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October 6, 2012 2:39am UTC

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!
Thanks!

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