InterFoam - Pressure

Miguel Hernandez · September 28, 2021, 10:53

Hello everyone,
I am studying the interFoam solver, I am asking the more experienced ones the difference between p and p_rgh.

Is p the total pressure?
Is p_rgh = p - rho g h ? and so, is p_rgh the dynamic component of the pressure?

Thanks, regards.

2538sukham · September 29, 2021, 05:42

I think if you see, pd or p_rgh are actually calculated as pd (or p_rgh) = p - rho*g*h . so its the pressure minus the hydrostatic term. please refer to Rusche thesis to learn how pressure is discretized and solved. If you study the interFoam code, you will find that pressure is reconstructed. Atleast this is much what i know. cheers

mAlletto · September 29, 2021, 11:25

By decomposing the pressure in this way the definition of the boundary conditions is easier. A constant p value at the exit cannot be applied for cases where the gravity place a role. Gower you can apply a constant p_rgh value

Anup Singh · October 1, 2021, 19:24

Yes, p is the total pressure.
P_rgh is the reduced pressure which is exactly the way you have defined.
But if you need a better explanation then there are other threads where it has been already discussed or refer to ferziger and peric.
You can also see its transformation from the following link where Hydrostatic pressure effects has been discused.
https://www.openfoam.com/documentati...orm-p-rgh.html
For its uses and understanding of interfoam: you can refer to the thesis of Henrik Rusche which explains it in great detail while following the framework of openfoam making it easier to understand.
Hope it helps

Miguel Hernandez · October 2, 2021, 07:09

Quote:

Originally Posted by Anup Singh

Yes, p is the total pressure.
P_rgh is the reduced pressure which is exactly the way you have defined.
But if you need a better explanation then there are other threads where it has been already discussed or refer to ferziger and peric.
You can also see its transformation from the following link where Hydrostatic pressure effects has been discused.
https://www.openfoam.com/documentati...orm-p-rgh.html
For its uses and understanding of interfoam: you can refer to the thesis of Henrik Rusche which explains it in great detail while following the framework of openfoam making it easier to understand.
Hope it helps

Thank you for the replay.

Now there is a thing I do not understand.
In a simulation I’m currently working on, in paraFoam I have always p_rgh greater then p... Is it possible?

Anup Singh · October 2, 2021, 07:27

Well, I am not entirely sure of the possibility but I also get it in my simulations.
If you wish to confirm your assumption then define another variable in parafoam as PP = p_rgh + rho g h
and see whether it is equal to p.
Also if you take a look at the equation then you will realize that even though you are adding (rho g h) to p_rgh. The contribution of g is -9.81 m/s2 which in scalar format equates to a negative contribution.

September 28, 2021, 10:53	InterFoam - Pressure	#1
Miguel Hernandez Member Miguel Hernandez Join Date: Feb 2021 Location: En mi casa Posts: 57 Rep Power: 6	Hello everyone, I am studying the interFoam solver, I am asking the more experienced ones the difference between p and p_rgh. Is p the total pressure? Is p_rgh = p - rho g h ? and so, is p_rgh the dynamic component of the pressure? Thanks, regards.

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September 29, 2021, 05:42		#2
2538sukham Member Michael Sukham Join Date: Mar 2020 Location: India Posts: 86 Rep Power: 7	I think if you see, pd or p_rgh are actually calculated as pd (or p_rgh) = p - rhogh . so its the pressure minus the hydrostatic term. please refer to Rusche thesis to learn how pressure is discretized and solved. If you study the interFoam code, you will find that pressure is reconstructed. Atleast this is much what i know. cheers

September 29, 2021, 11:25		#3
mAlletto Senior Member Michael Alletto Join Date: Jun 2018 Location: Bremen Posts: 616 Rep Power: 16	By decomposing the pressure in this way the definition of the boundary conditions is easier. A constant p value at the exit cannot be applied for cases where the gravity place a role. Gower you can apply a constant p_rgh value

October 1, 2021, 19:24		#4
Anup Singh New Member Anup Singh Join Date: Mar 2020 Posts: 22 Rep Power: 7	Yes, p is the total pressure. P_rgh is the reduced pressure which is exactly the way you have defined. But if you need a better explanation then there are other threads where it has been already discussed or refer to ferziger and peric. You can also see its transformation from the following link where Hydrostatic pressure effects has been discused. https://www.openfoam.com/documentati...orm-p-rgh.html For its uses and understanding of interfoam: you can refer to the thesis of Henrik Rusche which explains it in great detail while following the framework of openfoam making it easier to understand. Hope it helps

October 2, 2021, 07:27		#6
Anup Singh New Member Anup Singh Join Date: Mar 2020 Posts: 22 Rep Power: 7	Well, I am not entirely sure of the possibility but I also get it in my simulations. If you wish to confirm your assumption then define another variable in parafoam as PP = p_rgh + rho g h and see whether it is equal to p. Also if you take a look at the equation then you will realize that even though you are adding (rho g h) to p_rgh. The contribution of g is -9.81 m/s2 which in scalar format equates to a negative contribution.