CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > General Forums > Main CFD Forum

Significance of residuals

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   August 16, 2014, 05:54
Default Significance of residuals
  #1
Member
 
Michael Kruger
Join Date: Jul 2014
Location: South Africa
Posts: 38
Rep Power: 8
mich_K is on a distinguished road
I am busy with steady-state simulation of flow over a backward facing step and am interested in the turbulence of the flow.

Although my velocity residuals are coming down nicely, the pressure residual is jumping around erratically and not coming down as I would have hoped.

This made me wonder, if I am only really interested in the velocity field (in order to calculate wall shear stress and skin friction factor), how important is the pressure solution?

i.e. if my pressure residual is high, but my velocity residuals are very low, is my velocity solution still accurate?

Thanks in advance.
__________________
-------------------------------------------------------
Michael
mich_K is offline   Reply With Quote

Old   August 16, 2014, 07:01
Default
  #2
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 5,387
Rep Power: 57
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
you should specify the equation for the pressure you are talking about...Are you solving compressible o incompressible flow model?
FMDenaro is offline   Reply With Quote

Old   August 16, 2014, 07:04
Default
  #3
Member
 
Michael Kruger
Join Date: Jul 2014
Location: South Africa
Posts: 38
Rep Power: 8
mich_K is on a distinguished road
I am using simpleFoam, an incompressible steady state solver,

but I meant the question more in a general sense than to my specific problem.
__________________
-------------------------------------------------------
Michael
mich_K is offline   Reply With Quote

Old   August 16, 2014, 07:16
Default
  #4
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 5,387
Rep Power: 57
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
for steady state equations, the residual can be seen as the time derivative of the variable you want to solve. A non-vanishing residual is a unbalanced term you can see as a relevant time effect.

But, for the pressure equation in incompressible flow you have to drive the residual to a very small value. It is a Lagrangian-type constraint as ensures that your velocity field is divergence-free. Therefore, a meaningful residual in pressure equation strongly affects the velocity!
FMDenaro is offline   Reply With Quote

Old   August 16, 2014, 07:23
Default
  #5
Member
 
Michael Kruger
Join Date: Jul 2014
Location: South Africa
Posts: 38
Rep Power: 8
mich_K is on a distinguished road
Great thank you very much FMDenaro.
__________________
-------------------------------------------------------
Michael
mich_K is offline   Reply With Quote

Reply

Tags
open foam, residuals, simplefoam

Thread Tools Search this Thread
Search this Thread:

Advanced Search
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 Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
[Tutorials] Tutorial of how to plot residuals ! wolle1982 OpenFOAM Community Contributions 161 August 19, 2020 21:59
motorBike Residuals for SST k-omega... and mine JR22 OpenFOAM Running, Solving & CFD 6 August 1, 2013 09:08
judging convergence through residuals MachZero Main CFD Forum 7 December 25, 2012 12:18
Strange residuals of the Density Based Solver Pat84 FLUENT 0 October 22, 2012 15:59
Convergence - scaled vs unscaled residuals HS FLUENT 1 November 7, 2005 05:45


All times are GMT -4. The time now is 23:14.