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neutral boundary condition needed

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Old   April 16, 2008, 10:03
Default neutral boundary condition needed
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Bernd
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Hi fellow Fluent-Users,

I'm trying to simulate an incompressible streamwise flow along an unbounded corner in order to view the behaviour of the boundary layer in the corner.

For this, I use a cubical domain with a velocity inlet, 2 walls (on the left and on the bottom) and 3 pressure outlets (on the right, on top and at the exit).

My problem is that the pressure outlet boundary has an impact too big on the interiors. No matter how I scaled the cubicle, the isovels would still extend up tho the lateral outlets, which can't be right because there should be a free unidirectional flow. Have a look at this: http://img76.imageshack.us/img76/882...n3xpresed0.jpg

That's why I'm desperately looking for a way to use a kind of neutral boundary layer that puts no forces on the fluid an just lets it travel through without having an effect on it, as already described in this thread: http://www.cfd-online.com/Forum/flue...cgi?read=47598

Could I do this with the help of a UDF?

I'm thankful for any helping answers!
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Old   April 19, 2008, 08:51
Default Re: neutral boundary condition needed
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sa
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make the outlet further downstream so that there is no effect of outlet on ur solution
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Old   May 2, 2008, 09:22
Default Re: neutral boundary condition needed
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Bernd
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Thanks for the suggestion, but that won't help. As I said before, scaling has no effect on the problem I am encoutering.

I am now trying to define an "open boundary" that doesn't impose vicous stress on the fluid with the help of a UDF. As a matter of fact, I know that the equation needed for this boundary is

eta*(nabla(u) + (nabla(u))[transposed])*n = 0

with u and n being vectors.

Does anyone have a spontaneous idea how I can put this equation into a UDF? I have to admit that I have absolutely no experience with the programming of UDFs.

Thanks a lot in advance!
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