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Old   November 4, 2003, 05:40
Default Convergence Problem
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Johnny B
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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!
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Old   November 4, 2003, 07:36
Default Re: Convergence Problem
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BOB
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When you said you changed the URF. What exactly did you change them to?
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Old   November 4, 2003, 09:28
Default Re: Convergence Problem
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Johnny B
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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.
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Old   November 4, 2003, 10:43
Default Re: Convergence Problem
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BOB
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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
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Old   November 4, 2003, 10:51
Default Re: Convergence Problem
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Johnny B
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Thanks, Bob. I'll give it a go and let you know how I get on.

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Old   November 4, 2003, 17:01
Default Re: Convergence Problem
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johnnyb
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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.
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