Menu

Discussion Forum

New Discussion
Filtra

Discussion Closed This discussion was created more than 6 months ago and has been closed. To start a new discussion with a link back to this one, click here.

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

Posted 19 feb 2016, 15:24 GMT-5 Modeling Tools & Definitions, Parameters, Variables, & Functions Version 5.1 2 Replies

renphysics
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

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 giu 2016, 05:05 GMT-4
COMSOL Moderator

Hello renphysics

Your Discussion has gone 30 days without a reply. If you still need help with COMSOL and have an on-subscription license, please visit our Support Center for help.

If you do not hold an on-subscription license, you may find an answer in another Discussion or in the Knowledge Base.

Lluis Millet
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 8 years ago 22 giu 2016, 08:42 GMT-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

Please login with a confirmed email address before reporting spam

Posted: 8 years ago 28 giu 2016, 05:05 GMT-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

Note that while COMSOL employees may participate in the discussion forum, COMSOL® software users who are on-subscription should submit their questions via the Support Center for a more comprehensive response from the Technical Support team.

Suggested Content