant.
     
  8. Doug Lord
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    Doug Lord Flight Ready

    Surface running fully ventilated foil

    H, this is about a "surface running ,fully ventilated foil" invented by Jon Howes who is a member here. The foil was used on an almost production dinghy and on a windsurfer. It might work.....


    "As the foil was my concept I may be able to lay to rest some of the speculation regarding how it works.

    The facets towards the aft side of the upper surface are designed to ventilate progessively from rear to front as speed builds. When fully non-ventilated the section is highly cambered and generates a lift coefficient of around 0.77 and this gives a very low lift off speed.

    When fully ventilated (from the mid-chord aft facing step on the upper surface) the lift coefficient is 0.2. Between these two extremes there is a need for an intermediate condition so that, as the foil approaches the surface, sufficient speed has already been achieved to fully ventilate and no loss of lift occurs as the foil surfaces.

    I have missed out a lot of developmental steps here, but briefly, the pressure coefficient at the leading edge of the last facet is the lowest on the upper surface when completely unventilated and so giving an air path to this location (by an aft facing step or channel on the vertical strut) leads to early ventilation. The aft facing step on the facet tapers from root to tip and the step is swept (as is the foil). In combination these two features lead to a progressive ventilation growth across the span. By the time the ventilation is approaching the tip the next facet forwards is generating a healthy low pressure coefficient and the ventilation can either jump forwards from the aft facet or be fed to the step at the leading edge of the forward facet and a similar ventilation growth, and steady reduction in lift coefficient then occurs as speed continues to increase. By now the foil is approaching the surface and when it finally breaks out into surface running no sudden lift change occurs. The ventilation cavity follows the foil when submerged and, in design, is treated as an integral part of the foil section.

    As for drag, just like an aircraft, this is in two parts, lift induced drag and skin/profile drag. The profile drag of this family of sections is always higher than that of a conventional section if below cavitation speed, however the lift induced drag follows the same rules as any other lifting surface, ie, three things matter: wingspan, wingspan and wingspan. A comparison would be a lift to drag ratio (L/D) for a conventional section of around 100, for this section it is 22. When used on a 3D foil, the lift induced part becomes important and a conventional foil may then give around 12 to 15 whereas the ventilated section as we use it here gives about 10.

    In conclusion, upwind we will not beat a moth in moth conditions. With the wind free, after all the conventional foils have cavitated this thing will keep going and speed will be limited by nerve and structural failure. Note that this section was originally developed for my Monofoil project and is designed to behave beyond 100kts (getting there is another problem but if you don't design for it......). The foil on the dinghy will give simple sailing, low lift out speed, relatively shallow draft and, if the conditions and nerves allow, high speeds on the freer points of sailing.

    I notice the comments on combiantions of surface running foils with submerged foils below. We are experimenting with this on the dinghy rudder to give some tactile feed back to allow the correct incidence to be maintained. It could also be used to give crude ride height control to a submerged lifting foil but will obviously have problems if the speed increases sufficently for the submerged foil to lift the SR foil away from the surface. We put this in the patent application because you need to (?) but I do not have much faith in it as a design approach.

    On a sailboard it gets very interesting since cavitation will not occur, the board is kept away from wave impacts (and hence much of the source of drag on a planing board) and the span loading is reduced by at least a factor of two giving a much lower lift induced drag. If you want easy foiling go for the dinghy, if you want to scare yourself go for the board.

    Jon."
     

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  9. hump101
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    hump101 Senior Member

    Thanks Doug, that is really interesting as it is a different section, but similar in principle, to the wave piercing planing surfaces I used on my speedsailor, which I've previously discussed. My foils are intended to plane on the lower surface only at speed, so have dihedral and a conventional (sub cavitating) upper surface profile, since it only is required at low speed.

    However, for the F40 running offshore I need a fully submerged foil otherwise the motion will be excessive, hence the need to control the lift of a fully submerged foil with minimum energy.

    From my literature review and responses here I am going to experiment with a test section to derive some data, as i doesn't look like this has been tried before, or at least not reported.
     

