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Old   July 19, 2007, 12:58
Default Negative pressure
  #1
Luca
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Hi folk,

I'm simulating a valve with a big pressure (200 Bar)drop through a small passage with a incompressible fluid the process converges but in the results I found some big areas with negative pressure: total and static.

can I consider the results correct? Or what can I consider correct? for Example if I use a Inlet with mass flow and an Outlet whith atmosferic pressure, can I consider the pressure drop correct?

Thanks

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Old   July 19, 2007, 14:57
Default Re: Negative pressure
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Robin
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Hi Luca,

Since your fluid is incompressible the equation of state (i.e. fluid properties) are not dependant on pressure and therefore the solution is not dependant on pressure level (i.e. you will get the same solution regardless of what your reference pressure is, as long as the relative pressures remain the same). A negative pressure may be the result of liquid in tension but may also lead to cavitation in your real system. Note that if you include cavitation effects you solution WILL become pressure level dependant.

Regards, Robin
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Old   July 20, 2007, 02:27
Default Re: Negative pressure
  #3
Luca
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Hi Robin,

thanks for your exhaustive answer.

but I have to do an other question,

in this kind of analysis I study forces that are applaied on surface where there are negative and positive pressure and the last purpose of this analysis is to balance the force.

so are results (force) correct or not? and for this kind of analysis is better SST or a K-e?

Thanks
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Old   July 20, 2007, 15:38
Default Re: Negative pressure
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Robin
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The results are fine for this purpose but make sure that if the part has surfaces that are not exposed to the fluid (i.e. the outside of a pipe), you need to include the pressure on the outside. You'll get this if the domain reference pressure is equal to the outside pressure since the "Pressure" in CFX would then represent the gauge pressure.

Whether to run SST or k-e depends on many factors. In general I would go with SST since the results will be similar for the most part, but SST will also do a better job resolving separation. The SST prediction will also improve as you resolve more of the boundary layer, whereas k-e will do so only to a limit. This is a loaded topic, so I recommend reviewing the doc, literature and other posts on this forum to learn more.

Regards, Robin
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Old   July 24, 2007, 10:32
Default Re: Negative pressure
  #5
Luca
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Hi Robin,

Thanks for your help, your explanation is very clear.

But I have an other question, if a have a negative pressure on a surface where I have to check the force, CFX uses the negative pressure to calculate the force or not?

Thanks in advance. Luca
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Old   July 24, 2007, 11:11
Default Re: Negative pressure
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Robin
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Yes, CFX will use the negative pressure.
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Old   July 25, 2007, 04:08
Default Re: Negative pressure
  #7
Luca
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Hi,

Thanks for your answer,

but there is a method to calculate the force without negative pressure? Because in surface where I have to balance the force I have a part with negative pressure and in the same surface other parts with Positive pressure. I would set the minimum pressure (to calculate the force) to 1 Atm that is also the reference pressure.

Thank in advance. Luca
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