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Re: [Getfem-users] Computation of reaction forces associated to a dirich
Re: [Getfem-users] Computation of reaction forces associated to a dirichlet condition
Fri, 30 Jul 2021 13:07:59 +0200
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I don't know if this explains the difference obtained because it is
really important, but the residual of the linear system and the
multiplier are not really comparable. The residual is not a force
density, its components represent some equivalent nodal forces whose
intensity depends on the mesh size, whereas the multiplier is taken from
a finite element field and represents a force density. To obtain a force
density from the nodal forces, a mass matrix must be inverted on the
On 30/07/2021 12:41, David Danan wrote:
Dear GetFem community,
i am trying to compute the reaction forces associated to a dirichlet
condition, in order to do so, i have followed the advices given in
You can also find enclosed my test case, using the python interface.
The steps are the following:
-Solve a simple 2D elasticity problem on a rectangle (dirichlet on the
left, neumann on the top)
-Compute the reaction forces arising from the residual without the
dirichlet condition, by using the solution in displacement, the
tangent matrix and the right hand side term
-Retrieve the value of the multiplier associated to the dirichlet
In fact, i expected both quantity to be the same but they were not (2
order of magnitude different, i didn't expect such large values for
the multipliers) and i was wondering which one was correct.
If it helps, you can find both quantities in Mult.png (Multipliers is
the true multiplier and MultipliersResidual is the residual without
the dirichlet condition).
Next, i built a second model similar to the first one but without a
dirichlet condition. I added an explicit rhs using the
MultipliersResidual ( Multipliers gave strange results) and tried to
solve the problem, you can see the original solution "Displacement"
and newsolution "Displacement_test" result in Sol.png.
It seems i am missing something there, i expected the solutions to be
the same, not just close enough.
Can you enlighten me?
Yves Renard (Yves.Renard@insa-lyon.fr) tel : (33) 04.72.43.87.08
20, rue Albert Einstein
69621 Villeurbanne Cedex, FRANCE