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why doesn't ansys allow a negative pressure when cavitation is included

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Old   December 8, 2016, 09:17
Default why doesn't ansys allow a negative pressure when cavitation is included
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Hi,

I have done some numerical simulations in Ansys CFX on a high speed flow (7 m/s) in a small nozzle (mm scale). The pressure of the water in the nozzle is of importance. However, I am not sure how to interpret the results.

For the simulations without cavitation the pressure in the throat gets to -5 bar. A have read that a negative liquid pressure is physical (liquid tension) and is related to the onset to cavitation. However, when including cavitation in the simulation, the pressure profile is very different. Not only at the location where cavitation takes place (after the throat), but also before and at the throat the pressure is much higher. I would expect the pressure profile to be the same for the locations where there is no cavitation. It is almost like Ansys is not 'allowed' to present negative pressures when there is a gas in the system and therefore it shifts the graph upwards.

I have included graphs showing the pressure profile for the simulation with and without cavitation, the vapor fraction and a colormap showing the nozzle shape used.

So to summarize my question: if a negative water pressure is a physical phenomenon, why wouldn't Ansys allow it in a system where cavitation is included?

I have always considered negative pressure to be a non-equilibrium situation, thus could it be that there is a negative pressure in the throat, but that the formation of bubbles takes so much time that there is only cavitation after the throat?

Thank you a lot for your help!

greetings,
Joy

PS. the nozzle with the same conditions as in the simulation was used in experiments, however, it was not possible to measure any pressure data. It was attached to tap water, which would suggest an inlet pressure closer to 3 than to 8 bar.
Attached Images
File Type: png press_cavitation.png (6.4 KB, 43 views)
File Type: png press_nocavitation.png (7.1 KB, 39 views)
File Type: png vapor_cavitation.png (7.6 KB, 33 views)
File Type: jpg pressure_colormap.jpg (8.5 KB, 27 views)
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Old   December 8, 2016, 11:28
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Quote:
Originally Posted by vysje View Post
Hi,

I have done some numerical simulations in Ansys CFX on a high speed flow (7 m/s) in a small nozzle (mm scale). The pressure of the water in the nozzle is of importance. However, I am not sure how to interpret the results.

For the simulations without cavitation the pressure in the throat gets to -5 bar. A have read that a negative liquid pressure is physical (liquid tension) and is related to the onset to cavitation. However, when including cavitation in the simulation, the pressure profile is very different. Not only at the location where cavitation takes place (after the throat), but also before and at the throat the pressure is much higher. I would expect the pressure profile to be the same for the locations where there is no cavitation. It is almost like Ansys is not 'allowed' to present negative pressures when there is a gas in the system and therefore it shifts the graph upwards.

I have included graphs showing the pressure profile for the simulation with and without cavitation, the vapor fraction and a colormap showing the nozzle shape used.

So to summarize my question: if a negative water pressure is a physical phenomenon, why wouldn't Ansys allow it in a system where cavitation is included?

I have always considered negative pressure to be a non-equilibrium situation, thus could it be that there is a negative pressure in the throat, but that the formation of bubbles takes so much time that there is only cavitation after the throat?

Thank you a lot for your help!

