Convergence: Effect of residual and how to use under relaxation factor

asaha1 · December 14, 2018, 00:43

Hello,
I am new in CFD. I have read a few articles about the convergence. But I have some questions:
1) If I get my required convergence (e-5 in residual value) by changing the under-relaxation factor, is it mandatory to revert back the factor to its original value and checking whether the residual value is increasing or not? Or should I not bother about again reverting back and be happy with the converged solution..!!
2) Is there any procedure or set of procedures to get fully steady residual value..!!

Thanks in advance.
Regards.

Chandrachur · December 22, 2018, 04:23

You need not to change the URF. You can change the Residual values for your solution accuracy. If you are facing any problems (like Divergence in AMG etc.) then you can change the URF.

asaha1 · December 23, 2018, 07:31

Thank You for your response.. I have one more question..
Can we impose both velocity and pressure at the inlet for an incompressible pipe flow in fluent..! If yes then how..? Because I tried this so much but all my efforts went in vein..

Chandrachur · December 23, 2018, 07:51

If your flow is incompressible then please give the boundary condition as Velocity Inlet and Outflow boundary condition or else pressure outlet boundary condition. Another thing is you have to use either pressure inlet or velocity inlet. If you are using any of them, it will calculate based on other parameters

Sent from my BKL-L09 using CFD Online Forum mobile app

asaha1 · December 23, 2018, 08:05

Yes, I am giving velocity inlet but there is an option too for giving initial pressure inlet. But fluent is not taking my input pressure as inlet boundary... And it is calculating the pressure may be through iterative manner.. which is not satisfying my pressure condition... Though the pressure profile is not my prime concern and it is giving quiet good velocity contour... So can I ignore that or it is actually wrong what I am getting...!!

Chandrachur · December 24, 2018, 05:44

Quote:

Originally Posted by asaha1

Yes, I am giving velocity inlet but there is an option too for giving initial pressure inlet. But fluent is not taking my input pressure as inlet boundary... And it is calculating the pressure may be through iterative manner.. which is not satisfying my pressure condition... Though the pressure profile is not my prime concern and it is giving quiet good velocity contour... So can I ignore that or it is actually wrong what I am getting...!!

The option "Supersonic/Initial Gauge Pressure" in the velocity inlet window should be neglected in case of subsonic flow. You should give the velocity inlet value and put zero in the initial gauge pressure in case of subsonic flow

asaha1 · December 24, 2018, 06:44

Okay.. I will totally neglect it.. Thank You for your support..

December 14, 2018, 00:43	Convergence: Effect of residual and how to use under relaxation factor	#1
asaha1 New Member Argha Join Date: Dec 2018 Posts: 8 Rep Power: 7	Hello, I am new in CFD. I have read a few articles about the convergence. But I have some questions: 1) If I get my required convergence (e-5 in residual value) by changing the under-relaxation factor, is it mandatory to revert back the factor to its original value and checking whether the residual value is increasing or not? Or should I not bother about again reverting back and be happy with the converged solution..!! 2) Is there any procedure or set of procedures to get fully steady residual value..!! Thanks in advance. Regards.

December 22, 2018, 04:23		#2
Chandrachur New Member Chandrachur Chatterjee Join Date: Dec 2018 Posts: 4 Rep Power: 7	You need not to change the URF. You can change the Residual values for your solution accuracy. If you are facing any problems (like Divergence in AMG etc.) then you can change the URF.

December 23, 2018, 07:31		#3
asaha1 New Member Argha Join Date: Dec 2018 Posts: 8 Rep Power: 7	Thank You for your response.. I have one more question.. Can we impose both velocity and pressure at the inlet for an incompressible pipe flow in fluent..! If yes then how..? Because I tried this so much but all my efforts went in vein..

December 23, 2018, 07:51		#4
Chandrachur New Member Chandrachur Chatterjee Join Date: Dec 2018 Posts: 4 Rep Power: 7	If your flow is incompressible then please give the boundary condition as Velocity Inlet and Outflow boundary condition or else pressure outlet boundary condition. Another thing is you have to use either pressure inlet or velocity inlet. If you are using any of them, it will calculate based on other parameters Sent from my BKL-L09 using CFD Online Forum mobile app

December 23, 2018, 08:05		#5
asaha1 New Member Argha Join Date: Dec 2018 Posts: 8 Rep Power: 7	Yes, I am giving velocity inlet but there is an option too for giving initial pressure inlet. But fluent is not taking my input pressure as inlet boundary... And it is calculating the pressure may be through iterative manner.. which is not satisfying my pressure condition... Though the pressure profile is not my prime concern and it is giving quiet good velocity contour... So can I ignore that or it is actually wrong what I am getting...!!

December 24, 2018, 06:44		#7
asaha1 New Member Argha Join Date: Dec 2018 Posts: 8 Rep Power: 7	Okay.. I will totally neglect it.. Thank You for your support..