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

solution not converging for fine mesh..

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

Reply
 
LinkBack Thread Tools Display Modes
Old   February 1, 2010, 10:18
Post solution not converging for fine mesh..
  #1
New Member
 
saurabh deshpande
Join Date: Jan 2010
Posts: 4
Rep Power: 7
saurabh.deshpande88 is on a distinguished road
Hello everyone,I am doing analysis of flow around a NACA 0008 airfoil. Using gambit and fluent for the purpose.
When i use a coarser mesh (i.e. nodes=1,500) my solution is converging and i am getting the results.
But for a fine mesh (nodes=12,000) even after 1,60,000 iterations my solution is not converging and there is not a slightest sign that it will come down! In fact the "residual vs iterations" curve follows a zig-zag pattern. Why is it so?? please help. thanks..

IMP DATA:

residual: 1e-06

inviscid flow, angle of attack=20 degree

Discretization schemes: pressure= presto
momentum= second order upwind

Pressure velocity coupling: simple

underrelaxation factors have been reduced to the extent of 0.01 for pressure and momentum.
saurabh.deshpande88 is offline   Reply With Quote

Old   February 1, 2010, 16:50
Default
  #2
Senior Member
 
Join Date: Nov 2009
Posts: 411
Rep Power: 10
DoHander is on a distinguished road
Using a small mesh starts to smell like a DNS (Direct Navier Stokes) but if it is 2D and steady is not DNS . The moral is - if you use an extremely fine mesh this must be fine enough for representing the physics of your case. A fine mesh will usually catch small zones of separations and reattachments "invisible" for a coarse mesh - this is probably why you can't obtain a convergent solution (steady ?).

Do you use a turbulence model in your simulation ?

Do
DoHander is offline   Reply With Quote

Old   February 2, 2010, 11:23
Smile
  #3
New Member
 
saurabh deshpande
Join Date: Jan 2010
Posts: 4
Rep Power: 7
saurabh.deshpande88 is on a distinguished road
Hey thanks for the reply.

I got what you mean to say. due to "seperation and reattachment" of flow at micro level velocity changes continiously and solution is not getting converged.

This also explains why solution was converging for lower angles of attack (< 20 degree) for the same fine mesh. seperation starts as angle of attack increases and solution convergence stops.

i am not using a turbulent model. i am a begineer in the field of CFD. i am using inviscid model. should i change to turbulent models?

(But , actually if i am considering inviscid model then viscosity is zero and hence there should be no skin friction, no boundry layer formation and hence no seperation.. then why is it not converging? )
saurabh.deshpande88 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
grid dependancy gueynard a. Main CFD Forum 19 June 27, 2014 21:22
[ICEM] Unstructure Meshing Around Imported Plot3D Structured Mesh ICEM kawamatt2 ANSYS Meshing & Geometry 17 December 20, 2011 12:45
basic of mesh refinement arya CFX 4 June 19, 2007 12:21
Solution mesh independent Armin Gips FLUENT 2 August 13, 2001 10:35
Mesh Mignard FLUENT 2 March 22, 2000 06:12


All times are GMT -4. The time now is 18:08.