greetings,
Joy

PS. the nozzle with the same conditions as in the simulation was used in experiments, however, it was not possible to measure any pressure data. It was attached to tap water, which would suggest an inlet pressure closer to 3 than to 8 bar.
Well what pressure do you mean? Absolute pressure may not be negative. But Gauge pressure (i.e. relative to some reference value of pressure) may.
Note that term "negative pressure" is often used to refer to a situation in which an enclosed volume has lower pressure than its surroundings.
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Old   December 8, 2016, 13:10
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Practically speaking, there is no such thing as negative absolute pressure in the macro-sense. I have heard it can exist in the micro-world for minute time-scales during liquid vapor transition, but in your case, it is more likely a result of the numerical simulation. What I will typically do in problems where I expect high turbulence and/or cavitation is enable some advanced solver settings and clip the pressure at a low value (see attached picture). In my experience, this will give you a more realistic result without having to explicitly model cavitation. Then you can simply create plots in CFD post and determine where the local static pressure falls below the fluid vapor pressure. This will give you a good indication of where cavitation would be expected to occur. Please let me know if that helps. Best of luck!
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Last edited by bparrelli; December 10, 2016 at 10:20.
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Old   December 8, 2016, 13:44
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Quote:
Originally Posted by bparrelli View Post
There is no such thing as negative absolute pressure. It is purely a result of the numerical simulation.
I have read the same thing as VYSJE, that negative absolute pressure can happen (liquid tension) in some scenarios. Water Transport in trees is one scenario:
http://discovermagazine.com/2003/mar/featscienceof
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Old   December 8, 2016, 14:05
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We are talking about nozzle flow. Negative absolute pressure in a problem like this is not useful information, simply because it cannot be practically measured through experiment. At this point, we are getting into the semantics of how pressure itself is defined. This is counterproductive. If you are looking for a good way to model this problem and obtain a usable solution, then please refer to my previous post.

Last edited by bparrelli; December 9, 2016 at 12:13.
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Old   December 8, 2016, 14:37
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Quote:
Originally Posted by evcelica View Post
I have read the same thing as VYSJE, that negative absolute pressure can happen (liquid tension) in some scenarios. Water Transport in trees is one scenario:
http://discovermagazine.com/2003/mar/featscienceof
I think one should not take term negative pressure literally. It's just to designate different direction of action
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Old   December 8, 2016, 17:06
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Negative absolute pressure can and does happen. If you take a body of water and VERY quickly move the boundaries, the water will cavitate. But cavitation takes time as the water needs to get energy to do the phase change. The amount of time is tiny, of the order of nano to microseconds, but for those fractions of a second before it cavitates the liquid is sustaining a negative absolute pressure.

This effect can be ignored for most engineering work as the time scale is so small as to be negligible. But my field is MEMS design and we have effects which work in the nanosecond time scales in MEMS. So negative absolute pressure is an effect we need to account for in some cases.
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Old   December 8, 2016, 17:32
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Quote:
Originally Posted by ghorrocks View Post
Negative absolute pressure can and does happen. If you take a body of water and VERY quickly move the boundaries, the water will cavitate. But cavitation takes time as the water needs to get energy to do the phase change. The amount of time is tiny, of the order of nano to microseconds, but for those fractions of a second before it cavitates the liquid is sustaining a negative absolute pressure.

This effect can be ignored for most engineering work as the time scale is so small as to be negligible. But my field is MEMS design and we have effects which work in the nanosecond time scales in MEMS. So negative absolute pressure is an effect we need to account for in some cases.
Interesting. I didn't know this. Can this pressure be measured? Is it applicable to the nozzle problem in question here?
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Old   December 9, 2016, 00:03
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Quote:
Originally Posted by ghorrocks View Post
Negative absolute pressure can and does happen. If you take a body of water and VERY quickly move the boundaries, the water will cavitate. But cavitation takes time as the water needs to get energy to do the phase change. The amount of time is tiny, of the order of nano to microseconds, but for those fractions of a second before it cavitates the liquid is sustaining a negative absolute pressure.

This effect can be ignored for most engineering work as the time scale is so small as to be negligible. But my field is MEMS design and we have effects which work in the nanosecond time scales in MEMS. So negative absolute pressure is an effect we need to account for in some cases.
What does negative absolute pressure physically mean?
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Old   December 9, 2016, 01:03
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Is this relevant here? Unlikely. The effect I am talking about only occurs at tiny time scales, so for normal engineering cases like cavitation in hydrofoils, pumps or pipes it can be ignored. It only becomes important in things like MEMS simulations where you are using time step sizes of nanoseconds.

