Convergence problem
 

Hello Friends,

I was trying to do a steady state simulation of T junction fluid mixing using SST model (Mesh size - 4.5 Million nodes). I used automatic timescale option for both the fluid and solid domains. When I ran the simulation and looked at the out file, I observed that all the equations are converged except the T-Energy equation. I do not understand the reason why this happens. I have attached two pictures of how the RMS residual value kept on increasing as the iterations progressed. Please suggest me as to what is the mistake that I am doing with this simulation. Thank you for your help in advance.

Regards,
Karthick

I should write a FAQ on this, it is a very common question.

This is due to the fluid timescales being much shorter than the solid time scales. So the time step the solver is using for the fluid is far too slow for the solid. You need to do two things to address this:

1) Include imbalances in the convergence criteria. Imbalances are the best way of picking up whether the global conservation is OK, and this is a key issue for CHT simulations where the coupling between the fluid and solid domain is not captured properly by residuals alone.
2) Use a solid time scale factor. And be aggressive - normally factors like 100 or 1000 are used. Feel free to use "Edit run in progress" to adjust this as the simulation progresses as you tune it to a value which converges quickly but not too quick and goes unstable.