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  10. tspeer
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    tspeer Senior Member

    This form of direct lift control doesn't have a very promising history. The problem is, when air is introduced to an attached flow, it is immediately swept off the surface. In addition to supplying the air, you need to have flow separation and the ventilated area has to form a stable cavity. Because the flow is separated, there is a drag penalty, too.

    If you were to plot the pressure distribution around the section on a vertical axis (instead of the more common horizontal axis), what you will find is there are two loops that very nearly cancel each other out. The high pressure at the leading edge is almost matched by an increase in pressure toward the trailing edge. This results in low drag. And, if it weren't for the boundary layer, the two would exactly cancel and there would be no profile drag.

    When the flow is separated, there is low pressure in the cavity and you don't get the increase in pressure toward the trailing edge that you would with attached flow. The loss of higher pressure toward the trailing edge means the high pressure at the leading edge is not balanced and there is a pressure drag.

    So even if you managed to make the air control scheme work, the performance of the boat is likely to be disappointing.

    You should take the power you were going to put into pumping the air and use it to run an actuator. If the loads from moving the whole foil are too high, then just move part of the foil - a trailing edge flap like the Moth sailboats use. The Moth only has the force on the wand as the power to move its flap, which is not much if the wand isn't to have excessive drag itself. You can design the flap so the hinge moments are nearly balanced, reducing the power demands even further.

    I think you'll find that a trailing edge flap is the lowest power and least drag means of controlling the lift on the foil.
     
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  11. Doug Lord
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    Doug Lord Flight Ready

    Guy, I've had years of experience with a wand and flap system -taught by the Master, Dr. Sam. And I'd be willing to bet almost anything that a wand system with a flap is 100 times simpler than attempting to use air. Not only that but Dr. Bradfields old company still has the tooling(I think?) for the foils for Skat- his 40 footer. But even starting from scratch would be simpler and more effective than an air system, in my opinion.
    The wand system is entirely mechanical and quite simple when you get into it as well as being easy to install and remove and easy to retract, with the least possible mod to the boat.
    At any rate ,good luck!

    PS--This is Bradfield companies website-I don't know if the guys are still active since Sams death. The price for the SKAT T+ foil is too expensive if you're good with carbon but you could ,at least, get some reference on foil area. https://sites.google.com/site/hydrosail/HydroSail-Home

    Also some pictures of the Osprey foil and wand. The wand is mounted to the board-the only thing mounted to the boat is the foil and the shockcord.

    Picture SKAT flying and Osprey main foil:
     

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  12. hump101
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    hump101 Senior Member

    Many thanks, Tom, that is exactly the kind of information I was looking for, and you have confirmed my concerns over section drag. I should clarify that I was intending that the foil suck the air down, hence no pumping requirement, just valve control.

    The problem with using a flap, or any hinge system, that is permanently submerged is that I would need to keep it free of growth all the time. Keeping a smooth foil free of growth is easy, but hinges/pushrods, etc. are difficult as access is always poor.
     
  13. hump101
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    hump101 Senior Member

    Thanks again, Doug. I appreciate the simplicity of the wand system, but one of the aspects I am looking to introduce is the ability to process the signal from the heave sensor prior to actuating the foil - I don't want a foil lift profile that just follows the surface profile. I want to remove the noise from the signal and include other sources, such as accelerometer/gyro data, and then process the resulting data based on the characteristics of my boat to provide an "optimum" foil control.

    Primarily I want to avoid having any moving parts permanently submerged. My boat dries out on its mooring occasionaly, and since it can't take the bottom it sits on a mooring frame, which further limits space for foils. With this, and my beam bending limitations, I will have to custom make the foils around my geometry, so the Skat foils would not be suitable.
     
  14. Doug Lord
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    Doug Lord Flight Ready

    Guy, will you be using rudder T-foils as well?
     

  15. hump101
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    hump101 Senior Member

    Yes, and the intention is to control AoA on these as well, as a means to provide an average AoA setting on the main foil, but the pitch rotational inertia of the boat is too large to try and control main foil AoA using the rudder foils only, hence the need to have a rapid response lift control system on the main foil.

    An alternative would be to use a canard system, but this would involve some major structural changes to my boat, so I'm not keen on this.
     
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