dolfin team mailing list archive

[Johan Hake <johan.hake@xxxxxxxxx>]

On Thursday 07 January 2010 09:18:00 Ted Kord wrote:
> New question #96571 on DOLFIN:
> https://answers.launchpad.net/dolfin/+question/96571
> 
> Hi
> 
> I'm attempting to solve the following problem (using the C++ interface):
> 
> du/dt - div grad u = f(u)
> 
> Due to the non-linearity associated with f(u), I am using the parameters at
>  time t to calculate it thus avoiding the use of a nonlinear root-finding
>  algorithm.
> 
> The problem I have is how to define f(u).
> 
> Say, f(u) is:
>       0.1/(1 + exp(u - 2) )
> 
> How do obtain the scalar value 'u' to pass as the argument to f(u) at every
>  time step.

Have you tried to just define a scalar Function or Coefficient (new name) in 
ufl for f(). Then you subclass Expression in C++, something like:

  class F: public Expression
  {
    Function * _u;
  public:
    F:Expression(Function& u):Expression(),_u(&u){}

    void eval(Array<double>& values, const Array<const double>& x) const
    {
      double val;
      _u->eval(val, x);
      values[0] = 0.1/(1+exp(val-2));
    }
  };

  ...

  Function u_0(V);
  F f(u_0);


You should then have the values of the previous function available.

Johan


> Thanks.
> 
> Ted
>