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Old   August 25, 2006, 18:08
Default fixed velocity field
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Using CFX 10, I have developed a model to explore the fluid dynamics of a sheet of paper moving under a steam spray in the press section of a paper machine. I have modeled the paper using a porous domain with water as the main fluid (also have water vapour and air in multiphase homogenous model). I specify the paper velocity on the inlet and outlet on the leading and trailing edges of the paper. Additionally, I set a no-slip wall velocity on the non-interfacing side of the paper. With these conditions my velocity field is not uniform throughout the paper thickness (200 microns) due to the impingement of the steam onto the sheet surface. I was able to improve the uniformity of the velocity in the paper sheet somewhat by introducing a small momentum source following guidelines for setting Dirichlet velocity condition, but it is still not uniform. I have also tried only setting the Dirichlet condition and setting a free slip condition on the back side and leaving the edges of the paper as openings. I have also tried setting a 'free surface flow' on the interface between the paper surface and the surrounding air. I have tried various other things all with no success.

Can anyone give me some advice on fixing the velocity? I realize there is an option for specifying a 'rotating' frame of reference for turbo-machinery, but there does not seem to be the equivalent option for specifying a 'translating' frame of reference...

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Old   August 26, 2006, 09:45
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Modelling a moving non newtonian paper slurry as a combination of newtonian water flowing through a fixed porous matrix sounds like an exersize in futility to me.
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Old   August 28, 2006, 11:50
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It is not an excercise in futility. I am trying to refine a working model which has already given me tremendous payback in solving a complex (and yes, unconventional) problem.

Can someone else please take my enquiry seriously? I am open to productive advice, but spare me the glib responses.
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Old   August 28, 2006, 12:17
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Hi Glen,

Sounds like an interesting application. Where exactly do you see the non-uniform velocity? Is it only at the interface?

I suspect the problem is that you are using a porous domain. If you can do without the volume fraction, I would specify this as a subdomain, rather than a porous domain. In a porous domain, the porous loss model will be relative to the stationary frame of reference, not the moving frame as you suggested. If you specify this as a subdomain and apply your own loss model, you can adjust it accordingly for the rotating frame. This is essentially what you are doing when you specify a Dirichlet condition, but in that case you make the "losses" associated with deviations from the specified velocity very high.

If I'm way off, please try and describe what you are doing more accurately. I also highly recommend taking this up with technical support.

Regards, Robin

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