Problem Statement

  • This example briefly describes the creation of a basic input file and the six required components for utilizing MOOSE for solving a problem.
  • We consider the steady-state diffusion equation on the 3-D domain $$$\Omega$$$: find $$$u$$$ such that $$$$-\nabla \cdot \nabla u = 0 \in \Omega,$$$$ $$$u = 1$$$ on the bottom, $$$u = 0$$$ on the top and with $$$\nabla u \cdot \hat{n} = 0$$$ on the remaining boundaries.

  • The weak form (see Finite Elements: The MOOSE Way) of this equation, in inner-product notation, is given by: $$$$(\nabla \phi_i, \nabla u_h) = 0 \quad \forall \phi_i,$$$$ where $$$\phi_i$$$ are the test functions and $$$u_h$$$ is the finite element solution.

Input File Syntax

Mesh

  • The domain for the problem is created with the "Mesh" block in the input file.
  • Here the mesh, the mesh is read from the file mug.e.
[Mesh]
  file = 'mug.e'
[]

mug_mesh.png
mug.e mesh file

Variables

  • In this simple problem, a single variable, 'diffused,' is defined, which represents $$$u$$$ from the continuous problem.
  • The 'diffused' variable is approximated with linear Lagrange shape functions.
[Variables]
  [./diffused]
    order = FIRST
    family = LAGRANGE
  [../]
[]

Kernels

  • The weak form of problem statement is represented by a Diffusion Kernel object.
  • In general, users write their own Kernels and store them in their own MOOSE-based application, but in this case the Diffusion Kernel is already defined in MOOSE.

  • Within the input file, to invoke the use of this object a sub-block is defined, named "diff", that utilizes the Diffusion Kernel on the previously defined variable "diffused".

[Kernels]
  [./diff]
    type = Diffusion
    variable = diffused
  [../]
[]

Boundary Conditions (BCs)

  • Boundary conditions are defined in a similar manner as Kernels.
  • For the current problem two Dirichlet boundary conditions are required, again an object for this type of boundary is already defined in a C++ object within MOOSE: DirichletBC.
  • In the input file the two boundary conditions are applied utilizing a single C++ object as follows.
[BCs]
  [./bottom]
    type = DirichletBC
    variable = diffused
    boundary = 'bottom'
    value = 1
  [../]
  [./top]
    type = DirichletBC
    variable = diffused
    boundary = 'top'
    value = 0
  [../]
[]

  • Within each of the two sub-blocks, named "top" and "bottom" by the user, the boundary conditions are linked to the associated variable ("variable = diffused") and boundary.
  • The supplied mesh file, mug.e, prescribes and labels the boundaries "top" and "bottom", these are often numbers depending on how your mesh file was generated.

  • Note, the Neumann boundary condition for this problem is automatically satisfied, thus there is no need to define it. However, non-zero Neumann conditions as well as many others may be defined using existing MOOSE objects (e.g., NeumannBC) or using custom boundary conditions derived from the existing objects within MOOSE.

Executioner

  • The type of problem to solve and the method for solving is defined within the Executioner block.
  • For this problem the type is Steady and the method for solving is the default, Preconditioned Jacobain Free Newton Krylov.
[Executioner]
  type = Steady
  solve_type = 'PJFNK'
[]

Outputs

  • Here two types of outputs are enabled: output to the screen (console) and output to an Exodus II file (exodus).
  • Setting the "file_base" parameter is optional, in this example it forces the output file to be named "out.e" ("e" is the extension used for the Exodus II format).
[Outputs]
  file_base = out
  screen = true
  exodus = true
[]

Running the Problem

  • This example may be run using Peacock or by running the following commands form the command line.
cd ~/projects/moose/examples/ex01_inputfile
make -j8
./ex01-opt -i ex01.i
  • This will generate the results file, out.e, as shown in Figure 2. This file may be viewed using Peacock or an external application that supports the Exodus II format (e.g., Paraview).

results.png
Example 1 Results

Complete Source Files

ex01.i

main.C

ExampleApp.h

ExampleApp.C

Diffusion.h

Diffusion.C

Next Steps

  • Diffusion kernel is the only "physics" in the MOOSE framework
    • A large set of physics is included in the MOOSE modules
  • In order to implement your own physics, you must understand three things: