During various threads, the concept of "supercavitation" has been mentioned. I thought it might be interesting to start a separate discussion.
Very interesting Mat. Best explanation I have seen so far.
I would think this phenomenon would be easily tested and demonstrated in a 'water tunnel'. Did you find any pictures?
What causes the unstable tip vortex? Is it due to the sharp edge and the pressure difference between the sides?
If that occurs at 20 knots I wonder what's happening at the leading edge of our conventional fins. I know there is a general requirement to keep the leading edge somewhat rounded. Could that be the reason?
The problem seems to be that at high speeds you have supercavitation whether you like it or not so how do you handle it. Is that correct?
So back to the supercavitating foil - You have a sharp leading edge generating an oscillating vortex about the low pressure side. You say you need 100 knots to make the bubble stable. What's the problem with it being small but unstable? Does it cause vibration or damage the fin? If the chord is short enough would this help?
Why do you need a high angle of attack? Is that just to keep the back of the fin away from the collapsing bubble?
If the foil is asymmetrical that implies that the low pressure side is not going to be used as a high pressure face so you are free to fiddle with it.
We have conventional foils that appear to work up to 50 knots at least. We know that with supercavitation work is being done and in principle this work can be harnessed to do the work of a foil. So is the problem that conventional foils WONT work with supercavitation, or is it a problem of dealing with a transition state like going supersonic in air?
So... supercavitation occurs when one of those micro-bubbles is actually big enough to completely envelope the object. The purpose? -> It appears current research is focusing on reducing skin friction; in some studies it appears that at least 70% of the drag is directly related to skin friction - being able to reduce two thirds of your drag is a good design target.
For example, the current military research aims to produce torpedoes and subs which are capable of about 1000 knots underwater (I **** you not... Mach 1+... well Mach 1 airspeed... aka Mach .5 water).
The catch with supercavitation is that you need enough thrust to vapourise enough water to cause the envelope to form - then a bit more so that the bubble remains stable. And this is ignoring the need for directional control... Specifically, in tank simulations, about 100knots is needed to achieve stable cavitation bubbles for "normal" looking objects.
To actually achieve stable supercavitation, the current designs (specifically, torpedoes) all pretty much use some exhaust gas or some other gas, injected into the nose of the object, causing a gas layer between the device and the water - in effect a gas boundary layer. I guess, when you have a few billion dollars, you can make anything fast... The directional control is either provided by diversion of thrust using gimbled engines, some type of "thrusters" (aka microjets) or some fins which drag through the water.
Note that (AFAICT) there isn't much data on what a "normal" supercavitating design might look like - as opposed to a "normal" non-cavitating design pushed into supercavitation.
Regarding a "supercavitating foil design" - the idea is to cause a bubble to form around the fin. A very crude way to do this would be to simply use a rectangular leading edge - at speed, the water hitting it will diverge enough so that it wont touch the sides of the rectangle. eg:
After thinking about it for a while, the shape above was rough guess as to what might work to help cause pressure instability at a lower speed than would otherwise.
I think a sharp leading edge might work - although I am very skeptical... maybe a squared-off bit right on the edge...
Mathew said ...which gives us our current limitation of about 45-50 knots, ie: the cavitation point of sea water
Good design can extend this, bad design and cavitation starts much earlier, for example here is a naca 0015 cavitating at 15.6 knots
http://www.fluidlab.naoe.t.u-tokyo.ac.jp/Research/CavPictures/large/Cloud.birdseye2
I think Mal and Chris have fins that have a cavitation point over 50 knots and I have a design that will do 54 according to the same program but probably isn't as good getting there. I suspect they all look very similar.
Nebbs, you are wrong on the sailrocket foil.
This is what they are using. The one you referred to was a design study.
As they say on their site
..
"The Mk I design currently under test was built by DesignCraft and has the following details:
Span 700 mm
Area 0.137 m2
Section SR230-NC2 (subcavitating)
Construction Infused carbon shell on foam core
Subcavitating foil designs such as the Vestas SailRocket foil, attempt to avoid cavitation although it is generally accepted that cavitation is unavoidable at speeds significantly above 50 knots.
This is what the profile looks like (blue)
NB they could only get 45:1 out of it and not the 56 on the diagram. The green (6.5%) had a L/D of 45:1 also and red (3.9%) was 33:1
Yep, thats a very conventional looking subcavitating speed foil section. Looks like they have given up on the idea of supercavitiation? That puts them squarely back in the race with all the windsurfers. MI and Hydropter with cavitiation being one of the biggest limiting factors.
I have question about cavitation.
