Assemble only the time-dependet part of a bilinearfrom in timeloop and add this to a time-independet part (C++)

Question

Hi all,

for time-dependent problems, there is a small PYTHON example (on the fenics webpage http://fenicsproject.org/documentation/tutorial/timedep.html resp. in the file d2_d2D.py) for handling bilinearforms where only the stiffness matrix is time-dependent and the mass matrix does not depend on time. As shown in the example, it is (of course) more efficient to assemble the time-independent part only once before going into the time loop, where the time-dependent part has to be assembled, and add both (assembled) parts together.

My question is now the following: Is there a similar way (or even an example how) to implement such a problem in CPP? To my understanding, I have to create two *.ufl files, one for each part (mass- and stiffness-matrix) of the bilinearform which are added to one matrix. Is this correct? However, I think there is no operation like adding to matrices available (yet), is this right?

Cheers,
mischa

Answer 1

Yes, you need to create two UFL files.

You should be able to add matrices (see `GenericMatrix::operator+=), but I haven't tested it.

You could also copy your 'fixed' matrix, and pass the copy to the assembler and set

assembler.add_values = true;

This way, the assembler will not zero the matrix that you pass in, but will just add to entries.

Comments

Thank you very much!