Check viscous dissipation formula

[geodynamics]

Check viscous dissipation formula

[bangerth]

The code happens to be correct for the incompressible case, but is surely wrong for the compressible case.

[jdannberg]

Okay, I just had a look at the equation in Schubert, Turcotte & Olson, and I think equation (3) in the manual is correct even for the compressible case, but not for non-Newtonian fluids. In the book (part 1, page 202, equation 6.9.3), the shear heating term is written as \Phi = sigma_i,j * du_i / dx_j. For compressible Newtonian fluids we should have the correct formulation exactly as written in eq. (6.9.4) with bulk viscosity = 0.
However, it would be nice to have some more documentation about how this term is implemented (similar to the Compressible formula section in the manual). From the comments in assembly.cc, it was not clear for me why we have to replace div u...
In addition, we should think about what to do for the non-Newtonian case...

[jdannberg]

This should also be fixed now by #498, right?

[gassmoeller]

This is fixed with the new heating formulation.