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Old   April 5, 2010, 23:44
Default multiple fluids depending on time
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Hello.

I would like to ask a question regarding CEL.
I want to select alternatively two fluids (air and water) in the same domain depending on the simulation time.
(For example, among 20 sec of total transient simulation time, 0-15 sec: water and 15-20sec: air in the same domain).
Please let me know how to specify the fraction of two fluids using CEL w.r.t the simulation time in the same domain.
Thank you.

Jaho
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Old   April 6, 2010, 18:31
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Yes, this is easy. You just set the volume fraction to be a CEL expression which is 0 before the switch time and 1 after the switch time. In CFX v12 you can use an if statement to do this. If you are in an earlier version you will have to use the step function.

Alternately you can use a 1D CEL interpolation function.
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Old   April 10, 2010, 12:44
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Dear Glenn Horrocks,

Hi Glenn. I appreciate your kind help always.
If you don't mind, I would like to ask one more question.
Actually, my model is a casting die with a cooling channel. Among 87 sec as total cycle time, 83 sec is for cooling time. The remaing 4 sec is for ejection of melt (or part) and injection of new polymer and thus water supply should be stopped during 4sec.
So, I give some (positive) value of mass flow rate during 83 sec in the inet's BC of cooling channel and also give the zero value of mass flow rate during last 4sec in the inlet's BC.

When I simulate with the above condition, an error occurs in the CFX-solver manager as follows:

****** Notice ******
| A wall has been placed at portion(s) of an OUTLET
| boundary condition (at 20.0% of the faces, 31.1% of the area)
| to prevent fluid from flowing into the domain.
| The boundary condition name is: outlet.
| The fluid name is: Water.
| If this situation persists, consider switching
| to an Opening type boundary condition instead.

ERROR #001100279 has occurred in subroutine ErrAction.
| Message:
| Floating point exception: Overflow

Actually, the inlet is located on the bottom of channel and the outlet is on the top of channel. (So, the mass flow is zero in the inlet, the water can flow down to the inlet by gravity in real situation...I don't know if this is related to the above 'overflow'.)

Please give me an advice about how to solve the above problem...


(Actually, in order to solve this, I used the volumn fraction(with water supply for 83 sec and air suppy for 4sec), but there is an error too. Its error message is "Fluid solid interface are not supported with multiphase flow. Inferface "mouldcontactwater" connects a fluid domain with more than one phase to solid domain)

Thank you so much.

Sincerely,
Jaho
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Old   April 11, 2010, 06:50
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The overflow error simply means your simulation has diverged and you need to increase the numerical stability of the simulation.

Your statement about the inlet sounds like it may well be the cause. I would move the domain boundary back further so you have simpler boundary conditions. Hopefully you can locate a point where the stuff gets injected from but is otherwise shut off, maybe another port where the air or water escapes - having stuff enter and leave the same boundary is not recommended.
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Old   April 11, 2010, 19:28
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Hi Glenn,

Thank you so much for your help.
According to your advice, I switched the inlet location and the outlet location. It seems O.K now.
However, there is still the following notice.

****** Notice ******
| A wall has been placed at portion(s) of an OUTLET
| boundary condition (at 20.0% of the faces, 31.1% of the area)
| to prevent fluid from flowing into the domain.
| The boundary condition name is: outlet.
| The fluid name is: Water.
| If this situation persists, consider switching
| to an Opening type boundary condition instead.

If there is no error during the simulation in CFX-solver manager, then
is it O.K regardless of the above notice (Actually I cannot understand the meaning of the above notice)?

If you don't mind, could you please verify this (as well as the meaning if possible...)?

Thanks so much again and have a great day~!

Jaho
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Old   April 11, 2010, 20:18
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There have been many posts on the forum discussing these walls. Do a search on the forum for more information about them.
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Old   April 13, 2010, 15:37
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Thank you so much, Glenn.
Have a great day.
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