Update parameters in DirichletBC and a nonlinear form

Question

Hello
In my problem I have certain parameters involved in DirichletBC (defined through an Expression) and nonlinear form. In each time step, I have to set new values to these parameters. What is the best way to do this ? Do I have to redefine the DirichletBC and the form ?

Answer 1

Hi,

Let's say your expression for the boundary condition is $c_1 x+ c_2 y$. Where $c_1, c_2$ are your parameters.
You code might look something like this

#initial parameter values
c1 = 1.0; c2 = 1.0;    

#Apply parameters to your expression
f = Expression("{}*x[0] + {}*x[1]".format(c1,c2))
bc = DirichletBC(V, f, u0_boundary)

for i in range(max_iterations):
    #Solve problem with current bc
    <solver>

    #update boundary condition
    c1 = newValue; c2 = newValue;
    f = Expression("{}*x[0] + {}*x[1]".format(c1,c2))
    bc = DirichletBC(V, f, u0_boundary)

Comments

You can also use constants in an Expression which you have to give as parameters. This has the advantage that it does not require recompilation.

Example: e = Expression('a*x[0]', a=42)

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Thanks to sixtysymbols and Christian. I am using Christian's suggestion.