Odd residual behaviour for Two Layer K Epsilon
 

Hello,

I am currently pursuing my Master's thesis using CFD-ACE to simulate flow inside finned tubes. As results for realizable K-epsilon with two-layer wall function were way off, I'm currently using the standard "Two Layer K Epsilon" turbulence model.

The results are fine but my residuals always follow a similar behaviour: http://i40.tinypic.com/2ebd345.jpg

After normal behaviour for the first 100-500 iterations, the kinetic energy and afterwards the dissipation rate residuals start rising and fluctuating greatly while the others will no longer drop. Anyone have an idea what might be causing this?

Many thanks in advance.

The residuals fluctuate when the flow encounters the fins. Try similar geometry without fins and you wont see the fluctuations. You can tighten the relaxation factors to decrease the fluctuations. The fluctuations are not harmful. They only delay convergence.