Propellers

[PanamaMike]

Mike,
I believe that the water vaporizing because of the mechanical vaccum (in this case the negative pressure side of the prop) is the actual definition of cavitation.

Then you would respectfully indeed be mistaken. One fellow already out the
wikipedia def here. Or just check www.m-w.com:

Main Entry: cav·i·ta·tion : the process of cavitating : as A : the formation
of partial vacuums in a liquid by a swiftly moving solid body (as a propeller)
or by high-intensity sound waves; also : the pitting and wearing away of solid
surfaces (as of metal or concrete) as a result of the collapse of these
vacuums in surrounding liquid B : ...

You don't need to change the water to gas to cavitate -- it's a secondary part
of the process.

Regards,

--Mike

 

[cool_hardware52]

Everyone,
Let me stir the pot a bit for some fun.

It is interesting to note that the negative pressure side of the prop can commonly generate more power than the positive pressure side of the prop. This means that the prop can generate more power by sucking itself forward than by pushing itself forward.

So lets see here, the old hard boiled egg in to the milk bottle trick works because the vacuum (or relative lower pressure in the bottle) sucks the egg in?

Hummmm

The egg enters the bottle because the force on the outside of the egg (ambient pressure) is greater than the force resisting the egg inside the bottle (lower pressure, partial vacuum) If you put the milk bottle with the egg inside inverted in a bell jar and lower the pressure surrounding the milk bottle what happens to the egg? Why?


If things are really "sucked" into areas of lower pressure then we can assume all heavier than air aircraft are being "sucked" away from the surface of the earth, not lifted by the relatively greater pressure under the airfoil than above.


Tobin


Tobin

 

[cool_hardware52]

Main Entry: cav·i·ta·tion : the process of cavitating : as A : the formation
of partial vacuums in a liquid by a swiftly moving solid body (as a propeller)
or by high-intensity sound waves; also : the pitting and wearing away of solid
surfaces (as of metal or concrete) as a result of the collapse of these
vacuums in surrounding liquid B : ...

You don't need to change the water to gas to cavitate -- it's a secondary part
of the process.

Regards,

--Mike

How do you form a vacuum in a liquid without a phase change?

Tobin

 

[OceanObsessed]

Then you would respectfully indeed be mistaken. One fellow already out the
wikipedia def here. Or just check www.m-w.com:

Main Entry: cav·i·ta·tion : the process of cavitating : as A : the formation
of partial vacuums in a liquid by a swiftly moving solid body (as a propeller)
or by high-intensity sound waves; also : the pitting and wearing away of solid
surfaces (as of metal or concrete) as a result of the collapse of these
vacuums in surrounding liquid B : ...

You don't need to change the water to gas to cavitate -- it's a secondary part
of the process.

Regards,

--Mike

With all due respect... I would not trust wikipedia, it can be edited by anyone as with anything else on the internet. It says so right on the home page http://en.wikipedia.org/wiki/Main_Page.

I am not implying that you need to change the water to induce cavitation... only the pressure. Additionally, all I am saying is that I think that cavitation is a function of the vapor pressure of water.

I just looked up cavitation in my old Naval Architecture book (Introduction to Naval Architecture by Giller and Johnston, page 235). Here is its definition that was written by Naval Architects:

"When the minimum value of the absolute pressure on the back (of the propeller) is reduced below the vapor pressure of the water, which will occur at relatively high propeller speeds, vapor pockets or cavities are formed that disrupt the flow and reduce the propeller efficiency. This phenomenon, known as cavitation......"

So cavitation is a function of the vapor pressure of water as well as pressure.

 

[PanamaMike]

How do you form a vacuum in a liquid without a phase change?

Tobin

Make the void faster than the water vapor can fill it!

But no matter how fast you make the void, the amount of
water that changes phase does not substantially effect the
pressure in the cavitation, since at best the vapor pressure
of water will about 1/10th of an atmosphere. This can not,
substantially reduce the cavitation shockwaves.

The cavitation is due to creating a vacuum by the prop
throwing water out of its way faster than the ambient water
pressure can replace it. This creates a cavity/void/vacuum
which subsequently is partially filled with water vapor, but not
nearly enough to relieve the vacuum.

The phase change of water is basically irrelevant in the process.
You could cavitate in Hg or some other liquid with even lower vapor
pressures than water.

Regards,

--Mike

 

[PanamaMike]

So cavitation is a function of the vapor pressure of water as well as pressure.

Pressure is the first effect, vapor pressure of water the 2nd effect and not
required for cavitation.

It is also not required by the *definition* of cavitation.

Regards,

--Mike

 

[cool_hardware52]

Make the void faster than the water vapor can fill it!

