# Questions tagged [fenics]

Questions about the finite element method software FEniCS and its subpackages (Dolfin, UFL, FFC).

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### Poisson equation with pure Neumann boundary conditions (using FEM)

I am trying to derive the correct variational form for the Poisson equation with pure Neumann boundary conditions, and an additional contraint $\int_{\Omega} u \, {\rm d} x = 0$, as described in this ...
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### Tensor notation in fenics [closed]

Within Fenics what ordering is used for storing vectors as upper triangular matrices? Th:e question really is I define a green Lagrange strain tensor in the following way: ...
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### Integrating an Ordinary Differential Equation over a domain in Finite Element Methods

I'm trying to get a deeper understanding of the Finite Element Method (FEM) to better understand what I do in COMSOL and FEniCS. The sketch below shows the problem that I want to solve in FEM. The ...
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### PETSc Krylov Subspace and nullspace

How is the nullspace correction implemented in the PETSc conjugate gradient solver? I searched the source code and documentation, but could not find references on the actual implementation. For ...
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### Poiseuille flow

I have probably a very stupid problem. I can't solve a simple Poiseuille flow in a straight 2-D channel driven by a pressure drop. Results are complete nonsense. (Setting zero pressure on the outflow ...
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### CFD: multiphase flow modeling of a laminar flow reactor

I am planning to model the laminar flow reactor shown in the picture below using computational fluid dynamics (CFD). The laminar flow reactor is used to study a multiphase flow: a layer of sheath air ...
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### FEniCS : How to interpolate data at vertices of (3D) cells?

I am trying to get an interpolation function $f$ (in 3D) at all vertices of cells. I extract all vertices of cells and then I assign the value to each vertex: if it's in a sphere of radius $R$, then I ...
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### compute max(||u||) for vector field

Given a vector-valued Dolfin function u from the function space V*V with ...
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### Fast function evaluation on a grid

I am trying to calculate a 3D Function f on a large set of points, say a uniform cubic grid. Besides, the cubic grid points do not coincide with the mesh vertices where the Function f is defined. For ...