Basis Functions and Shape Functions

  • While the weak form is essentially what you need for adding physics to MOOSE, in traditional finite element software more work is necessary.
  • We need to discretize our weak form and select a set of simple "basis functions" amenable for manipulation by a computer.


Copyright Oden, Becker, Carey 1981

Shape Functions

  • Our discretized expansion of $$$u$$$ takes on the following form: $$$$u \approx u_h = \sum_{j=1}^N u_j \phi_j$$$$

    • The $$$\phi_j$$$ here are called "basis functions"
    • These $$$\phi_j$$$ form the basis for the "trial function", $$$u_h$$$
    • Analogous to the $$$x^n$$$ we used earlier
  • The gradient of $$$u$$$ can be expanded similarly: $$$$\nabla u \approx \nabla u_h = \sum_{j=1}^N u_j \nabla \phi_j$$$$

  • In the Galerkin finite element method, the same basis functions are used for both the trial and test functions: $$$$\psi = \{\phi_i\}_{i=1}^N$$$$
  • Substituting these expansions back into our weak form, we get: $$$$\left(\nabla\psi_i, k\nabla u_h \right) - \langle\psi_i, k\nabla u_h\cdot \hat{n} \rangle + \left(\psi_i, \vec{\beta} \cdot \nabla u_h\right) - \left(\psi_i, f\right) = 0, \quad i=1,\ldots,N$$$$

  • The left-hand side of the equation above is what we generally refer to as the $$$i^{th}$$$ component of our "Residual Vector" and write as $$$R_i(u_h)$$$.

  • Shape Functions are the functions that get multiplied by coefficients and summed to form the solution.
  • Individual shape functions are restrictions of the global basis functions to individual elements.
  • They are analogous to the $$$x^n$$$ functions from polynomial fitting (in fact, you can use those as shape functions).
  • Typical shape function families: Lagrange, Hermite, Hierarchic, Monomial, Clough-Toucher
    • MOOSE has support for all of these.
  • Lagrange shape functions are the most common.
    • They are interpolary at the nodes, i.e., the coefficients correspond to the values of the functions at the nodes.

Example 1D Shape Functions

Linear Lagrange

Quadratic Lagrange

Cubic Lagrange

Cubic Hermite

2D Lagrange Shape Functions

Example bi-quadratic basis functions defined on the Quad9 element:

  • $$$\psi_0$$$ is associated to a "corner" node, it is zero on the opposite edges.
  • $$$\psi_4$$$ is associated to a "mid-edge" node, it is zero on all other edges.
  • $$$\psi_8$$$ is associated to the "center" node, it is symmetric and $$$\geq 0$$$ on the element.