> I'm using step-20 to calculate the dual-mixed formulation of the > Laplace-equation in context of mechanics with zero boundary values for the > scalar component. Here only the definitions change (pressure becomes > displacement and velocities become stresses), the weak formulation stays > nearly the same (only two algebraic signs change because of my classical > formulation and substitution). > > In step-20 the boundary value/ curve for the pressure is a polynomial > function and the applied force/ right-hand-side is zero. > > If I change the pressure boundary value to zero (i.e. g=0) and the force to > non-zero as I need it for my formulation, the membrane breaks off at the > boundary. This happens both if the force is applied on the hole domain > [-1,1]^2 and if it is only applied on a subdomain like a circle around > (0,0) with radius 0.5. I'm wondering about this because the natural > boundary condition for the scalar component of the dual-mixed > Laplace-problem is the zero value (cf. D. Braess, "Finite Elemente", > Springer 2007, p. 140).
What exactly do you mean when you say the "membrane breaks off"? I can't tell where the problem is (though what you describe should work at least in principle) but here are a few things you could try: - Throw out the solver and replace it with SparseDirectUMFPACK (see step-29; the class can also deal with block matrices). This way you know that the problem is not in the solver but in assembling the matrix. - Try a forcing function that has mean value zero. If the linear system is indefinite, you may get still get the right solution if you have such a rhs function, and knowing that may give you some insight where to search next. - What happens as you refine the mesh -- does it converge to something? Best W. ------------------------------------------------------------------------- Wolfgang Bangerth email: [email protected] www: http://www.math.tamu.edu/~bangerth/ _______________________________________________ dealii mailing list http://poisson.dealii.org/mailman/listinfo/dealii
