# Discussion Forum

## Unidirectional vs Bidirectional Constraints

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John Lovell

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

Ivar Kjelberg

June 18, 2010 5:52am UTC in response to John Lovell

Re: Unidirectional vs Bidirectional Constraints

Hi

For me this means either two equations linking in both directions, or just in one direction.
It's easier to spot when you set up your coupling variables yourself, the ones internally defined by COMSOL are often well hidden in all the internal variables (physics - equations System ... in v3.5)

Good luck
Ivar

John Lovell

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

Ivar Kjelberg

June 19, 2010 5:34am UTC in response to John Lovell

Re: Unidirectional vs Bidirectional Constraints

Hi

Yes coupling is delicate, and there are several ways (even mor I believe in 4 than in 3.5) and it takes time to catch the subtilities of each.

My way is, once having passed a few hours fighting, to make a simple representative case, and send it to "support", so if you manage, perhaps deposit it here on the forum too, then we other can learn too ;)

Have fun Comsoling
Ivar

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