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SRFSimpleFoam vs SimpleFoam (MRF)

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Old   February 14, 2014, 16:50
Default SRFSimpleFoam vs SimpleFoam (MRF)
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Oscar Urbina
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Hello Foamers,

i have a couple of questions about the rotating reference frame solvers in OpenFoam.

i'm simulating a blade in half a cylinder, now the blade is static (represented by a force),

and the entire domain has to move.

i've been reading the threads and OpenFoamWiki,

so from my understanding it would be the same if i use SRFSimpleFoam or if i use SimpleFoam.

so my questions are:

In SRFSimpleFoam i can define the axis of rotation but i'm unable to define the origin of my rotation, so is the origin going to be (0,0,0) all the time? or if there a way to set it?

when i see SimpleFoam with the MRF option (MRFSimpleFoam), i can define the axis and the origin.

would it be the equivalent of SRF if i define the entire domain as the cellzone that rotates?

also when i use simpleFoam (domain with no obstacles in constant rotation), my inlet bc is fixedvalue (5,0,0), and my omega is 700rpm, but when i see my boundary in parafoam after a couple of iterations it becomes (0, something, something).
i understand if it adds the rotational forces but why is setting my axial inflow to 0 when it's suppose to be fixed to 5?

and in the internal field my axial velocity tends to 0 and gets negative values even.


if someone could throw some light into this i would really appreciate it

thanks

Oscar
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Old   March 10, 2014, 12:53
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Pengchuan Wang
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Hi Oscar,

If you put all the cells in the MRFzones, theoretically SRF and MRF will give you the same result.

I did a very simple test before, that is using simpleFoam, SRFSimpleFoam and MRFSimpleFoam to run the same case. That means three different solvers solving the same problem, though the setup for each solver is different. Unfortunately, MRFSimpleFoam cannot give me the correct result. The results from simpleFoam and SRFimpleFoam are the same, and agree with my analytical solution. But the result from MRFSimpleFoam does not. And right now I still don't know the reason why the results are different. If you have any news, please update me.

Thanks,
Pengchuan
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Old   March 11, 2014, 10:37
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Oscar Urbina
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Hi Pengchuan,

I found a solution to my problem,

first i discarded SRFSimpleFoam, the solution is too unstable at least for my case.

i used SimpleFoam with the MRF option.

now as i said before if you set all the cells in the rotationg frame it will crush your boundary conditions, i think it's because MRF is made for more than one frame, so if you put only one frame these kind of problems arise.

the solution for me was to leave at least a one cell distance between the rotating frame and the BCs, this way your BCs are respected and the overall solution is not affected (it behaves as SRF).

hope it helps,

Oscar
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Old   March 11, 2014, 10:41
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Pengchuan Wang
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Hi Oscar,

Thank you for your reply. Then your results are exactly the same? I mean the results with MRF and without MRF. For me, the results are different, and without MRF is better than the result with MRF.

Thanks,
Pengchuan
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Old   March 16, 2014, 14:53
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Oscar Urbina
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Hi,

i think they are suppose to be different

in simpleFoam without MRF you only have the inertial frame, and in simpleFoam with MRF you have the inertial frame plus the rotating frame in other words you add the centrifugal and coriolisis forces.

the only case in which both will have the same results is if you have a domain with no obstacle cause then the forces won't have any effect over the flow.

as soon as you add an obtacle let say a force, the solutions will be different.

you should read this article in openFoamWiki, here they explain how the MRF works.

http://openfoamwiki.net/index.php/Se...RF_development

hope it clears out some doubts,

good luck

Oscar
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Old   March 16, 2014, 15:21
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Pengchuan Wang
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Hi Oscar,

Thank you for your reply. Yes, you are right, they use different method to solve the equation. However, though they use different methods, they are actually solve the same PDE, since they are simulating the same problem. No matter what method you use, the results should be the same. But right now, I got different results from them, that's what makes me confused.

Thanks,
Pengchuan
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Old   May 13, 2015, 10:51
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Hi Oscar,

in your first Message you wrote:

Quote:
Originally Posted by oscar_urri View Post
In SRFSimpleFoam i can define the axis of rotation but i'm unable to define the origin of my rotation, so is the origin going to be (0,0,0) all the time? or if there a way to set it?
Did you find an answer to the question about setting the origin in SRFSimpleFOam?

Cheers, Dennis
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