CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > STAR-CCM+

Star ccm+Unsteady residuals

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Display Modes
Old   July 6, 2017, 05:46
Default Star ccm+Unsteady residuals
  #1
New Member
 
gemxx
Join Date: Feb 2015
Posts: 24
Rep Power: 4
gemxx is on a distinguished road
Hi all,

I simulate the flow around a submerged body for acoustic analysis. The analysis converges during steady state, but the residuals, especially x-y-z momentum residuals, are changing rapidly when the unsteady state analysis starts . I attach some pictures about residuals and physical models which I selected.

Please advise as to what the reason of this change on residuals. What can I do to reduce the residuals at the beginning of the unsteady state ?
Attached Images
File Type: jpg Residuals.jpg (138.1 KB, 50 views)
File Type: jpg Physical Models.jpg (49.1 KB, 25 views)
gemxx is offline   Reply With Quote

Old   July 6, 2017, 11:10
Default
  #2
Senior Member
 
Lane Carasik
Join Date: Aug 2014
Posts: 651
Rep Power: 8
lcarasik is on a distinguished road
Quote:
Originally Posted by gemxx View Post
Hi all,

I simulate the flow around a submerged body for acoustic analysis. The analysis converges during steady state, but the residuals, especially x-y-z momentum residuals, are changing rapidly when the unsteady state analysis starts . I attach some pictures about residuals and physical models which I selected.

Please advise as to what the reason of this change on residuals. What can I do to reduce the residuals at the beginning of the unsteady state ?
This is expected behavior. When running unsteady simulations, your residuals will observe a "saw-tooth" pattern. Just ensure the residuals are dropping significantly between each time step.
lcarasik is offline   Reply With Quote

Old   July 6, 2017, 11:17
Default
  #3
New Member
 
gemxx
Join Date: Feb 2015
Posts: 24
Rep Power: 4
gemxx is on a distinguished road
Quote:
Originally Posted by lcarasik View Post
This is expected behavior. When running unsteady simulations, your residuals will observe a "saw-tooth" pattern. Just ensure the residuals are dropping significantly between each time step.
So, it is necessary more converged residuals at steady state to obtain at least 10^-4 convergence for unsteady state. Right ?
residuals are not dropping significantly between each time step. After 100-200 steps , they remains constant. I think that is the problem.
gemxx is offline   Reply With Quote

Old   July 6, 2017, 11:53
Default
  #4
Senior Member
 
Matt
Join Date: Aug 2014
Posts: 624
Rep Power: 9
fluid23 is on a distinguished road
It could also be a product of your time step. Either too large or too small of a time step and your residuals will fail to converge. You will still see the so called 'sawtooth' pattern but your residual magnitudes should be reducing at least 3 orders of magnitude as well. I suspect if you fix your time step you will get better convergence on residuals.
fluid23 is offline   Reply With Quote

Old   July 10, 2017, 14:13
Default
  #5
New Member
 
Vincent
Join Date: Jul 2017
Posts: 9
Rep Power: 2
Vincent_GDC is on a distinguished road
You should not pay lots of attention to the residuals, I think your residuals looks fine. Its the relative error that is important.
Vincent_GDC is offline   Reply With Quote

Old   July 10, 2017, 15:49
Default
  #6
New Member
 
gemxx
Join Date: Feb 2015
Posts: 24
Rep Power: 4
gemxx is on a distinguished road
Quote:
Originally Posted by MBdonCFD View Post
It could also be a product of your time step. Either too large or too small of a time step and your residuals will fail to converge. You will still see the so called 'sawtooth' pattern but your residual magnitudes should be reducing at least 3 orders of magnitude as well. I suspect if you fix your time step you will get better convergence on residuals.
I tried 10^-6 and 6x10^-5 for acoustic analysis. Do you think that it is too small ? The total time of analysis is 0.12 second
gemxx is offline   Reply With Quote

