# Weak costraint to solve laminar NS?

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 June 9, 2012, 02:19 Weak costraint to solve laminar NS? #1 Member     kiwi Join Date: Mar 2012 Location: South East Asia Posts: 54 Rep Power: 6 Hi, dear all, Recently go through some article during lunch, have your guys heard about "weak constraint"? I heard that weak constraint can served as Dirichlet condition to solve the CFD convergence issues. it may be applied as boundary condtion to make the model converge faster. is there any book i can refer to ?i believe this term can be explained through mathematical, but how about CFD? Thanks.

 June 9, 2012, 05:31 #2 Senior Member   cfdnewbie Join Date: Mar 2010 Posts: 557 Rep Power: 12 Not sure if I understood you correctly, but was it about weak and strong boundary conditions? Sounds like....

June 9, 2012, 05:46
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kiwi
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Quote:
 Originally Posted by cfdnewbie Not sure if I understood you correctly, but was it about weak and strong boundary conditions? Sounds like....
Thanks for cfdnewbie interest.

Because i am curious what is different if we add in weak constraint. As in CFD, we have inlet and outlet condition. We can Neumann/dirichlet condition, and after few iteration, it may help to solve the problem.

where is the application of this weak constraint term? or does it means that if i combine Neumann condition and weak constraint, if can form a good boundary condition?

 June 9, 2012, 05:53 #4 Senior Member   cfdnewbie Join Date: Mar 2010 Posts: 557 Rep Power: 12 a strong bc formulation means that you prescribe the value of the function directly, a weak bc means that you prescribe the flux that fits to the state you would like to achieve. Let's say you would like to enforce a no-slip wall. You can then force the noslip through the state directly (strong bc), or indirectly through the fluxes. The latter does not guarantee (for all resolutions) no slip on the wall, but it will make convergence easier.

June 9, 2012, 09:48
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kiwi
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Quote:
 Originally Posted by cfdnewbie a strong bc formulation means that you prescribe the value of the function directly, a weak bc means that you prescribe the flux that fits to the state you would like to achieve. Let's say you would like to enforce a no-slip wall. You can then force the noslip through the state directly (strong bc), or indirectly through the fluxes. The latter does not guarantee (for all resolutions) no slip on the wall, but it will make convergence easier.
i get it. thanks

 June 9, 2012, 13:19 #6 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 2,498 Rep Power: 31 I think that the issue has a mathematical implication, prescribing the flux as BC on the boundaries requires to satisfy a compatibility constraint involving the surface integral of the flux, this remember the compatibility constraint in the pressure equation for incompressible flows.

June 10, 2012, 09:49
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kiwi
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Quote:
 Originally Posted by FMDenaro I think that the issue has a mathematical implication, prescribing the flux as BC on the boundaries requires to satisfy a compatibility constraint involving the surface integral of the flux, this remember the compatibility constraint in the pressure equation for incompressible flows.
Hi, FMDenaro,

thanks for your thought, however, I have some confusion.
well, I thought when you prescribe the flow rate or surface integral of the flux(flow rate) on the inlet, CFD solver automatically assign a pressure term on the inlet BC.
If you add in a compatible constraint, will it cause NS equation to be overdefined or overconstraint, and hence results in convergence issues?

i am newbie, sorry for the confusion.

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