Bear with me, I'm new to all this!

I have a simple problem, but am having difficulties achieving convergence. I have a two blade paddle in a 1 litre cylindrical tank. The shaft rotates at 200 rpm. I have used the MRF method with the k-epsilon turbulence model. After 3,500 iterations, the simulation appeared to have converged. I then switched to second order discretization and ran another 3,500 iterations. Again I had decent convergence.

However, when I repeated the above using the k-omega model, I got decent convergence after 3,500 iterations, but when I switced to second order discretization and ran another 3,500 iterations, I didn't get a converged solution. I amended the under-relaxation factors for pressure, body forces, turbulence ke and turbulent viscosity, switched to second order discretization and ran another 3,500 iterations, but still I didn't get a converged solution.

What am I doing wrong? Please go easy on me if I'm being daft!

When you said you changed the URF. What exactly did you change them to?

Original URFs Pressure 0.6, Density 1, Body Forces 1, Momentum 0.3, Turbulence KE 0.8, Specific dissipation rate 0.8, Turbulent viscosity 1.

Revised URFs: Pressure 0.4, Density 1, Body Forces 0.5, Momentum 0.3, Turbulence KE 0.3, Specific dissipation rate 0.4, Turbulent viscosity 0.3.

Try this: Pressure:0.7 Momentum:0.3 Density:0.8 BF:0.8 Turb KE:0.6 Diss Rate:0.6 Turb Visc:0.8

Thanks, Bob. I'll give it a go and let you know how I get on.

I have re-run a further 3,500 iterations using the urfs suggested by BOB. For the first 3,500 using first order discretization, I got scaled residuals of below 1e-06 for omega and z velocity, just over 1e-06 for k, x and y, and approx 1e-06.5 for continuity. All were still dropping. However, for the second 3,500 iterations using second order discretization, I got scaled residuals of between 1e-04.5 and 1e-04.8 for all. The residuals reached a minimum after about 6,200 iterations (i.e. 2,700 after switching to second order discretization) and then started to rise.

For information, the 2nd order discretization factors used are as follows: Pressure: PRESTO!, Pressure-velocity coupling: SIMPLE, Momentum: 2nd order upwind, Turb KE: 1st Order upwind, Spec. Dissipation Rate: 1st Order upwind.

Any thoughts / words of advice much appreciated.