Residual target for turbulence transport equations

saisanthoshm88 · November 7, 2012, 10:44

The CFX documentation states that the CFX solver internally uses a residual target of 10 times the specified value for the turbulence transport equations..

Could some one please explain the reason for this.

I know that residuals are not the only criteria to judge convergence and that one needs to look up for monitor points, flow imbalances and even see from the post processing if the obtained results make a complete physical sense.

Despite knowing all this I still have this question about the residuals.

My only interest is to know why such relatively large residual target is internally chosen for the turbulence transport equations by the CFX solver.

Could some one please provide an explanation.

ghorrocks · November 7, 2012, 17:40

The turbulence equations do not directly affect the flow. They only have a weak coupling by affecting the viscosity or diffusivity (for 2-eqn models anyway). So as the coupling is weak then convergence can be looser and accuracy will not suffer (for most applications anyway).

saisanthoshm88 · November 8, 2012, 02:03

Many thanks for the clarification glenn

November 7, 2012, 10:44	Residual target for turbulence transport equations	#1
saisanthoshm88 Senior Member --------- Join Date: Oct 2010 Posts: 303 Rep Power: 18	The CFX documentation states that the CFX solver internally uses a residual target of 10 times the specified value for the turbulence transport equations.. Could some one please explain the reason for this. I know that residuals are not the only criteria to judge convergence and that one needs to look up for monitor points, flow imbalances and even see from the post processing if the obtained results make a complete physical sense. Despite knowing all this I still have this question about the residuals. My only interest is to know why such relatively large residual target is internally chosen for the turbulence transport equations by the CFX solver. Could some one please provide an explanation. __________________ Best regards, Santhosh.

November 8, 2012, 02:03		#3
saisanthoshm88 Senior Member --------- Join Date: Oct 2010 Posts: 303 Rep Power: 18	Many thanks for the clarification glenn __________________ Best regards, Santhosh.

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November 7, 2012, 17:40		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	The turbulence equations do not directly affect the flow. They only have a weak coupling by affecting the viscosity or diffusivity (for 2-eqn models anyway). So as the coupling is weak then convergence can be looser and accuracy will not suffer (for most applications anyway).