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outlet boundry condition-refrence pressur

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Old   December 17, 2012, 13:11
Default outlet boundry condition-refrence pressur
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hamed
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Hi all.
I am trying to model cavitation around a butterfly valve which is installed in a pipe line and I have got som problem with reference pressure.

First, what should i set for the reference pressure of water domain ?
And second
I donít know what is going to happen at the outlet boundary of the pipe so how can I define an average static pressure or velocity there?

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Old   December 17, 2012, 16:21
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Set the reference pressure to be your outlet absolute pressure - should be OK for most cases. Or if this model is approximately at ambient pressure then just one atmosphere.

If you do not know what is happening at the outlet boundary then you cannot set a boundary there. By definition, boundary conditions are placed where you know what is happening so you can replace with with a mathematical BC. So what do you know about your simulation? Can you draw an image?
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Old   December 19, 2012, 10:51
Default cavitation modeling problem with pressures
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thanks for your clue.
my problem in just like the tutorial in cfx guide chapter 11 flow trough a butterfly valve but modeling cavitation by means of information's in chapter 21:cavitation around hydrofoil.

there is another question which is ,what does the reference pressure refer to ?are we talking about absolute pressures when we set the reference pressure to zero or that is for something else?sorry about my rookie questions but it is so important in cavitation modeling as u know better than me .

best regard
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Old   December 19, 2012, 11:46
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Hi Hamed,

This is on the documentation. The solver computes gauge (or manometric, or relative) pressure. To get the absolute pressure you need to add the reference pressure to it.

The idea is that it is a lot easier to loose information due to round off errors when computing differences between 100000.1 Pa and 100000.0 Pa then it is to compute the differences between 0.1 Pa and 0.0 Pa. So we use a reference pressure to offset those 100000 Pa. In the end the result is the same.
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Old   December 19, 2012, 13:06
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pleas corect me if I am wrong
so when we set the refrence pessureto 1 atm and we need an static gauge pressur of 0.5 at at the outlet then we should just set the pressure to 0.5?by the way tanks for reply.
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Old   December 19, 2012, 13:11
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If you set a gauge pressure of 0.5 atm at the outlet and a reference pressure of 0.5 atm, that means your absolute pressure at the outlet will be 1 atm.

The other (better) option is to set a reference pressure of 1 atm and a gauge pressure of 0.
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Old   December 20, 2012, 03:02
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i am now pretty sure about what i am doing with my model .tanks a lot for your help.
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