CFD Online Logo CFD Online URL
Home > Forums > OpenFOAM

Unstable flow simpleFoam 2nd order

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

LinkBack Thread Tools Display Modes
Old   August 26, 2009, 08:12
Default Unstable flow simpleFoam 2nd order
New Member
Valentin Fischer
Join Date: Aug 2009
Location: Germany
Posts: 9
Rep Power: 7
Valle is on a distinguished road
Hello everyone,

I'm a newbie in OpenFoam calculation, so I try to compare CFX with OpenFoan results in order to delve myself into OpenFoam numerics. For that I choosed a simple geometry and "simple" physics (simpleFoam) to calculate a TT-junction.
The turbulence model is k-epsilon.
The time discretization was fixed to deltaT = 0.001;End Time=10.
All schemes are Gauss linear (corrected for laplacian), except for div(phi,k) > Gauss upwind and div(phi,epsilon) > Gauss upwind;
Tetrahedral mesh.

After calculating and comparing the problem (figures CFX/ OpenFoam), I determined velocity spots in the results of the OpenFoam calculation which don't existed in the CFX calculation. In order to understand this difference between OpenFoam and CFX I changed nearly all parameters in the fvSolution and fvSchemes, but the results were always nearly the same. These spots are always present and I don't really understand why this difference between OpenFoam and CFX.

In my opinion, the velocity spots aren't caused by Postprocessor interpretation, because calculating the problem 1st order, (div(phi,U) > Gauss upwind) no velocity spots could be determined during the complete calculation. Another hint for this discretization problem was the residuum developing (figure Screenshot). When switching to 2nd order div(phi,U) > Gauss linear) the residuum increased and started fluctuating which indicates the 1st/ 2nd order problem in simpleFoam, too. Plotting the results of the 2nd order calculation showed a high unstable flow for a "simple" geometry and physics.
So I tried numerically to stabilize the calculation, but no chance whatever I tried... Should I calculate transient???
It can't be possible that a 2nd order calculation of an incompressible and isothermal problem is so hard to get it converge...

Can someone help me to understand this difference in the results?

Attached Images
File Type: jpg openfoam.jpg (32.4 KB, 34 views)
File Type: jpg cfx.jpg (37.5 KB, 32 views)
File Type: jpg Screenshot.jpg (40.0 KB, 33 views)
Valle is offline   Reply With Quote


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
2nd Order Scheme for SimpleFoam bastil OpenFOAM Running, Solving & CFD 0 February 22, 2009 17:20
2nd order interpolation for NS solver Quarkz Main CFD Forum 6 July 15, 2005 21:50
1st order temporal & 2nd order spatial Prateep Chatterjee FLUENT 0 January 19, 2003 01:31
computation about flow around a yawed cone Tylor Xie Main CFD Forum 0 June 9, 1999 07:33
CFD of turbulent flows raj calay Main CFD Forum 23 April 23, 1999 05:03

All times are GMT -4. The time now is 19:07.