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Homework Help: Finite element in MATLAB

  1. May 4, 2013 #1
    1. The problem statement, all variables and given/known data
    Consider the problem
    $$-u''\left(x\right) = 1, \;\; 0 < x < 3, \;\; u \left(0\right) = 0, \; -u' \left(3\right) = u\left(3\right)+1.$$
    Formulate a MATLAB code to produce the solution and plot the solution from 0 to 3.

    2. Relevant equations

    3. The attempt at a solution
    Multiply by a function v and integrating from 0 to 3 to get
    $$v \left(3 \right)u \left(3 \right)+v \left(3 \right)+u'\left(0\right)v \left(0 \right)+\int_0^3 v'u' \; dx = \int_0^3 v \;dx$$
    In this case the bilinear form is
    $$a\left(u,v\right) = v \left(3 \right)u \left(3\right)+u'\left(0 \right)v\left(0 \right) + \int_0^3 v'u' \; dx$$
    and the linear functional is
    $$F(v) = \int_0^3 v \; dx - v\left(3\right).$$
    The variational formulation is to find u such that $$a\left(u,v\right) = F(v).$$
    I produced a MATLAB plot when we have the following boundary conditions
    $$u(0) = 0, u(3) = 1$$
    given in the attached file. Note that here we have the full stiffness matrix K, which involves the integrals
    $$\phi_0 \quad \text{and} \quad \phi_n$$
    and is useful for Neumann and Robin BVPs. The actual coefficient matrix A for the Dirichlet BVP is the matrix obtained by deleting the first and last rows and columns of K. The function f is defined
    separately given by f.m. How do I alter the code to incorporate the boundary conditions at x = 3. I know that there must be an extra part in the line K = spdiags... to make the matrix of boundary terms in a(u,v) and we delete the line A = ... and we alter the 3 lines that make the load vector bd = .... given in F(v). Someone please help.

    Attached Files:

    • simpleFEM.m
      File size:
      897 bytes
    • f.m
      File size:
      40 bytes
    Last edited: May 4, 2013
  2. jcsd
  3. May 5, 2013 #2
    I am completely lost.
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