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SPF.rho and mat1.def.rho(P,T)

Posted Dec 29, 2016, 4:19 a.m. EST Fluid & Heat Version 5.2a 5 Replies

Alexandre Fête
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Dear all,
I have a very simple laminar flow model with one inlet and one outlet (that I use to fix the pressure to 0.4mbar). Everything works fine apart from the fact that when I plot SPF.rho and mat1.def.rho(P,T) (both constant over the all domain) spf.rho is twice bigger that mat1.def.rho... do you have any explanation?
I use Dioxygen (gas) as a fluid.
Thanks in advance
Alex
5 Replies Last Post Dec 31, 2016, 8:18 a.m. EST
Sven Friedel COMSOL Employee

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Posted: 7 years ago Dec 29, 2016, 7:19 a.m. EST
Hi,
where do you plot those quantities - in a domain or on a line?
Sven
Hi, where do you plot those quantities - in a domain or on a line? Sven
Sven Friedel COMSOL Employee

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Posted: 7 years ago Dec 29, 2016, 7:23 a.m. EST
Note that the SPF interface comes with a reference pressure level of 1[atm], it may be that you evaluate your material properties at 1[atm]+0.4[mbar] rather than at 0.4[mbar].
Note that the SPF interface comes with a reference pressure level of 1[atm], it may be that you evaluate your material properties at 1[atm]+0.4[mbar] rather than at 0.4[mbar].
Alexandre Fête
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Posted: 7 years ago Dec 29, 2016, 10:08 a.m. EST

Note that the SPF interface comes with a reference pressure level of 1[atm], it may be that you evaluate your material properties at 1[atm]+0.4[mbar] rather than at 0.4[mbar].


Yes indeed... I realize that I was not understanding correctly the role of the reference pressure level in the computation. I set it to 0.4 mbar which means that the actual pressure in the vessel was 0.8mbar instead of 0.4 hence the factor two in SPF.rho. I set it to the base pressure of my system 1e-8 [mbar] and it works now !

As I understand it know the reference pressure level is a pressure which is there indepedently of the computation. Then the computation "adds" or "remove" pressure based on the inlet and outlets of the system (correct me if I'm wrong..).

Thanks for educating me and sorry for this question that I should have solved myself..
[QUOTE] Note that the SPF interface comes with a reference pressure level of 1[atm], it may be that you evaluate your material properties at 1[atm]+0.4[mbar] rather than at 0.4[mbar]. [/QUOTE] Yes indeed... I realize that I was not understanding correctly the role of the reference pressure level in the computation. I set it to 0.4 mbar which means that the actual pressure in the vessel was 0.8mbar instead of 0.4 hence the factor two in SPF.rho. I set it to the base pressure of my system 1e-8 [mbar] and it works now ! As I understand it know the reference pressure level is a pressure which is there indepedently of the computation. Then the computation "adds" or "remove" pressure based on the inlet and outlets of the system (correct me if I'm wrong..). Thanks for educating me and sorry for this question that I should have solved myself..
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 7 years ago Dec 30, 2016, 5:30 a.m. EST
Hi all

There is exactly a Blog on this subject just issued:

www.comsol.eu/blogs/how-to-assign-fluid-pressure-in-cfd-simulations/

COMSOL, as many Physics in general use "gauge values" for variables with large offset, such as the pressure and the temperatrure in SPF, magnetic fields in ACDC ...

--
Good luck
Ivar
Hi all There is exactly a Blog on this subject just issued: https://www.comsol.eu/blogs/how-to-assign-fluid-pressure-in-cfd-simulations/ COMSOL, as many Physics in general use "gauge values" for variables with large offset, such as the pressure and the temperatrure in SPF, magnetic fields in ACDC ... -- Good luck Ivar
Alexandre Fête
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Posted: 7 years ago Dec 31, 2016, 8:18 a.m. EST
Very good thanks !
Alex
Very good thanks ! Alex

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