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Old   April 13, 2005, 19:49
Default Divergence
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Docker
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Can anyone tell me the fundamental reasons why divergence occurs if one tries to use a mass flow at one boundary and pressure at the other?

Regards, Docker
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Old   April 13, 2005, 23:12
Default Re: Divergence
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pUl|
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I do not think divergence errors are directly related to boundary conditions chosen. Instead they are linked to your problem definition. Personally, I follow some of the guidelines in the Fluent manual and things shape up very well. For instance, once you find continuity diverging after a particular amount fo time, you might want to:

a. Lower some URF's, say momentum and body forces (judiciously) at the problem start and then increase them by small amounts.

b. Try to solve for lesser equations till the flow develops. Like turn off "volume fraction" solution till the initial flow field has developed.

For my own multiphase case, the 6.2 manual now says that one can now restrict the application of lift forces only to the specific areas in the domain where the shear rates are significant, which is quite sensible and IMO will avoid divergence issues. But they have not mentioned how this can be done. Perhaps through a UDF?
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Old   April 14, 2005, 07:16
Default Re: Divergence
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Docker
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Thanks very much for taking the time to respond.

I am new to all this.

On a seperate issue of mine, which is causing the most trouble: I've set up an unsteady run with an inlet manifold as my domain. Pressure inlet and UDF for pressure outlet. I now want to be able to turn the outlet from a 'pressure outlet' to a 'wall', when then valve is closed. So I need to able to change the boundary condition at certain time steps.

Any ideas? Heard of a method using a 'hand-written' journal file?

Thanks again,

Docker
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Old   April 15, 2005, 06:04
Default Re: Divergence
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Abu-Khawlah
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Hi, there

I think you should only run your case with pressure-outlet for define time, then re-define the outlet as a wall and start the problem again, Fluent will use your last case data as an intial data for the new case, Howeve, if you need to open and close the out let valve very frequently, then I thing you need to use a UDF macro.

All, the best

Abu-Khawlah
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Old   April 15, 2005, 07:47
Default Re: Divergence
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Docker
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A very good idea.

Unfortunately you pre-empt the problem. The valve does open and close frequently. And there are 4 inlet valves (outlets to the domain)!

How, in a macro can you define a change in boundary condition. Now I don't mean a change in a value - that is easy, I mean a change form 'pressure-outlet' to a 'wall' (at stated time steps)?

This is the thing that I do not understand? Am I missing something, or is this tricky?

Thanks for your help,

Docker
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Old   April 15, 2005, 09:20
Default Re: Divergence
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Abu-Khawlah
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I dont know, if you can change the boundary condition status using the UDF, However, I mean you should use a proper UDF macro to represent you physical bounday conditions, like maybe you can define your outlet as an WALL and link it with a UDF source macro allowed the fluid to leave the system once it hit that wall and also pre-define the surface with pressure profile that change with the physical status of your case (open valve=>run the sorce code+applie a pressure) or (close valve=>disactivate the source code+solid wall pressure)

I hope it is cleare for you, it is just a suggestion, I never didt before, so if you will go for it, make sure it will give you the same pressure distribution like real case, I suggest you run different cases with and without avalves to be a reference cases for you

All the best

AbuKhawlah
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Old   April 17, 2005, 05:46
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Old   April 17, 2005, 19:02
Default Re: Divergence
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Docker
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Thats clear, yes. I know what you mean re the pressure in the UDF. I am using that technique. And, yes, it is a good suggestion to try both methods, to compare.

Thanks for the suggestions.

Docker
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