During various threads, the concept of "supercavitation" has been mentioned. I thought it might be interesting to start a separate discussion.

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Ian K said in another thread...
Looked up Wiki to see what there is out there on supercavitation - not much. From what I can gather its main purpose is to get a superbubble on the low pressure side which doesn't collapse and cause damage until the foil is clear. Deliberately inducing cavitation also bypasses a normal foils unpredictable behavior at ~50 knots. But the compromise is they have to have a high angle of attack and so a lower lift to drag ratio.

So getting the craft foiling might be the crux of the matter. Maybe it'll do 5 knots or 55 knots but nothing in between? 55 knots or bust. Good on 'em. Oi! Oi! Oi!.

I did some reading on this during summer - like you said, there isn't much info out there. What I could find said that a minimum of about 100knots is required for a stable cavitation bubble, increasing to 150 for big things like torpedoes. However just about all of the evidence suggests that some form of induced tip bubble needs to be generated, eg by diverting some torpedo exhaust gas, to the tip.

I did get the picture that a triangular shaped foil might work, with the pointy end forwards and with a sharp tip. The idea is that the tip creates a pressure vortex which quickly becomes unstable thus causing cavitation bubbles at very low speeds (say 20 knots boat speed). Then with enough speed, due to the sides of the triangle being sloped outwards, the tip-bubbles start to coalesce forming something stable. The rear-end of the foil shouldn't trail off like a normal foil as the negative pressure gradient will cause bubble instability. eg:



Of course a windsurfer probably couldn't use this foil as the horsepower needed to overcome the initial drag would be huge (although it might work if you could hold down a formula sail...).

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?

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NotWal asked some questions...

Interesting questions... I guess its probably worth first describing what cavitation actually is, for those not up with it.

Cavitation occurs when the pressure drops low enough so that either the air dissolved the water is released and/or some water itself changes into steam. Since the bubbles are tiny - think nanometers to micrometers in diameter - they instantaneously collapse to produce extremely high temperatures at their core (eg: 5000 degrees) and a very strong shock wave.

The theoretical speed at which cavitation will occur, can be calculated for a given foil shape (aka pressure distribution) and given water state. On a fin, cavitation can occur due to foil shape, irregularity in fin surface and outright speed. Some of the fin designers (eg: Mal Wright, Chris Lockwood) are using various software packages to produce foils which aim to reduce drag and increase lift - the aim is to produce a foil shape which controls the pressure distribution so that it isn't too extreme thus causing premature cavitation.

Of course there are other issues to fin design which affect the foil profile such as flex - which may prematurely cause cavitation - or not... And to make it complicated, a large part of these calculations make some assumption about the water state, eg: if the water is warm, then cavitation occurs earlier; if it contains lots of air, it may occur earlier, etc.

So... when cavitation starts occurring, drag goes up drastically ie: orders of magnitude more drag. And since the the water is disturbed, the pressure changes and then so does lift, etc. Note that flow doesn't "detach" like it does in a spin-out, rather "drag" simply increases from about the widest point in the fin (well not exactly - its way more complicated than that...).


One a speed-boat, the bubble implosion is often what causes the pitting on speed-boat propellers. But since there is so much horsepower available, its easily possible to push past the increase in drag, this causes the number and size of the bubbles to increase.

On a windsurfer, we dont have loads of extra power, possibly just a little bit more -> so it makes increases in speed pretty hard - which gives us our current limitation of about 45-50 knots, ie: the cavitation point of sea water.

Note that some of the 45kn speeds being achieved, may not be limited due to cavitation, but maybe simply due to ventilation or foil (aka pressure) instability.

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.

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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.

I'm not sure whether a supercavitating foil is capable of working similarly to a "slower foil" ie: producing side-lift... AFAICT, the research is simply focusing on reducing skin-friction.

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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?

Anything could work "in supercavitation mode" - just like sticking an Ares rocket engine to your Volvo would make it go supersonic - any shape will supercavitate, but not very well...

As you hinted, there is a problem when transitioning from "starting-cavitation" to stable supercavitation - basically the drag increases dramatically during this phase, thus requiring a suitable increase in thrust.

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What causes the unstable tip vortex? Is it due to the sharp edge and the pressure difference between the sides?

yep - well... maybe... Thinking about it, the tip instability might be able to be modeled using XFoil...

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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?

Well... I was a little hopeful of that "20 knots" figure... I think a sharp leading edge might work - although I am very skeptical... maybe a squared-off bit right on the edge...

At very high speeds, yes you always have supercavitation - but without a rocket motor, your probably not going to come anywhere near it with conventional engines.

