# Discussion Forum

## Simultaneous Boundary Conditions

Hello,
I am making an exercise to reproduce a model with a set of PDE using math model in COMSOL, the model include a set of boundary conditions like this:
y=0 ---> Ψ = 0
y=0 ---> ∂Ψ/∂y = - S ∂θ/∂x
y=0 ---> ∂θ/∂y= 0
When I introduce the second one for the same differential equation overwrite the first one,
How can I introduce this simultaneous conditions?,
or simply COMSOL can not manage this?

3 Replies Last Post 2017年9月15日 GMT-0400下午8:36
Posted: 2 months ago

I believe it is possible to do so. But maybe one should use some alternative form like constraint?

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Pu, ZHANG
Huazhong University of Science and Technology

Posted: 2 months ago

I believe it is possible to do so. But maybe one should use some alternative form like constraint?

Thanks

I indroduce the second one as constraint, 0 = psiy + S * thetax in some time mixing 0 = thetay + psiy + S* thetax but solving with the last one can produce values for tethay=/=0

Posted: 2 months ago

I guess that your governing equation including two function y & theta. So For the first one, you can apply directly by using Dirichlet BCs. But in case of second & third one, it should be better if you apply them as weak form by using Weak form Contribution. Because your BCs is the derivative form ( except for Neumman BCs ) Theorically, all the governing, BCs equation in COMSOL will be convert to Weak form. So onece you convert them to weak form, it means that you have already physically described your system BR.