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[Question #121743]: How to specify a variable coefficient tensor

 

New question #121743 on DOLFIN:
https://answers.launchpad.net/dolfin/+question/121743

Hi

I'd like to specify a variable coefficient tensor, but not in the ufl form file. A snippet of the ufl form file is:

==
D = TensorConstant("triangle")

....

a = u*v*dx + dt*theta*inner( D*grad(u), grad(v) )*dx
L = u_old*v*dx - dt*(1-theta) * inner( D*grad(u_old), grad(v) )*dx + dt*theta*f*v*dx
==

D is a 2x2 tensor which I construct from main.cpp in a subclass of expression. So, I have:


class VariableCoefficientTensor : public Expression {
public:
    ....
    void eval(Array<double>& values, const Array<double>& x) const {
        ...
        values[0] = some_value;
        values[1] = some_value_too
};

int main()
{
        ...
    	VariableCoefficientTensor D;
	a.D = D;
        ...
}
==

When I execute the program, I get:

terminate called after throwing an instance of 'std::runtime_error'
  what():  *** Error: Invalid value rank for coefficient 0, got 0 but expecting 2. Did you forget to specify the value rank correctly in an Expression sub class?


What's the correct way to go about this?

Thanks

Jack


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