Low under relaxation factor

swati_mohanty · June 11, 2009, 01:01

Dear all,
i am running a VOF model for air water system and find that the residuals for the continuity equation do not converge using default under relaxation factors. I have changed the under relaxation factor for momentum to 0.01 and find that the convergence is much better. However, is it advisable to use such a low URF.

Thanx in advance

SM

Piotrek · June 11, 2009, 09:13

dear swati,

By lowerign URF to such a low value, you limit any chance for the results to change. It makes in difficlut to estimate if the results are changing and you need considerable number of iteration to pass in order to see any influence.
I suggest lowering UDF gradually. Starting from default value and lowering by about 10-20% once the residuals are stable - not changing.

If you see no change in relevant variables on chosen surface, after a number of iterations you can judge, that the run is converged.

Hope that helps/

June 11, 2009, 01:01	Low under relaxation factor	#1
swati_mohanty New Member sm Join Date: Jun 2009 Posts: 15 Rep Power: 17	Dear all, i am running a VOF model for air water system and find that the residuals for the continuity equation do not converge using default under relaxation factors. I have changed the under relaxation factor for momentum to 0.01 and find that the convergence is much better. However, is it advisable to use such a low URF. Thanx in advance SM

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June 11, 2009, 09:13		#2
Piotrek New Member Join Date: Jun 2009 Posts: 15 Rep Power: 17	dear swati, By lowerign URF to such a low value, you limit any chance for the results to change. It makes in difficlut to estimate if the results are changing and you need considerable number of iteration to pass in order to see any influence. I suggest lowering UDF gradually. Starting from default value and lowering by about 10-20% once the residuals are stable - not changing. If you see no change in relevant variables on chosen surface, after a number of iterations you can judge, that the run is converged. Hope that helps/