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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?

100 knots is minimum value that has shown in tank results (plus high speed photography), to produce a stable bubble around the object. As you said, the sharp tip is simply there to cause a pressure variation at a much lower speed - whether that pressure change is of the good kind is a separate question... and you would need to bubble on both sides of the foil, not just the low pressure side.

An unstable bubble will collapse earlier in the flow, thus causing water to touch the foil - increasing drag very dramatically (aka shock waves, turbulent water, etc). Vibration and damage is probably a minor problem compared to the thrust issue...

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Why do you need a high angle of attack? Is that just to keep the back of the fin away from the collapsing bubble?

Not sure what is being asked... the aft-part of foil shape? then yes, ie: simply to ensure that there isn't a negative relative-pressure gradient.

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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.

Note sure about asymmetry... since there is pretty much no knowledge of side-lift when supercavitating, we cant even assume there is a "low pressure side"... although I do suspect there is, eg:




Here I am trying to show what happens when we change the angle of attack... normal flow should cause the bubble to eventually collapse somewhere aft of the foil. When the angle changes, there would be change in pressure from each side, giving us some control (these are awful pics - the fluid line shouldn't touch...).

I also suspect that the tail of the bubble might elongate somewhat - which may cause the equivalent of directional thrust in a direction off-axis to the direction of travel.

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

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yoyo said...

Nebbs, you are wrong on the sailrocket foil.

Whoops! I stand corrected

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.

Supercavitationexpialidocious!

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LeStef said...

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?

"Ventilation" and "cavitation" are 2 different phenomena. "Ventilation" is common or garden spinout. Its cause by sucking air mostly from the surface and a bit of entrained air out of the water. "Cavitation" is the phenomenon described above by Matthew. It is largely vaporised water in tiny areas of intense low pressure. The bubbles are continually collapsing and reforming following local pressure fluctuations. Sound waves can cause it.
People often use the term "cavitation" improperly to refer to ventilation.

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.

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bigbear said...

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.

Outboard motors have a huge amount of power compared with sailing craft. They just suck up the extra drag and keep spittin' out thrust.

Prop on large ships also cavitate. It is one the the sounds that sub drivers listen for on their sonar.

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.

And to get back upwind you'd just block the pipe?????

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.

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mathew said...

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.

first of all thnx a lot for pointing out that there is a difference between cavitation and ventilation.
lot of confusion out there and a lot people talking about cavitation actually mean ventilation..

i don't quite get the last bit of your quote above... air injection on a fin and sailing it that way is exactly the same as sailing a fin in spin-out mode. so of course you can't inject air and not sail in spin-out mode.

of course it won't work best with a standard fin. to make a superventilated fin work at all on a windsurfer you would have to about double to triple the size and ride it at very high angles of attack. so a symmetrical configuration is basically ruled out.
on a traditional fin only about one third of the power comes from the compression side.
you can optimise the shape the of the pressure side quite a bit on a ventilated fin and surely have to adjust the angle it's being sailed at for best L/D ratio, but if my research is pointing in the right direction then you would still struggle to get up to a L/D ratio of around 10 more likely around 6-8.
fully submerged fins with no cavitation can be better by a factor of 10... for a power limited craft like a windsurfer, which needs to play the efficiency game to go fast, this will be very hard to make work for achieving new speed records.

kitesurfers have a lot more power available and can use the low efficiency ventilated surface system, which an edged kiteboard represents. ventilated surfaces are great in respect of the power being very dependable as there can't be any spin-outs and there is no cavitation problem no matter how fast you go. it's just not very efficient...
that is also the biggest potential improvement in kitesurfing speed designs. they have the potential to increase their efficiency by a large amount. given the speeds they are doing now i think we will see 60kn peaks within a year.

Boogie

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?

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yoyo said...

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.

As a complete layman here, doesn't this highlight a basic fault with this whole argument.

In the case of the torpedo you gain massive speeds by enclosing it in a slippery bubble of cavitating gas, yep I understand that.
BUT, in order to facilitate steering, you have part of a foil extending outside the bubble (which is going to be subject to the vagaries of drag etc. as per any non supercavitating foil).

THEREFORE, in the case of a windsurfer foil, if you were going dead downwind and didnt require the ability to turn or steer, you would gain massive speeds by encasing your fin in a supercavitation bubble (forgetting the problems with apparent wind on your sail etc etc.) BUT by wanting to go across the wind and have some semblance of steering control the bubble becomes useless.

it seems this whole argument has forgotten the need for control, or am I missing something