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June 17, 2010 7:48pm UTC

Unidirectional vs Bidirectional Constraints

Does anyone have an easy to read summary of the difference between unidirectional and bidirectional constraints? I thought I understood, but then saw a Tutorial module from Comsol that used uni when I would have used bi, so I switched and now my answer is different. Confusingly the equations look identical when viewed as "show equation"

My immediate application is coupling Brinkmann to Turbulent Navier Stokes. The wall-condition interface doesn't let me specify whether the NS wall-condition is uni or bi. The Brinkman condition on pressure does allow choice.

Thanks, John

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June 18, 2010 9:13pm UTC in response to Ivar Kjelberg

Re: Unidirectional vs Bidirectional Constraints

Hi Ivar, yes, that's what I thought too.

In my oft-repeated example, I have a "offline" model where I compute fully developed inlet conditions and then pass those as Dirichlet's to the model I really want to solve. I certainly dont want anything from the latter messing up the former, so the latter gets unidirectional. No problem, all works fine. That second model also has coupled Brinkmann, Navier-Stokes.

But the online notes seem to imply its addressing a slightly different problem, namely when you want to assign both a Dirichlet and a Neumann condition. For example, to couple physics A to B with a Dirichlet and physics B to A with a Neumannn. The way the notes explain, its got something to do with whether that Neumann condition is the "natural" Neumann associated with the PDE.

And in the COMSOL tutorial example coupling Brinkmann to Navier-Stokes, they use unidirectional constraints on both sides. Just on a whim I changed my (formerly) bidrectional constraint into unidrection and the answer is totally different.

So now I'm confused!

Regards, John

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