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Gravity volume force in non-isothermal laminar flow

Posted Mar 12, 2013, 5:29 a.m. EDT Computational Fluid Dynamics (CFD) Version 4.3a 5 Replies

Simon Tågerud
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Hello

I'm trying to simulate non-isothermal laminar flow through a tube, and I use 2D-axisymmetrical to decrease the computing time. The model works fine with laminar flow and heat transfer, but when I add gravity, volume force, in the laminar flow interface (-g_const*spf.rho) a solution cannot be found. I get the following error:

- - -

Failed to find a solution.
Singular matrix.

There are 16 void equations (empty rows in matrix) for the variable mod1.p.
There are 1 void equation (empty rows in matrix) for the variable mod1.spf.pmfinl1.
Returned solution not converged.
- Feature: Stationary solver 1 (sol2/s1)
- Error: Failed to find a solution.

- - -

A finer mesh made no difference, but decreasing the gravity to "-0.1*g_const*spf.rho" returned a converged solution.

Any ideas to what might be the problem would be appreciated.

/Simon Tågerud
5 Replies Last Post Mar 21, 2013, 10:53 a.m. EDT
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Mar 13, 2013, 11:05 a.m. EDT
Hi

is your gravity also 2D-axi symmetric ? that is along "Z" the axis, if not you really need to go to 3D.

If its 3D axi, then be sure you have added a body force and an initial relative pressure difference that keeps your initial flow, at sea level 1m water hight adds quite some pressure drop ;)

--
Good luck
Ivar
Hi is your gravity also 2D-axi symmetric ? that is along "Z" the axis, if not you really need to go to 3D. If its 3D axi, then be sure you have added a body force and an initial relative pressure difference that keeps your initial flow, at sea level 1m water hight adds quite some pressure drop ;) -- Good luck Ivar
Simon Tågerud
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Posted: 1 decade ago Mar 15, 2013, 5:21 a.m. EDT
Thanks for the reply.

The gravity is only working in the z-direction. I have attached a simplified test model I used to investigate the problem. It seems the volume force is working when heat is not implemented.

Thanks
Simon
Thanks for the reply. The gravity is only working in the z-direction. I have attached a simplified test model I used to investigate the problem. It seems the volume force is working when heat is not implemented. Thanks Simon

Attachments:

Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Mar 20, 2013, 4:48 p.m. EDT
Hi

you have among other a confusion about the presure, and the sign of gravity I believe, betwen the body force and the pressure. In CFD you have the gauge pressure (set in the BC's) and the absolute pressure (set in the main CFD physics node) Normally athmospheric presure decreases with altitude, so you model is slightly confusing ;)

THen why use SPF and NITF or was it "just" t test the flow convergence ?

--
Good luck
Ivar
Hi you have among other a confusion about the presure, and the sign of gravity I believe, betwen the body force and the pressure. In CFD you have the gauge pressure (set in the BC's) and the absolute pressure (set in the main CFD physics node) Normally athmospheric presure decreases with altitude, so you model is slightly confusing ;) THen why use SPF and NITF or was it "just" t test the flow convergence ? -- Good luck Ivar
Simon Tågerud
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Posted: 1 decade ago Mar 21, 2013, 8:51 a.m. EDT
Hi

I'm made some changes, and put gauge (relative) pressure everywhere, that is the dependent variable p (p3 for the laminar flow physics). It should calculate the absolute pressure, which seems to be needed only for the density calculation at the material node. Changing the pressure in the material density function from pA (absolute pressure) to (p+p_ref) with p_ref being 1 [atm], or nitf.pA didn't make any difference either. The problem still doesn't converge no matter what configuration I use for the pressure.

The device I'm trying to model is placed upside down compared to the geometric model. This was corrected simply by changing the sign of the gravity force. I have tried both with positive and negative gravity, still with no solution.

I have been experimented with both using SPF together with hear transfer (ht), and to exchange them with the equivalent interface NITF, in hope that NITF has better integration between the physical effects. In the test file, the SPF was used to investigate if the solution converged if heat flow was neglected, which it did.

By the way, I made a support case with comsol about this problem, and are awaiting their response.

Thanks
/Simon Tågerud
Hi I'm made some changes, and put gauge (relative) pressure everywhere, that is the dependent variable p (p3 for the laminar flow physics). It should calculate the absolute pressure, which seems to be needed only for the density calculation at the material node. Changing the pressure in the material density function from pA (absolute pressure) to (p+p_ref) with p_ref being 1 [atm], or nitf.pA didn't make any difference either. The problem still doesn't converge no matter what configuration I use for the pressure. The device I'm trying to model is placed upside down compared to the geometric model. This was corrected simply by changing the sign of the gravity force. I have tried both with positive and negative gravity, still with no solution. I have been experimented with both using SPF together with hear transfer (ht), and to exchange them with the equivalent interface NITF, in hope that NITF has better integration between the physical effects. In the test file, the SPF was used to investigate if the solution converged if heat flow was neglected, which it did. By the way, I made a support case with comsol about this problem, and are awaiting their response. Thanks /Simon Tågerud
Simon Tågerud
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Posted: 1 decade ago Mar 21, 2013, 10:53 a.m. EDT
Hi again

I got help from the support, and it seems they pinpointed the problem. Appearantly the gravity force (source term) was to large, and thus the solution did not converge. This was solved by ramping up the volume force in the same way as when having a large reaction term in knowledge base entry 103.

www.comsol.com/support/knowledgebase/103/

Thanks
Simon Tågerud
Hi again I got help from the support, and it seems they pinpointed the problem. Appearantly the gravity force (source term) was to large, and thus the solution did not converge. This was solved by ramping up the volume force in the same way as when having a large reaction term in knowledge base entry 103. http://www.comsol.com/support/knowledgebase/103/ Thanks Simon Tågerud

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