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axial scaling of navier-stokes and brinkman equations

Posted Jun 17, 2010, 11:59 a.m. EDT 2 Replies

Adam Davidson
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Hi,

im using the free and porous media flow physics system in Comsol 4.0. The thing is im considering hollow fibres where the length of the domain is around 1000 times bigger than the radius (2d axisymmetric coordinates). Trying to mesh and solve something like this is troublesome. I can use convection-diffusion equations in a scaled form but im wondering if anyone knows how to do it with the NS and brinkman equations?

Thanks,
Adam
2 Replies Last Post Jun 23, 2010, 7:30 a.m. EDT
John
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Posted: 1 decade ago Jun 17, 2010, 2:09 p.m. EDT
I'm using a mapped mesh for much the same thing: brinkman in porous and RANS NS in fluid. My aspect ratio also about 1000. Results are surprisingly stable using a quad mesh, mapped so as to have lots of elements near the interface. If I mesh too finely at the interface then it becomes unstable again, presumably reaching some limit of aspect ratio.

I have a different question though: what is the difference between using the free/porous interface versus having two separate physics models. Are the equations on the interface different? I force continuity of pressre and velocity and basically allow a discontinuty in viscous stress. I dont know if that is correct!!

Thanks, John
I'm using a mapped mesh for much the same thing: brinkman in porous and RANS NS in fluid. My aspect ratio also about 1000. Results are surprisingly stable using a quad mesh, mapped so as to have lots of elements near the interface. If I mesh too finely at the interface then it becomes unstable again, presumably reaching some limit of aspect ratio. I have a different question though: what is the difference between using the free/porous interface versus having two separate physics models. Are the equations on the interface different? I force continuity of pressre and velocity and basically allow a discontinuty in viscous stress. I dont know if that is correct!! Thanks, John
Adam Davidson
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Posted: 1 decade ago Jun 23, 2010, 7:30 a.m. EDT
Hi John,

thanks for the reply. ive started using the mapped mesh now too and it seems ok although its impossible to show a 2d surface plot with non-equal axes, even though you can view the geometry this way. i was getting strange results for the free and porous media flow physics so I thought id try it your way. can i ask how do you set up the continuity of flow and pressure at the boundary between the free medium and porous matrix?

Thanks,
Adam
Hi John, thanks for the reply. ive started using the mapped mesh now too and it seems ok although its impossible to show a 2d surface plot with non-equal axes, even though you can view the geometry this way. i was getting strange results for the free and porous media flow physics so I thought id try it your way. can i ask how do you set up the continuity of flow and pressure at the boundary between the free medium and porous matrix? Thanks, Adam

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