| Hello, I'm quite new on working with deal.ii since I'm working on my master's thesis by supervision of Prof. Dr. F. T. Suttmeier in Siegen, Germany. 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). Can anybody explain this to me? Thanks a lot! Nicole Droege
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