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How to impose a Neumann boundary condition for one dependent variable, but not for others?

Posted 19 févr. 2016, 15:24 UTC−5 Modeling Tools & Definitions, Parameters, Variables, & Functions Version 5.1 2 Replies

renphysics
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I am working with "General Form PDE", and want to solve two coupled ODEs for u1(x) and u2(x):

grad(u1x) = f1(u1, u2, u1x, u2x)
grad(u2x) = f2(u1, u2, u1x, u2x)

At the boundary x=0, I want to impose a Dirichlet boundary condition for u1, and a Neumann boundary condition for u2:

u1(0) = 1
u2x(0) = 1

To impose a Neumann boundary condition, we can select "Physics">"Boundaries">"Flux/Source", and fill in the "Boundary Flux/Source" section. But we have to fill in the two text fields for u1 and u2 at the same time; I cannot find a way to impose a Neumann boundary condition for u2 only, but not for u1.

Note that u1(0)=1 does not mean u1x(0)=0, for example, if the near boundary behavior of u1(x) is 1+c*x+....

How to impose a Neumann boundary condition for one dependent variable, and a Dirichlet boundary condition for another dependent variable?
2 Replies Last Post 28 juin 2016, 05:05 UTC−4
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Hello renphysics

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Lluis Millet
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Posted: 8 years ago 22 juin 2016, 08:42 UTC−4
Hey, I have the exact same problem. If you did find a solution, please report it, it would be really helpful. Otherwise, does anybody know how overcome these limitation?
Hey, I have the exact same problem. If you did find a solution, please report it, it would be really helpful. Otherwise, does anybody know how overcome these limitation?
Magnus Ringh COMSOL Employee

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Posted: 8 years ago 28 juin 2016, 05:05 UTC−4
Hi,

You should be able to do this by adding a Dirichlet Boundary Condition node under the Flux/Source node, and then select only the dependent variable for which you want to apply a Dirichlet boundary condition. For the other dependent variable, the Neumann boundary condition is then still active. See the attached screenshot.

Best regards,
Magnus Ringh, COMSOL
Hi, You should be able to do this by adding a Dirichlet Boundary Condition node under the Flux/Source node, and then select only the dependent variable for which you want to apply a Dirichlet boundary condition. For the other dependent variable, the Neumann boundary condition is then still active. See the attached screenshot. Best regards, Magnus Ringh, COMSOL

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