I am trying to numerically solve a 3D diffusion type equation with a spatially varying diffusion $\nabla\cdot(K \nabla T)=\partial T/\partial t$ with K a given function of space with different initial value (e.g., at time $t=0$ the function $T$ decays exponentially with depth $|z|$) and boundary conditions (e.g., at the top $z=0$ layer, $T$ is a given function of time). I have perused some of the earlier posts regarding numerical solutions of related problems and seem to consistently get (compiling and output) errors (including those of NDSolve ‘FEM’ FEMStiffnessElements operator failure). Any suggestions will be greatly welcomed.


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The error FEMStiffnessElements operator failure comes up when K is not defined properly. The first application example from the DiffusionPDETerm

model = DiffusionPDETerm[{u[x], {x}}, {{If[x <= 3/4, 1, 2]}}]

It's impossible to be more specific unless you provide your specific equation and code that you have.


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