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Can the MRF and porousZone models be combined in one zone?

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Old   January 18, 2016, 07:10
Question Can the MRF and porousZone models be combined in one zone?
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Erik Löfgren
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Hello!

I have been trying to run a couple of cases in OF v.2.3.1 (solver porousSimpleFoam) where a certain cell zone is simultaneously modeled as a porous zone and an MRF zone. I.e. a rotating porous medium with no walls in the rotating cell zone whatsoever. The expected behavior would be that the rotating porous medium exerts some forcing on the fluid within the pores and generates a certain degree of co-rotation. However, the results I have been getting show zero fluid velocity in the porous/MRF zone in the absence of any moving walls or other forcing which would make sense if the equations of the porous medium does not apply to the relative velocity, but the absolute. This means that the porous medium itself is stationary.

I haven't been able to determine this from the source code, but I thought that maybe someone within the community had some insight into the implications of applying the MRFzone and porousZone tools to the same cell zone. Would it be possible for me to rewrite the porousZone code to use the relative velocity for the calculation of force, if this is not already implemented?

Best regards
Erik
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Old   April 5, 2016, 02:03
Default Relative Velocity Resistance Formulation
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Erik Löfgren
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The feature that I'm looking for is apparently implemented in ANSYS Fluent and it's called "Relative Velocity Resistance Formulation". Is there anyone else interested in having this feature implemented in OpenFOAM?
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Old   April 7, 2016, 13:47
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António Soares
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I'm trying to define a case using porousSimpleFoam and form what I've seen so far it's possible to implement MRF and porous zone since version 2.2.0.

Is this what you're referring to?
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Old   April 7, 2016, 16:54
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Erik Löfgren
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Hi! No, that link is not quite the case I'm trying to simulate. I agree that both MRF and porous media can be implemented in the same case, but as far as I can tell the porous media can't be made to rotate in a real sense. If you define a rotating zone as a porous medium the resistance due to the medium is calculated with respect to the stationary frame (as far as I can tell). So it's like any rotors are "sliding" through the porous medium rather than having the porous medium corotating with the rotating zone. It would be nice to have the option to chose wether or not the porous resistance should be computed w.r.t. the stationary or the rotating frame like in Fluent.
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Old   April 7, 2016, 19:22
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Ah I see, so what you want something like a dynamic mesh in which the porous cellzone is the moving element?
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Old   April 11, 2016, 05:20
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Yeah something like that. Even better if it's with the MRF model. That is, if you would make a spherical/cylindrical domain that is completely porous and rotating using the MRF model, and the boundary conditions are just slip walls, I would want the steady state solution to be a rigid-body rotation of the fluid (dragged along by the rotating porous medium). But If I set this case up right now using the current MRF/porous models, the steady state is just a fluid at rest.
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Old   April 11, 2016, 05:37
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Well, if you can in fact program that feature into OF I'm pretty sure the community would be thankful for it. It might take more work than merely programming a new solver (I'm not sure, but I assume it would take some actual programming instead of just putting in new equations into the library files) but it would be well worth the effort, and I'm sure that if you do it through the online repository someone would help you out.
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