Note: This discussion is about an older version of the COMSOL Multiphysics® software. The information provided may be out of date.

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.

problem on boundary definition of a 4th order PDE

Hi, I am a newer for COMSOL and I am learning to using COMSOL for solving PDEs.
My PDE is as follows:
uxxxx+2*uxxyy+uyyyy = f

with two types of boundary:
1) u = 0 && uxx+uyy = 0
2) u = 0 && ux*nx+uy*ny = 0 (Neumann Condition)

I split it into two second-order equations:
uxx+uyy = w
wxx+wyy = f

I have got a correct result with the the first type boundary by defining Dirichlet boundary conditions, i.e., u = 0 && w = 0, but I do not have an idea about how to deal with the second type boundary. So far, I have tried following methods, but all failed:
1) add a Neumann Condition (zero flux) or constraint , but it will overriden the Dirichlet boundary conditions. X
2) add a flux/source, but it do not change anything. X
3) add a weak constraint, the solution can not be found. X

Any help will be appreciated!

0 Replies Last Post Oct 5, 2012, 10:39 PM EDT
COMSOL Moderator

Hello Chao Pan

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.

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.