Ivar, could you please say something about the solver error?

Hi,
I would like to model friction stir welding process. I used general heat transfer module and incompressible navier stokes module together. I do not have any problem with the heat transfer module, when I solve seperately, the heat transfer model gives reasonable results. But when I add the navier stokes model I get an error. NaN or failed to find.....(I can give later on detailed error message)

I checked all the boundary conditions, all of them are consistent with the real world. I added some pressure boundary conditions to ease the numerical calculations.

I think the problematic thing is the way I define the viscosity. The viscosity is a function of flow stress and strain rate. And again the flow stress is a function of temperature and strain rate. So the viscosity changes with the strain rate and temperature.

I ticked higly nonlinear problem alternative, I ticked complex number alternative, I tried several different solvers. But I could not manage to avoid this error.

I looked up for the error code in the manuals. I realized that this error occurs because of the viscosity equation. This equation has first derivatives of velocity components. In the manuals it says either change the expression or the initial values of the velocity components. I did change the initial values, but I got the same result. I also have to use the viscosity expression. What can you recommend? What can I do to eliminate this error.

Ivar, could you please say something?

I want to continue to have fun comsoling...but it is hard to do it now.:)

Atilla.