But no matter how fast you make the void, the amount of
water that changes phase does not substantially effect the
pressure in the cavitation, since at best the vapor pressure
of water will about 1/10th of an atmosphere. This can not,
substantially reduce the cavitation shockwaves.

The cavitation is due to creating a vacuum by the prop
throwing water out of its way faster than the ambient water
pressure can replace it. This creates a cavity/void/vacuum
which subsequently is partially filled with water vapor, but not
nearly enough to relieve the vacuum.

The phase change of water is basically irrelevant in the process.
You could cavitate in Hg or some other liquid with even lower vapor
pressures than water.

Regards,

--Mike

Are you claiming that you can achieve a true vacuum, i.e. the absence of anything, because the water can't undergo a phase change fast enough?

Tobin

 

[PanamaMike]

Are you claiming that you can achieve a true vacuum, i.e. the absence of anything, because the water can't undergo a phase change fast enough?

Tobin

Nope, not at all. The vapor pressure can be arbitrary low,
not to mention that all the gas partial pressures don't
have to be in equilibrium.

It's like this. Image a hand bicycle pump. You work the
piston back and forth and, with two one-way values, you can
pump air in the tire. Now forget the tire and imagine that
the piston plunger is down so that the volume of the piston
is minimized. Now seal off both check valves and pull the
plunger up. You can, leaving a vacuum in the piston. OK,
sure, it's not a perfect vacuum, there's various contaminants
in the piston that out gas and the void in the pump will be
"filled" with this stuff(but it won't come up to atmospheric
pressure unless there's a leak.) The pressure in the pump
will depend on these partial pressures. But they could be
zero and you'd *still* be able to pull the plunger back no
matter what these partial pressure were.

Similarly propeller cavitation does not *require* a phase
change and it is not part of its definition. The partial
pressures could be zero and you could still though any old
dense fluid away from the prop leaving voids. When the
voids "fill" its still basically a vacuum, just not a
perfect one.

Regards,

--Mike

 

[ClayJar]

How do you form a vacuum in a liquid without a phase change?

Kinetics.

Take something like a classic liquid column barometer. If you start with a vacuum above the liquid surface inside the sealed top of the column, the equilibrium state is for vapor from the liquid to fill the space with the partial pressure of the vapor above the liquid surface equal to the vapor pressure of the liquid at the temperature of the liquid surface. However, the velocity of each particle of the liquid (say, each molecule of water) is finite, and so, you will not instantaneously reach the equilibrium state.

Any study of cavitation will be heavily into kinetics, with equilibrium states possibly only illuminating the limiting terms.

 

[cool_hardware52]

Nope, not at all. The vapor pressure can be arbitrary low,
not to mention that all the gas partial pressures don't
have to be in equilibrium.

It's like this. Image a hand bicycle pump. You work the
piston back and forth and, with two one-way values, you can
pump air in the tire. Now forget the tire and imagine that
the piston plunger is down so that the volume of the piston
is minimized. Now seal off both check valves and pull the
plunger up. You can, leaving a vacuum in the piston. OK,
sure, it's not a perfect vacuum, there's various contaminants
in the piston that out gas and the void in the pump will be
"filled" with this stuff(but it won't come up to atmospheric
pressure unless there's a leak.) The pressure in the pump
will depend on these partial pressures. But they could be
zero and you'd *still* be able to pull the plunger back no
matter what these partial pressure were.

Similarly propeller cavitation does not *require* a phase
change and it is not part of its definition. The partial
pressures could be zero and you could still though any old
dense fluid away from the prop leaving voids. When the
voids "fill" its still basically a vacuum, just not a
perfect one.

Regards,

--Mike

I can agree that a propeller, impeller, sliding piston, sliding vane, diaphram, any other manner of dynamic or positive displacement pump is capable of creating pressure differential, that's why you use a pump in the first place.

If this pump is operating in a liquid, and the low pressure (inlet side) pressure drops below the vapor pressure of that liquid you will generate a vapor that will attempt to fill that void. Is it possible that there will still be a net reduction in the pressure in this vapor due to the dynamics of the system? Sure why not?

Would it be possible to create this low pressure area without creating any vapor? Seems unlikely.



BTW prop "cavitation" is not always a bad thing. We used to drill outboard prop hubs to intentionally allow some of the exhaust to exit at the root of the blades. (Works only with through the hub exhausts)

At low speeds this casues the prop to "slip" a little. This way you could get away with a steeper pitch prop than one would normally fit to a given boat. Allowing the prop to slip at low RPM would keep a 2 stroke from "bogging" out of the hole. This is like starting a 125 dirt bike in 3 gear by slipping the clutch enough to keep the motor "on the pipe"

Maybe not the most efficient approach, but you gain top end speed due to the steeper pitch.

Works best if you have aluminum props, lots of time for testing and tig welder when you make the holes too big


Tobin