Under Relaxation factor for UDS

arash.mp · January 29, 2019, 14:15

hi everyone,
i have a problem in ANSYS FLUENT V.18
i want to solve a UDS in a domain. i start from an under relaxaition of 0.1 for UDS and in some steps make that to 1. my problem is that for every under relaxation my answers differ from each other! for examlpe when i set UR to 0.2 and LET THE SOLUTION TO CONVERGE (ie. every control point and RESIDUALS are CONSTANT FROM THAT TIME) the answers are COMPLETELY DIFFERENT from the answers i get from UR of 0.2, 0.5 etc. and my results are correct when i set the UR on 1!
please help me.
thank u.

LuckyTran · January 30, 2019, 07:47

Details? Steady? Transient?

What's a control point? Do you monitor the uds per iteration? All the residuals are constant? But the residual for the uds you should be able to see change.

Well of course changing the urf changes the solution. But it is odd that the converged solution is different. Which hints that your solution isn't converged. Which actually hints that you have a numerical oscillation somewhere.

arash.mp · January 30, 2019, 08:11

thanks for your answer,
my simulation is a 2D-Axisymetric-steady.
i track "average UDS magnitude" and "maximum UDS magnitude" (per iteration) on several lines in my zone as control points and all of them converge to a different value for every URF!it means that they become straight lines and don't change anymore! but when i increase the URF they sharply change! until i set the URF to 1 and get the correct value!

LuckyTran · January 30, 2019, 08:50

Just for debugging purposes. Freeze all the equations except the UDS and do the same test. Freezing the other solvers freezes the coupling and dependency of the UDS solution on any of the other equations and let's you confirm that the UDS urf is the issue and not the coupling with other equations (which is always an issue).

arash.mp · January 31, 2019, 03:33

yes i did as u said! first i let the flow equations to be completely converge and residuals dropped to e-10! then i solved only the UDS equation and like before for different URF for UDS i got different answers

January 29, 2019, 14:15	Under Relaxation factor for UDS	#1
arash.mp New Member Join Date: Dec 2018 Posts: 17 Rep Power: 7	hi everyone, i have a problem in ANSYS FLUENT V.18 i want to solve a UDS in a domain. i start from an under relaxaition of 0.1 for UDS and in some steps make that to 1. my problem is that for every under relaxation my answers differ from each other! for examlpe when i set UR to 0.2 and LET THE SOLUTION TO CONVERGE (ie. every control point and RESIDUALS are CONSTANT FROM THAT TIME) the answers are COMPLETELY DIFFERENT from the answers i get from UR of 0.2, 0.5 etc. and my results are correct when i set the UR on 1! please help me. thank u.

January 30, 2019, 07:47		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,682 Rep Power: 66	Details? Steady? Transient? What's a control point? Do you monitor the uds per iteration? All the residuals are constant? But the residual for the uds you should be able to see change. Well of course changing the urf changes the solution. But it is odd that the converged solution is different. Which hints that your solution isn't converged. Which actually hints that you have a numerical oscillation somewhere. arash.mp likes this.

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January 30, 2019, 08:11		#3
arash.mp New Member Join Date: Dec 2018 Posts: 17 Rep Power: 7	thanks for your answer, my simulation is a 2D-Axisymetric-steady. i track "average UDS magnitude" and "maximum UDS magnitude" (per iteration) on several lines in my zone as control points and all of them converge to a different value for every URF!it means that they become straight lines and don't change anymore! but when i increase the URF they sharply change! until i set the URF to 1 and get the correct value!

January 30, 2019, 08:50		#4
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,682 Rep Power: 66	Just for debugging purposes. Freeze all the equations except the UDS and do the same test. Freezing the other solvers freezes the coupling and dependency of the UDS solution on any of the other equations and let's you confirm that the UDS urf is the issue and not the coupling with other equations (which is always an issue).

January 31, 2019, 03:33		#5
arash.mp New Member Join Date: Dec 2018 Posts: 17 Rep Power: 7	yes i did as u said! first i let the flow equations to be completely converge and residuals dropped to e-10! then i solved only the UDS equation and like before for different URF for UDS i got different answers