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Discussion Starter · #1 ·
I've seen more than once in threads where answers might be unclear, or even contradictory, only because different people mean different things when using the same "technical" words. Hopefully this will help people understand some things better in the future. Let me say that there isn't one "right" way to use these words. After all, words can only mean what people think they mean - what other choice is there? That said, here I go:


First the easy one - Cavitation.

In engineering, cavitation happens when something moving through water (or another liquid) reduces the local pressure so low that the water "boils". You might have seen the experiment in a school science class where a glass of water is put in a vacuum chamber. When the pressure gets low enough, the water boils even though it is at room temperature. This is basically what happens with cavitation.

In common usage, even by outboard motor companies and other experts, cavitation is used to describe the situation where air (or exhaust) gets in the propeller or travels down the low pressure side of a rudder.

In engineering, air getting where you don't want it is called "ventilation". If your outboard motor doesn't "bite" in reverse, or the engine revs up when the propeller gets too close to the surface, then you are experiencing "ventilation". If the front side of your propeller develops pits in the metal, then you most likely had cavitation occurring.


Laminar, turbulent, and separated flow:

In engineering speak, separated flow is flow where some of the fluid is traveling backwards - that is there are vortexes or eddies. If the telltales on your sail are flipping all around, that is separated flow. Separated flow will usually occur behind anything that ends rather abruptly. For example, thru-hull fittings, and masts.

The term "separated flow" doesn't seem to be used in common usage. Instead people call this "turbulent flow". This is where confusion can start.

In a strict engineering sense, turbulent flow has a somewhat different meaning. It is flow that is still following along some surface, but it's not doing it completely smoothly. There is some mixing between the layers of the flow close to the surface, but no big eddies. In common usage this type of "turbulent flow" is often referred to a "laminar flow".
In engineering, laminar flow has a mush more limited meaning. It is flow where there is no mixing between different layers of the flow. That is, if there is laminar flow, then the water that is 1/100 of an inch away from the hull of your boat at point A is still 1/100 of an inch away at point B. I'm not aware of a term in common usage for this type of flow.

So, how can you tell what another person means by one of these words? Well, you can't always, but here are some things that might help.

On a boat traveling at 6 knots, even a boat with the best designed and smoothest hull, laminar flow (in the engineering sense) will only occur for a maximum of 8 inches back from where the water first touches the boat. On the hull itself, this is such a short distance that it is not often significant. On a high aspect fin keel, maintaining laminar flow 8 inches back from the leading edge could be significant if you are racing. Note that any roughness from bottom paint, or small amounts of marine growth will shorten the length of laminar flow - likely to zero inches. As for sails, if there is 15 knots of apparent wind, laminar flow could extend as far as three feet back from the leading edge, but probably less as the leading edge of most sails is not completely smooth. Anything further aft of these dimensions all the flow will be turbulent or separated (again using the engineering definitions).

The common illustrations that show a transition from laminar to turbulent flow contribute to the varying definitions above. They make turbulent flow look like separated flow. I tried to find better illustrations that showed all three flow regimes, but didn't find anything really good.

I did find this gif. though. If you watch the upper rear half of the foil you can see the flow go from turbulent to separated. Even at the start, notice that the white area at the back of the foil is thicker than at the front. This much thickening is probably due to turbulent flow. Notice how it gets even thicker (it appear a bit pink on my screen) as the foil starts to tilt. Here the flow is still following the top of the foil with no backward flow over the top of the foil. But, in an engineering sense, the flow is nun the less turbulent not laminar. However, very quickly, the "tale" behind the foil starts to wave and then you can see the flow reverse on the top of the foil. This is separated flow. In airplane speak, the wing has "stalled".

http://web.mit.edu/13.021/13021_2003/Lifting surfaces/movies/separation at angles of attack.gif

This gif. comes from this page which has some additional interesting ones:

lecture 24C

This brings up the final word I will mention - "stalled". In an aerodynamic sense, a wing stalls when its angle of attack gets too high and the air flow over the wing "separates". If you apply this definition to a sail boat, the sail will stall if you turn too far OFF of the wind. In fact, if you have telltales on near the leading edge of you jib, when the outer ones flip all around, the sail is "stalled" in an aerodynamic sense. Of course, in sailing, "stalling" is often use to describe the condition where you are pointed too CLOSE to the wind and lose headway. This is almost the opposite of the aeronautical definition.
 

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What??? no Reynolds Number discussion??? :p ;)
 

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Bombay Explorer 44
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Using an 'understanding' of laminar and turbulent flow allows you to 'prove' that bumblebees can't fly.