When I have a "spin-out" ie when I push too hard on the back foot, or try too hard going upwind or crossing heavy bubbly water (it happened to me trying to cross the wake of a big Hobart ferry and totally went side-ways at full speed with no control); I thought that I had cavitation on the fin wich sort-of made it "inexistant" (going side-ways).
How would you control a board at 100 knots with a supercavitating fin ?
Don't we need a balance betweend less (or no) drag and a minimum of it to be able to control the run?
Lift is only about 1/3 when using just the positive side. Lift is basically directly proportional to AoA. If you look at the above pick the foil is working at about 30 degrees. Obviously you cant sail at 30 degrees for long before tripping a rail.
In your case it probably was aerated water rather than cavitation.
How is it, that the blades of a prop on any decent size outboard motor, don't suffer from cavitation? I'm sure they pass through the water at more than 50kn.
have a look at their surface, especially on the tips. Any tiny pits are caused by cavitation.
Tiny pits don't stop them from pushing the boat along. Has anyone ever tried to shape a fin like a porp blade? It may be the ultimate in assametric fins.
Kind of related to bigbear's question...
One variation in the type of prop-transom-setup is the use of surface-piercing props - this configuration generally doesn't suffer from cavitation!
And yet they tend to be used on the fastest speed boats due to their low-drag and high efficiency properties, eg: low drag and good efficiency due to prop/foil-shape, low drag due no other bits of steel sticking in the water, increased efficiency as the prop is a zero degrees to the water surface (as opposed to about 15 degrees). Basically this design is one of the most efficient layouts for a prop-driven boat - at high speed - at low speed they tend to suffer in other aspects of boating.
The reason for no cavitation is they use the air as a way of "superventilating" the back of the prop (ie: dragging air into the down-stroke), thus reducing the low pressure -> so you get no cavitation - aka, lower drag than when cavitating, but higher drag than when not cavitating.
However, I dont think it would be possible to use a foil in superventilating mode for windsurfing... superventilation occurs mainly on the lower pressure side of prop due to air injection -> I cant think of a way of air-injection on a fin, which wont cause spin-out.
Here is some general info on surface-piercing boats and superventilation: people.well.com/user/pk/SPAprofboat.html
Maybe at cavitating speeds water inertia is enough stop spinout. In that case you could ventilate the low pressure side.
How do I put that another way? Spinout happens when a void appears on the low pressure side of the fin removing support for the pressure bulb from the high pressure side of the fin causing water that would normally support the fin to move away, ok?
The way water supports a fin is two fold. Both water pressure and water inertia play a role. The faster you go the more inertia takes over - (f=ma i.e The force supporting the fin equals the mass of water by its acceleration as the fin pushes it away)
Spinout is a failure of water pressure not inertia. Once you are going fast enough to be supported by inertia ventilation shouldn't matter.
What would happen if you put that supercavitating fin design on a normal windsurfer, with an air pipe going down to the low pressure side?
Wouldn't it then behave as if it is cavitating, when it is actually just ventilating?
I would guess that you'd discover you're going along a lot slower than you were with a conventional fin... but if cavitation really is your problem (ie you regularly crack 45 knots on a GPS) then you might get a speed advantage.
Regarding angle of attack, why not angle the fin in the finbox? That way your board is still going straight, relative to the water. Less chance of tripping a rail I'd guess.
I think that the 'supercavitating fin' in the diagram above is stalled...
because I just came across this:
Decrepit: Yes, getting back upwind would be a problem
Not sure what I'm looking at there. Is that the end of a long prismatic shape up against the glass wall of a tank? It looks like a tile skimming across the surface of the water.
I think it is from here
cse.umn.edu/aem
Basically a foil on a supercavitating torpedo that extends through the cavitation bubble into the water for steering the torpedo.
I found it here:
cse.umn.edu/aem
I think the picture is looking at the tip of a supercavitating foil, with the water surface 'below' it in the picture. So the foil is surface piercing, much like a windsurfing fin when you're flying over chop.
I wonder what a "learner" superventilating fin would look like?
What I mean is a fin that could be sailed (although inefficiently) at 30 knots with a high angle of attack without suddenly losing the lot like our present fins do.
It might allow the same sort of incremental development which has made our subsonic windsurfers such a joy..
I think we've come full circle. Isn't a planing hull just a superventilating foil? With adjustable area. We've sacrificed the drag penalty for the control advantage. Are kiters using the one superventilating foil for both lift and side force?
How do the blade riding moths on the AMAC thread go in heavy weather? Do they lose out on the control issue to planing hulls?
