On 05/17/2018 10:05 PM, Pai Liu wrote:
I found that I forget to add other boundary conditions, and in this case the
stiffness matrix is in fact singular.
From my understanding, if I want to constrain a dof "i" to zero, instead of
adding a constraint of dofs "i, j, k, ...", I may only need the add_line function.
Yes, that is correct.
Do you have an elasticity problem with only Neumann (traction) boundary
conditions? In that case, it is not enough to only pin the displacements at
one vertex of your domain. That kills the translation mode. But you still need
to kill the rotation mode as well, which is one additional degree of freedom
in 2d (for a total of 3) and 3 more degrees of freedom in 3d (for a total of 6).
Best
W.
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Wolfgang Bangerth email: [email protected]
www: http://www.math.colostate.edu/~bangerth/
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