# neutral boundary condition needed

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 April 16, 2008, 10:03 neutral boundary condition needed #1 Bernd Guest   Posts: n/a 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!

 April 19, 2008, 08:51 Re: neutral boundary condition needed #2 sa Guest   Posts: n/a make the outlet further downstream so that there is no effect of outlet on ur solution

 May 2, 2008, 09:22 Re: neutral boundary condition needed #3 Bernd Guest   Posts: n/a 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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