What does negative pressure physically mean? Good question and I am not sure anybody has a complete answer to that. For instance most EOS fall apart below zero absolute pressure. I visualise it as similar to tensile stress in a solid body.
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Old   December 9, 2016, 05:58
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thanks a lot for all your replies! I find the discussion very helpful. The simulation is a part of my master's assignment and there are already very diverse opinions on negative pressure among the professors here.

I am guessing that the simulation with cavitation does not include the time needed for cavitation to take place and therefore assumes it to take place immediately. However in my case the time for the flow to pass through the whole nozzle is around 0.45 ms. If the time scale for cavitation is indeed nano to miliseconds, it would be important.

Ghorrocks, do you know how to estimate the order of magnitude of the time scale for cavitation, or is it always 10^-3? I assume it is dependent on the liquid properties (gas fraction and such), do you know if demineralizing or degassing the water will have a significant influence?
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Old   December 9, 2016, 06:04
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I did not say milliseconds. I said microseconds to nanoseconds. If your flow time is 0.45ms then it does not appear to be a significant effect.

I would have to look up the typical time delays - it is an equivalent effect in pressure changes to what superheat is in temperature changes.
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Old   December 9, 2016, 08:58
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You are right, I have misread your comment and it is indeed of a different order of magnitude.

I have another question however. When looking into tensile stresses on a liquid I have found that the adhesive forces in a liquid will break (thus the liquid will cavitate) when a certain tensile strength is overcome, similar as in a solid. These tensile stresses are theoretically of order size 10^8 Pa and practically closer to 10^5 Pa, because of impurities in the liquid. But can it then be that the liquid pressure is in between 0 and -1 bar without cavitating?

Also, what is the role of the critical pressure of water here? I believe this is what Ansys uses in order to determine when the liquid will start to cavitate.

I'm sorry to ask these physical questions on the Ansys forum. Its just the interpretation of the numerical results which I struggle with.

Last edited by vysje; December 9, 2016 at 10:35.
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Old   December 9, 2016, 12:23
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I was thinking about this today, and thought I'd throw this one out there for some food for thought:

Consider air behaving as an ideal gas, but existing at a pressure of -10 psia and a temperature of 535 R. The density would be:

rho = (144 in^2/ft^2)*(-10 lbf/in^2) / [(53.3521 ft-lbf/lbm-R) * (535 R)]
rho = -0.05 lbm/ft^3

Now, I know the ideal gas EOS is not a good model for these conditions, but what the hell does negative density mean? Is that anti-matter?
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Old   December 10, 2016, 05:25
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Ideal gas is one of the eos which can't handle negative pressures. In fact I think it only applies to liquids. The example I gave is of the phase transition from liquid to gas, and the negative pressure occurs in the liquid.
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Old   December 11, 2016, 11:29
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Quote:
Originally Posted by ghorrocks View Post
Negative absolute pressure can and does happen. If you take a body of water and VERY quickly move the boundaries, the water will cavitate. But cavitation takes time as the water needs to get energy to do the phase change. The amount of time is tiny, of the order of nano to microseconds, but for those fractions of a second before it cavitates the liquid is sustaining a negative absolute pressure.

This effect can be ignored for most engineering work as the time scale is so small as to be negligible. But my field is MEMS design and we have effects which work in the nanosecond time scales in MEMS. So negative absolute pressure is an effect we need to account for in some cases.
Very interesting comments, but a question is rising :
The absolute negative pressure implies in physics that intermolecular bond force is larger than outside force acting on the fluid molecules, which will violate the basic assumption of fluid continuum in the Navier-Stokes equations. I do not think any CFD can predict such a very rare situation outside of its theoretical boundary. I believe the absolute negative pressure from CFD is an error.
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Old   December 11, 2016, 17:18
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I do not think any CFD can predict such a very rare situation outside of its theoretical boundary.
I agree. I do not know of any other CFD solver which can handle it either.

Quote:
I believe the absolute negative pressure from CFD is an error.
For almost all cases I agree, and in the case which is the topic of this thread that is definitely the case. The exception I discuss above is such an unusual, specialised case that it would only be of interest to a very few people.
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