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PIMPLE algorithm relaxation Factors

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Old   April 5, 2013, 14:45
Default PIMPLE algorithm relaxation Factors
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Mojtaba Amiraslanpour
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Hi guys.
I am using pimpleFoam for my case and I noticed that there are two keywords defined for relaxationFactors in fvsolution. fields and equations.

Quote:
relaxationFactors
{
fields
{
}
equations
{
"U.*" 0.9;
"k.*" 0.7;
"epsilon.*" 0.7;
}
}
What are the differences between these two? there is no such a thing in SIMPLE algorithm.

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Old   April 6, 2013, 00:31
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Hi Mojtaba,

Quote:
there is no such a thing in SIMPLE algorithm.
You can also find the keywords defined in the tutorials of simpleFoam.

Quote:
What are the differences between these two?
Regarding your questions, you might want to read the Ph.D. thesis of Dr. Jasak:
http://www.h.jasak.dial.pipex.com/

especially
  • keyword "equations": eq. (3.95) and (3.96) on page 115
  • keyword "fields": eq. (3.145) on page 149

Hope this helps,
Fumiya
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Old   April 6, 2013, 04:55
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Quote:
Originally Posted by fumiya View Post
Hi Mojtaba,


You can also find the keywords defined in the tutorials of simpleFoam.


Regarding your questions, you might want to read the Ph.D. thesis of Dr. Jasak:
http://www.h.jasak.dial.pipex.com/

especially
  • keyword "equations": eq. (3.95) and (3.96) on page 115
  • keyword "fields": eq. (3.145) on page 149

Hope this helps,
Fumiya
Thank you Fumiya,
as I could get,
  • under-relaxation factors which are placed in keyword "equations" are the general factors which are used to under relax the system of algebraic equations created by the discretisation.
  • under-relaxation factors which are placed in keyword "fields" are used to under relax new pressure equations. as in Dr.Jasak Thesis:
Quote:
In order to obtain a better approximation of the “correct” pressure field, it would be necessary to solve the pressure equation again
therefore another relaxation factors for p and U are defined.

So for instance is it ok to say:

Quote:
relaxationFactors
{
fields
{
U 0.7
p 0.3
}
equations
{
"U.*" 0.9;
"k.*" 0.7;
"epsilon.*" 0.7;
}
}
Bests.
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Old   April 6, 2013, 07:26
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Hi Mojtaba,

Quote:
Originally Posted by Mojtaba.a View Post
therefore another relaxation factors for p and U are defined.
I think you understand correctly.

Quote:
Originally Posted by Mojtaba.a View Post
So for instance is it ok to say:

Quote:
relaxationFactors
{
fields
{
U 0.7;
p 0.3;
}
equations
{
"U.*" 0.9;
"k.*" 0.7;
"epsilon.*" 0.7;
}
}
I think you should put U 0.7; in the equations dictionary, because the variable U is solved in UEqn.

Hope this helps,
Fumiya
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Old   April 6, 2013, 15:47
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Quote:
I think you should put U 0.7; in the equations dictionary, because the variable U is solved in UEqn.
Yes you are right. I thought there is an another relaxation factor envolved in new pressure equation, but there is not.

p^{new}=p^{old}+\alpha_{p}*(p^p-p^{old})
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