, FreeCAD 0.20.1 is out:
It fixes many things in FEM. For example you will now see by highlighting what fluid boundaries are in the constraint. Therefore please upgrade.
The bad news is that I had to revert your patch because it introduced a regression_
there are 2 bodies, a fluid and a solid material. Solid 2 is bound to the fluid while the solid material is not bound to anything. In this case the solid material is automatically bound to the remaining body. Your patch broke the analysis for this case.
The good news is that I could fix it now properly (2 days after the release): git commit b716644e27
Therefore please use the latest patch collection:
For the convergence, for the nonlinear tolerance Elmer's default of 1e-7 is sensible, for the linear tolerance, also the default of 1e-10 is OK. For the linear method BICGstab is slightly faster than BICGstabl but BICGstabl is sometimes better to find a convergence.
The relaxation factor is THE most important parameter.