Old   July 10, 2017, 15:56
Default
  #7
New Member
 
Vincent
Join Date: Jul 2017
Posts: 9
Rep Power: 2
Vincent_GDC is on a distinguished road
Don't focus so much on the residuals. It is better to create reports and monitors, from that you can determine if the results are reasonable. Also in the monitor settings set the setting from auto to off. That gives a better understanding of the residuals.
Vincent_GDC is offline   Reply With Quote

Old   July 11, 2017, 02:34
Default
  #8
New Member
 
gemxx
Join Date: Feb 2015
Posts: 24
Rep Power: 4
gemxx is on a distinguished road
Quote:
Originally Posted by Vincent_GDC View Post
Don't focus so much on the residuals. It is better to create reports and monitors, from that you can determine if the results are reasonable. Also in the monitor settings set the setting from auto to off. That gives a better understanding of the residuals.
Thank you for your advises. I'll set the setting from auto to off in the monitor setting. But still x-y-z momentum is high. I add the last residuals in attachment.

I also wonder the mesh structure is appropriate at the sharp bodies. I added two mesh picture and I marked the complex parts of the geometry. Do you think that the mesh structure is good especially at the marked parts ? How can I generate continuous prism layer layers along the geometry ?
Attached Images
File Type: jpg Force.jpg (119.0 KB, 15 views)
File Type: jpg Mesh 1.jpg (147.9 KB, 18 views)
File Type: jpg Mesh2.jpg (138.0 KB, 18 views)
gemxx is offline   Reply With Quote

Old   July 11, 2017, 02:37
Default
  #9
New Member
 
gemxx
Join Date: Feb 2015
Posts: 24
Rep Power: 4
gemxx is on a distinguished road
Quote:
Originally Posted by gemxx View Post
Thank you for your advises. I'll set the setting from auto to off in the monitor setting. But still x-y-z momentum is high. I add the last residuals in attachment.

I also wonder the mesh structure is appropriate at the sharp bodies. I added two mesh picture and I marked the complex parts of the geometry. Do you think that the mesh structure is good especially at the marked parts ? How can I generate continuous prism layer layers along the geometry ?
I also add the last residuals after I change the setting from auto to off.
Attached Images
File Type: jpg residuals2.jpg (128.8 KB, 12 views)
gemxx is offline   Reply With Quote

Old   July 11, 2017, 03:08
Default
  #10
New Member
 
gemxx
Join Date: Feb 2015
Posts: 24
Rep Power: 4
gemxx is on a distinguished road
Quote:
Originally Posted by MBdonCFD View Post
It could also be a product of your time step. Either too large or too small of a time step and your residuals will fail to converge. You will still see the so called 'sawtooth' pattern but your residual magnitudes should be reducing at least 3 orders of magnitude as well. I suspect if you fix your time step you will get better convergence on residuals.
When I reduce the time step from 10^-5 to 10^-3, I get better convergence on residuals. The Prism layer near wall thickness is 1.9 mm. If I set the time step to 10^-3, the courant number is 1.5x10^-3 . Do you think that the value of CFL is good for density based implicit solver ? I perform an acoustic analyse. If the time step is 10^-3 , the maximum frequency resolution is 100 Hz. It is too small, right ?
gemxx is offline   Reply With Quote

Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Problem in PERIODIC BC in STAR CCM mohammad STAR-CCM+ 7 January 28, 2015 10:38
STAR CCM & WIN 8 pro hyderkhan74 STAR-CCM+ 3 March 5, 2014 05:56
Star CCM 8.02 compatible version of Netbeans trsakthipriya STAR-CCM+ 2 July 10, 2013 05:14
how to open star ccm 6.04 version file in older versions pritam.edke Siemens 2 June 25, 2012 03:14
error in star ccm+ unsteady problem Yue Siemens 1 April 2, 2007 11:15


All times are GMT -4. The time now is 12:06.