Convergence Problem
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 kepsilon 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 komega 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 underrelaxation 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! 
Re: Convergence Problem
When you said you changed the URF. What exactly did you change them to?

Re: Convergence Problem
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. 
Re: Convergence Problem
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

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

Re: Convergence Problem
I have rerun 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 1e06 for omega and z velocity, just over 1e06 for k, x and y, and approx 1e06.5 for continuity. All were still dropping. However, for the second 3,500 iterations using second order discretization, I got scaled residuals of between 1e04.5 and 1e04.8 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!, Pressurevelocity coupling: SIMPLE, Momentum: 2nd order upwind, Turb KE: 1st Order upwind, Spec. Dissipation Rate: 1st Order upwind. Any thoughts / words of advice much appreciated. 
All times are GMT 4. The time now is 01:59. 