CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > CFX

Upwind scheme - convergence problem

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   March 31, 2015, 23:47
Default Upwind scheme - convergence problem
  #1
Senior Member
 
Join Date: Aug 2014
Posts: 160
Rep Power: 12
MissCFD is on a distinguished road
Hello,

I simulate a passage of stator and a passage of rotor for a hydraulic turbine so I use water. I use a mass flow rate condition for inlet and pressure condition for outlet.

I don't understand why when I use an upwind advection scheme I have a wall which take place in outlet and my calculation differ, crash. This doesn't happen when I use a high resolution or blend factor = 1 for advection scheme.

With an upwind advection scheme, I introduce numerical diffusion so my calculation should converge more easily, isn't it ?

What can I do to avoid that a wall which take place in outlet of my domain ? Reduce the time scale ?

Thank you for your help
MissCFD is offline   Reply With Quote

Old   April 1, 2015, 05:23
Default
  #2
Senior Member
 
Thomas MADELEINE
Join Date: Oct 2014
Posts: 126
Rep Power: 12
Thomas MADELEINE is on a distinguished road
For what I understood of the advection scheme :
upwind => blend factor of 0 => first order
HR => blend factor vary between 0.5 and 1 (normally close to 0.9) => relaxation term between first order and second order
blend factor of 1 => second order

The equations will be modeled at each integration by a first/second order function.
first order functions are easier to calculate and converge faster but they are less precise and can't bend so much.
So if you have big gradient the first order will be unable to converge and your simulation will crash.

solution :
stay at an order bigger than 1 (maybe 1.5 => blend factor at 0.5) or refine your mesh.

If I said something wrong please correct me, it is what I understood but I am not 100% sure i get it all.
Thomas MADELEINE is offline   Reply With Quote

Reply

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
Convergence problem in Fluent for quenching process kaeran FLUENT 4 December 1, 2014 03:14
Accuracy problem of HO schemes on unstructured mesh, HO scheme gives 1st order result gemini Main CFD Forum 12 December 27, 2011 22:01
Force can not converge colopolo CFX 13 October 4, 2011 23:03
convergence problem limseokmin FLUENT 3 November 14, 2004 13:43
Help!explicit multistage scheme with upwind scheme D .T. Main CFD Forum 1 June 4, 2003 04:42


All times are GMT -4. The time now is 13:04.