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

Why Relaxation factor (URF) for presure in SIMPLEC is set to 1?

Register Blogs Community New Posts Updated Threads Search

Like Tree1Likes
  • 1 Post By sbaffini

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   June 19, 2021, 04:28
Default Why Relaxation factor (URF) for presure in SIMPLEC is set to 1?
  #1
Senior Member
 
Mandeep Shetty
Join Date: Apr 2016
Posts: 188
Rep Power: 10
granzer is on a distinguished road
simplec_delet.jpg
Hello,
Why is the URF for pressure term, when using SIMPLEC algorithm, is usually set as 1? i.e.Why doesn't the pressure term require under relaxation? Why is it not recommended to change it from its default value? (There is no such recommendation when using the SIMPLE algorithm.)
granzer is offline   Reply With Quote

Old   June 19, 2021, 05:55
Default
  #2
Senior Member
 
sbaffini's Avatar
 
Paolo Lampitella
Join Date: Mar 2009
Location: Italy
Posts: 2,192
Blog Entries: 29
Rep Power: 39
sbaffini will become famous soon enoughsbaffini will become famous soon enough
Send a message via Skype™ to sbaffini
It is very well illustrated in the new edition of the Ferziger et al. book. Basically, the reasoning can be split in two parts:

1) All these methods (SIMPLE, SIMPLEC, PISO, etc.) involve a velocity correction (the term that actually links velocity and pressure) proportional 1/(Ad+Ad*) where Ad is the matrix coefficient on the diagonal and Ad* is the sum over the coefficients of the neighbors. Now, Ad+Ad* might be 0, so here arises the need for velocity under-relaxation, which would make Ad+Ad*>0 no matter what.

2) Now, the main difference between SIMPLE and SIMPLEC is that the former actually has Ad* = 0. So the pressure correction from SIMPLE is proportional to 1/Ad while the one from SIMPLEC is proportional to 1/(Ad+Ad*). As Ad* actually cancels out some or all of Ad, turns out that the amount of pressure correction needed for SIMPLEC is much smaller than what needed from SIMPLE to produce the same velocity correction. So, the latter needs to have the pressure correction reduced with respect to SIMPLEC. Actually, one can show that the pressure relaxation factor to be used in SIMPLE that would match the SIMPLEC behavior is \alpha_p=1-\alpha_u.

So, while this is not, I think, a proper demonstration of why it is needed in SIMPLE, it is quite close to. It is not needed in SIMPLEC (for orthogonal grids) and SIMPLE is made like SIMPLEC by using the value above.
granzer likes this.
sbaffini is offline   Reply With Quote

Reply

Tags
simplec, under relaxation factor, urfs


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
Possible bug with stitchMesh and cyclics in OpenFoam Jack001 OpenFOAM Pre-Processing 0 May 21, 2016 09:00
Ansys SIG$ILL error loth ANSYS 3 December 24, 2015 06:31
[mesh manipulation] mirrorMesh and undoing the joining of patches chegdan OpenFOAM Meshing & Mesh Conversion 3 October 21, 2015 09:09
Help with GNUPlot Renato. Main CFD Forum 6 June 6, 2007 20:51
Problems with installation jonititan OpenFOAM Installation 4 November 6, 2005 05:16


All times are GMT -4. The time now is 08:58.