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Houman September 5, 2006 09:11

Hi everyone,

Does anybody know how can I use "Extrapolation" in my model at the Opening boundary condition!?!? unfortunately there is no information about this command on the CFX Manual and I don't have any support how could help me with that :(

FYI, I need to use it(Extrapolation) for my additional variables at the opening... so that I don't have to set a fixed value...

Would appreciate any comments or help...

thanks in advance, houman

Michael B September 6, 2006 03:32

Re: Extrapolation!!!!
Do you mean a Neumann boundary condition (Setting the gradient and not the value itself)?

Houman September 6, 2006 04:01

Re: Extrapolation!!!!
Dear Michael,

Thanks for your reply... I'm not sure about the definition of "Neumann boundary condition" but what I need is to be able to have the same values of my "Additional Variables", as input to the opening boundary (at top of my Bubble Column), as they (Additional Variables) are inside (at the very top of) the column, and not something totally different (new) coming to the domain (just because there is an opening boundary).

Please let me know if you need more details about the problem... would appreciate your reply,


Phil September 6, 2006 08:33

Re: Extrapolation!!!!
If you have flow coming in, you need to set the additional variable value, otherwise you don't have a well-posed problem (except for exceptional cases). Leaving it unset is similar to having an inlet with temperature unset - this is appropriate occassionally, but not in general.

On an opening, of course, the specified values are used only when the flow is coming in.

Houman September 6, 2006 08:52

Re: Extrapolation!!!!
Hallo Phil,

thanks for your reply, I do have an input in that opening that means, as you said I have to set the AV value at the opening, but since the value I had to set (because of my model case) is not constant I needed to find a way to have a variable value for my AV. Which was actually the same value of AV on the way out of the column (from Opening)...

I guess I found a way: I set an Expression value for the AV at the opening: probe(AV)@opening [kg m^-3]

so this expression will take a probe of AV at the specified point (opening) and uses the value for the input AV...

I started the run and it looks ok :)

best regards, houman

Houman September 6, 2006 11:18

Re: Extrapolation!!!!
Hi Phil,

the run stoped because of an error :( so, I still need help in this regard... the error was:

ERROR #001100279 has occurred in subroutine ErrAction. | | Message: | | An error: Illegal variable: in call back function probe on locale | | MONP3

Do you have any idea what might be the cause!?

best regards, houman

Phil September 7, 2006 13:31

Re: Extrapolation!!!!
This still sounds too Neumann-like, but you should be able to set it up. Make sure you put the correct AV name in. Or use areaAve(av_name)@boundary_name to use the average value over the boundary.

Houman September 8, 2006 03:27

Re: Extrapolation!!!!
Hi Phil,

thanks for your reply and your suggestion, I'll try it and let you know what's the output...

best regards, houman

Houman September 8, 2006 03:34

Re: Extrapolation!!!!
Hi again,

the run again stopped:

(An error: Illegal variable: in call back function areaAve on locale Top )

I guess the only problem is that the Solver doesn't accept any Expression containing the same AV to be set as a Value for that same AV... which is now unfortunately the case...

So, do you have any idea how we can overcome this problem!? Would appreciate any comments...

best regards, houman

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