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-   -   Pressure BCs or Periodic BCs for model of frost on vertical surface? (https://www.cfd-online.com/Forums/fluent/73165-pressure-bcs-periodic-bcs-model-frost-vertical-surface.html)

 andy.eco March 1, 2010 06:41

Pressure BCs or Periodic BCs for model of frost on vertical surface?

Dear Fluent Community,

My simulation on Fluent 6.3 is the melting of frost on heated vertical surface.

1) I set up top face as "Pressure inlet", and bottom one as "Pressure outlet".

This setting seems unstable because it got "reverse flow" most of the time. I think the reason for that problem is the distance between two boundaries is too small ( in this case, the distance is 30mm). Therefore, I converted these faces into "Periodic Boundaries" and the model runs better (no reverse flow). However, my control volume is not really a repetitive part of a larger domain, so I would like to ask for your advices on how to set up BCs for this problem.

2) In "Periodic Condition" panel, I set Flow Direction as following: X=0, Y=-1 (Y is also set up for -9.81m/s^2 in Operation condition) and got velocity vector of melted frost pointing upward (?). Can you tell me what wrong with my setting? The pressure gradient is set 0.

3) I set up frost layer as a "fluid" material. On vertical plan, because this fluid volume has no bottom (the bottom now is set as periodic boundary) so I think that the fluid may run down continuously and has non-zero velocity (due to gravity). However, this doesn't happen.

Of course, in reality, frost does stick to the vertical fin surface and drop down only when it is melted til certain amount of it volume. Can you explain why fluid can "stand still" without bottom wall like this?

Eco

 andy.eco March 6, 2010 21:22

Periodic BCs are not for vertical flow?

Hi everybody,

Today I had a talk with an experienced CFD user about my questions above and here are some of his opinions that I would like to share with you.

First, from Fluent user guide, they recommend periodic BCs for fully developed flow or flow that behaves with repeating manner. For vertical plan with gravity turned ON, it's clear that flow will not guarantee such a "repeating manner".
Second, frost melting is melted uniformly on the surface.
And third, for VOF models, FLUENT recommends not using periodic BCs.

For other questions, I am still looking for help.

Thank you.
Eco

 sosososo1114 March 26, 2010 03:02

I suspect that you start a multi-phase solver. It seems that peoridical BC is unaccetabal under multi-phase condition according to the fluent user's guide. By the way, could you mind to tell me how you set your initial condition? I am trying to model defrost process. Yet before this, I want to get some information about the end of the frost. And is the wall temperature in your model constant?

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