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

Convergence problem

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

Reply
 
LinkBack Thread Tools Display Modes
Old   July 20, 2010, 05:31
Default Convergence problem
  #1
Member
 
lalupp
Join Date: Jul 2010
Location: India
Posts: 38
Rep Power: 8
lalupp is on a distinguished road
I am developing a 3d code with SST turbulence model (incompressible,unsteady) which is built based on OPENFVM. My problem is that my k and omega equations are giving convergence for the first time step but diverges in the next iteration onwards (segregated approach).

Is it because of lack of proper updation procedure or
if some modification in the linear equation especially in the source term is necessary for the implementation of wall boundary condition
I rechecked my k and omega equation several times but not giving
My problem is flow past a sphere


but please help
Thanks in advance
lalupp is offline   Reply With Quote

Old   July 21, 2010, 17:01
Default
  #2
Member
 
Join Date: Apr 2009
Posts: 35
Rep Power: 9
artemiss1984 is on a distinguished road
Dear friend your statement is general.
It can occur because of several reasons.

What is is your initialization for k and omega?
What is flow Reynolds number? Probably your simulation has vortex shedding process.
The SST turbulence model uses k-w near the wall and k-e far from the wall.what is your methodology for matching this two turbulence models?
be more specific for your problem.


QUOTE=lalupp;268153]I am developing a 3d code with SST turbulence model (incompressible,unsteady) which is built based on OPENFVM. My problem is that my k and omega equations are giving convergence for the first time step but diverges in the next iteration onwards (segregated approach).

Is it because of lack of proper updation procedure or
if some modification in the linear equation especially in the source term is necessary for the implementation of wall boundary condition
I rechecked my k and omega equation several times but not giving
My problem is flow past a sphere


but please help
Thanks in advance[/QUOTE]
artemiss1984 is offline   Reply With Quote

Old   July 21, 2010, 17:02
Default
  #3
Member
 
Join Date: Apr 2009
Posts: 35
Rep Power: 9
artemiss1984 is on a distinguished road
Dear friend your statement is general.
It can occurs because of several reasons.

What is is your initialization for k and omega?
What is flow Reynolds number? Probably your simulation has vortex shedding process.
The SST turbulence model uses k-w near the wall and k-e far from the wall.what is your methodology for matching this two turbulence models?
tell more detail about your problem.


QUOTE=lalupp;268153]I am developing a 3d code with SST turbulence model (incompressible,unsteady) which is built based on OPENFVM. My problem is that my k and omega equations are giving convergence for the first time step but diverges in the next iteration onwards (segregated approach).

Is it because of lack of proper updation procedure or
if some modification in the linear equation especially in the source term is necessary for the implementation of wall boundary condition
I rechecked my k and omega equation several times but not giving
My problem is flow past a sphere


but please help
Thanks in advance[/QUOTE]
artemiss1984 is offline   Reply With Quote

Old   July 23, 2010, 00:33
Default Re Convergence problem
  #4
Member
 
lalupp
Join Date: Jul 2010
Location: India
Posts: 38
Rep Power: 8
lalupp is on a distinguished road
Thanks
I got my k and omega equation from the following link
http://turbmodels.larc.nasa.gov/sst.html

For near wall , omega was assigned a large value (equation below) and k was set to zero. I used a blending function to blend k-w and k-epsilon equation as per Nasa site (which depends on the distance to wall) . Other boundary conditions are inlet , outlet etc
I checked my code several times but not giving convergence
Re ~= 1e6 , vortex shedding present
Thanks
lalupp is offline   Reply With Quote

Old   July 23, 2010, 06:33
Default
  #5
Member
 
Join Date: Apr 2009
Posts: 35
Rep Power: 9
artemiss1984 is on a distinguished road
Dear friend.

my first suggestion is about your numerical simulation if your code is well prepared.

1.check the CFL condition (the relation between time step and grid cell sizes)
2. what is the properties of fluid? at given Re number use of air with density of 1kg/m3 help to better convergence rather than water with density of 1000 kg/m3.
3.if you are using iterative solution check for the under relaxation values.Don't select high values like usual commercial software(s).I doubt the reality of this great values in commercial software(s).
artemiss1984 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
convergence problem when use pisoFoam, LES for wind tunnel case Forrest_Lei OpenFOAM 3 July 19, 2011 06:00
convergence problem commonyue Main CFD Forum 1 December 1, 2009 04:54
Convergence of CFX field in FSI analysis nasdak CFX 2 June 29, 2009 01:17
3D Fluid Flow Convergence problem Emily FLUENT 2 March 21, 2007 23:18
Non Convergence of 3D Heat transfer cfd problem Balraj Main CFD Forum 3 December 9, 2004 01:24


All times are GMT -4. The time now is 16:32.