Mean while the bees got on with life.
 

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Keep doing it to me. I may go crazy but I am trying to understand Skene's Elements. Now to compare cavitation and separated flow. Any descriptions numbers that will push me to think I appriciate.
 

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Discussion Starter · #5 ·
What??? no Reynolds Number discussion??? :p ;)

Oh, you want to discuss Reynolds numbers? OK, here we go ... Just kidding - I'm not that mean. ;)


Using an 'understanding' of laminar and turbulent flow allows you to 'prove' that bumblebees can't fly.

Mean while the bees got on with life.
I'm fairly sure you said this in jest, but it still gives me an excuse to ramble on some more. :D

According to Snopes, this legend likely started in the 1930's. The science of aerodynamics has improved a bit since then. Further, in areas of engineering study, there is a tenancy to do research in areas that are likely to be useful and profitable. Until fairly recently, with the advent of robotic insects, there was not much of a push to study aerodynamics at the scale of insects.

Not surprisingly (to me anyway), all the theories and equations that do a good job of describing the flight of airplanes don't always work as well when used on something the size of an insect. As Snopes puts it:

"So, no one "proved" that a bumblebee can't fly. What was shown was that a certain simple mathematical model wasn't adequate or appropriate for describing the flight of a bumblebee."


Anyway, today aerodynamic theory can "prove" that a bumblebee can fly.
 

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Well cavitation is hardly to be compared with boiling...
It is different... In both cases, bubbles form in the liquid but differently...
In boiling water the bubbles are made of water vapor because the energy cracks up the liquid aggregate of it - this could happen due to energy put into the system (heating) or by reducing the energy outside of the water system (reducing air pressure)...

In cavitation now also bubbles form due to lowering the pressure rapidly by a fast moving object for instance... But those bubbles are a vakuum - big difference, because the vapor filled bubbles you get if you boil the water have the same pressure as the surroundings and the vakuum bubbles from cavitation are essentialy pressureless...
Those then implode fractions of a second after they have formed and that causes a schockwave in the water which then causes all the damage to i.e. the prop blades...

How can one now describe how those vakuums are formed?
The pressure is reduced rapidily and very localy and so the adhaesion (the force that keeps the molecules together in its liquid aggregate) between the water molecules is overcome and the molucules are ripped apart in certain areas... As soon as the pressure difference is lowered again, the pressure from outside let these bubbles implode - schockwave - damage...
Essentialy quite easy, is'nt it? ;)
 

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Yes it is quite easy and nice to know except doesn't work to remove barnacles from the prop.
Who knows with all new paint markers on market writing Reynolds numbers all over the prop just might do better job.
 

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Oh, you want to discuss Reynolds numbers? OK, here we go ... Just kidding - I'm not that mean.
Dimensionless numbers are great for scaling. At a macro level (fins and whole boats) Froude numbers are more interesting.

Note that there are still limits to scaling (just as there are to the application of any model--the bumblebee flight "conclusion" came from the application of an inappropriate model, laying the foundation for bad science on the Internet years before the Internet came to be).

In naval architecture, model testing is often used to good effect. We recognize the scaling limitations that derive from the incompressibility of water and scale using multiple factors, usually based on the aforementioned dimensionless numbers.

For entertainment value, using density ratios a ship model scaled up directly would be the equivalent of a full-size ship sailing through jello.
 

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Perhaps this should go in the 'Rule of Thumb' thread but: When an engineer begins by saying, "Now here's something interesting" what follows often isn't.
 

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it is not only engineering terms that are misused and misunderstood in the world of boats.
The two that drive me nuts are "electrolysis" and "osmosis".
 

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I'm fairly sure you said this in jest, but it still gives me an excuse to ramble on some more. :D
Tongue was firmly in cheek.

Anyway, today aerodynamic theory can "prove" that a bumblebee can fly.
The bumblebees are very glad to know this, it confirms something they have always suspected.


Lets talk about low speed reynolds numbers though. Michael Selig is the man.
 

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it is not only engineering terms that are misused and misunderstood in the world of boats.
The two that drive me nuts are "electrolysis" and "osmosis".
Why that?
Electrolysis is quite simple and well understood...
Two differnet metals brought together and an electrical current flows, "dissolving" the weaker metal... Stick a copper and an iron nail into a citrone and you could light a LED with the current flowing between the two until the iron nail has rotted away...
Osmosis is also pretty simple but i do not know why it should occur in GRP...
And it does not... ;)
It occures between the GRP and the gelcoat... And then it is absolutely logical and easy to comprehend... ;)
 
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