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Model flow over / through expanded metal walkways.

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Old   January 9, 2008, 10:06
Default Model flow over / through expanded metal walkways.
  #1
andy2o
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Hi All,

I'm new here, I've searched the archives for answers to my question and not found anything, so here's my first post.

I need to model ventilation air flow and heat transfer within a confined machinery space to an unusually high degree of fidelity. Within the domain there are a number of expanded metal walkways. I expect fans and heaters to drive forced and natural convection flows through the holes in the walkway.

(Just incase you don't know what an "expanded metal walkway" is, they are effectively metal sheets a few millimeters thick with diamond shaped holes in - you can find plenty of examples by doing a Google image search for "expanded metal walkway".)

Can anyone suggest how best to capture the effect of the these walkways on the flow in CFX11?

I'm not concerned about heat transfer to the walkways - they'll soon be in thermal equilibrium - but their influence on the flow should be assessed, and modelled if necessary.

-Would you use a very thin slab of porous region material?

-Any other ideas?

-Do any references come to mind?

(I certainly can't mesh the holes in the walkway explicitly of course!!)

Many thanks for you help and comments.

Best regards, Andy
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Old   January 9, 2008, 18:00
Default Re: Model flow over / through expanded metal walkw
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Glenn Horrocks
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Hi,

This is a standard requirement - you model them as momentum sinks. Firstly you need to establish the flow versus pressure drop curve for the mesh. You can get this data from experiments, a detailed simulation of flow through a mesh or just guess it by extrapolating from perforated sheet or something similar where you have some results.

Then you pick a momentum source/sink which matches the flow versus pressure curve. You define a volume (and I think you can do a thin surface which would be useful here) in the mesh and apply the momentum sink to the region. This is explained in the documentation.

Regards, Glenn Horrocks
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Old   January 10, 2008, 06:28
Default Re: Model flow over / through expanded metal walkw
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andy2o
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Glenn - many thanks for your reply.

I was aware of the momentum sink for volumes - that's one option I'd certainly been considering (using correlations to find dp) but if it can be done with a thin surface instead of meshing a thin 3-D slab that would be much better...

I'll look into momentum sinks at thin surfaces - I've not noticed this in the docs before but I'll look again now.

So, if anyone has done a momentum sink at a thin surface before before, I'd love to see a reference or hear how you did it!

Many thanks, Andy.

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Old   January 10, 2008, 12:05
Default Re: Model flow over / through expanded metal walkw
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Bart Prast
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I don't think you can do a moment source on a thin surface. The thin surface has volume 0 so a source term wouldn't do much
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Old   January 10, 2008, 18:51
Default Re: Model flow over / through expanded metal walkw
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Glenn Horrocks
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Hi,

Bart is right - a source/sink has to be applied to a volume so if you can specify the mesh as a volume this will work. If you want to specify the mesh as a thin surface you will need to use an interface with a pressure change applied to it.

Glenn Horrocks
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Old   January 11, 2008, 05:13
Default Re: Model flow over / through expanded metal walkw
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andy2o
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Thanks to you both Bart and Glenn - I'll set up a simple sample geometry and run some tests. (If I find anything I think is of general use I'll post a follow-up reply here for others to read in the archives.) Andy
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