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

Unstable Spallart-Allmaras

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

Reply
 
LinkBack Thread Tools Display Modes
Old   October 10, 2012, 16:00
Default Unstable Spallart-Allmaras
  #1
New Member
 
Peter Doings
Join Date: Sep 2012
Location: Portugal
Posts: 4
Rep Power: 4
PeterDoings is on a distinguished road
Hello,
I'm trying to implement the Spalart-Allmaras model in my code, but I ran into some difficulties.
It seems the diffusion coefficient, which depends on the eddy viscosity, makes the solution process very unstable. Atm I'm using a structured grid with Finite Volume discretization and linear interpolation at the boundaries to calculate the fluxes, with GMRES to solve the system of eqs.
Will using a higher order scheme help? Is the use of flux limiters recommended? If so, which one is the best? I plan on applying the method to unstructured grids later on, by the way.

Thank you for your comments.
PeterDoings is offline   Reply With Quote

Old   October 10, 2012, 16:11
Default
  #2
Senior Member
 
Chris DeGroot
Join Date: Nov 2011
Location: Canada
Posts: 387
Rep Power: 6
cdegroot is on a distinguished road
Higher order schemes are almost always more difficult to converge. Don't move on to higher order schemes until you can get a first order upwind scheme to converge. That being said, a flux limiter will be helpful when you are working with the higher order schemes.

Also it sounds like your problem is with diffusion so changing the advection discretization probably will not help. Maybe you need to apply under-relaxation to the eddy viscosity to smooth things out and keep things stable. That is to say, you should blend in some of the previous estimate to prevent the eddy viscosity from changing too abruptly between iterations.
cdegroot is offline   Reply With Quote

Old   October 10, 2012, 21:06
Default
  #3
New Member
 
Peter Doings
Join Date: Sep 2012
Location: Portugal
Posts: 4
Rep Power: 4
PeterDoings is on a distinguished road
Well, I'm already using quite low values for the under-relaxation factor.
Another question: limiting the viscosity to positive values seems to help. Is this frowned upon or too unrealistic to be used in practice?
PeterDoings is offline   Reply With Quote

Old   October 11, 2012, 04:12
Default
  #4
Member
 
SergeAS's Avatar
 
Serge A. Suchkov
Join Date: Oct 2011
Location: Moscow, Russia
Posts: 73
Blog Entries: 5
Rep Power: 5
SergeAS is on a distinguished road
Send a message via Skype™ to SergeAS
Take a look http://turbmodels.larc.nasa.gov/spalart.html especially notes.
For test your implementation take a look http://turbmodels.larc.nasa.gov/flatplate_sa.html
http://turbmodels.larc.nasa.gov/shear_sa.html http://turbmodels.larc.nasa.gov/bump_sa.html http://turbmodels.larc.nasa.gov/bump3d_sa.html
SergeAS 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
Spalart and Allmaras Paper Mohamed Main CFD Forum 7 July 10, 2014 17:39
CFX Spalart Allmaras turbulence models aweizazuji CFX 9 September 24, 2013 11:53
Convergence on unstable steady states ekor FLUENT 0 August 24, 2010 06:50
spalart allmaras paper hafiz Main CFD Forum 1 April 10, 2007 04:47
extracting K and Epsilon from Spalart Allmaras Sundar Main CFD Forum 4 July 31, 2006 03:02


All times are GMT -4. The time now is 08:33.