CFD Online Logo CFD Online URL
Home > Forums > Software User Forums > ANSYS > CFX

Convergence Issues

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

LinkBack Thread Tools Display Modes
Old   November 7, 2013, 16:29
Default Convergence Issues
New Member
Join Date: Oct 2013
Posts: 4
Rep Power: 6
ferocity is on a distinguished road
I've been having some issues getting my model to converge. I am trying to model a podded turbine engine in flight at 35000 ft Mach 0.75, zero angle of attack. The podded engine is essentially a NACA symmetrical airfoil that has been cut in half and spread apart, with a mass flow outlet at the engine face, and a momentum source inlet at the engine nozzle. The momentum source has a higher total temperature, total pressure, and velocity than ambient because the engine is doing work on the flow. The insides of the engine are not being modeled however.

The engine is being modeled as a 2D section, 3 cells wide, about 100k cells in total. Min orthogonal quality is about 0.22 on some inflation layer cells at the leading edge. First layer thickness is about 0.00005 ft to achieve a y+ around 5. Any thinner and the orthogonal quality was very poor. The width of the 2D section has been set to create the necessary area to match the mass flow rate of the engine inlet and nozzle and Mach number from hand calculations.

The model appears to converge well, but will suddenly become unstable and fail. I have tried simplifying the model down to inviscid (by using free slip walls), laminar, upwind advection, etc. The timescale has also been varied from 1e-5s to 1s; the smaller timescale will simply delay the solution becomes unstable. The solution always seems to diverge at about the same number of solutions (~40 for auto timescale factor 1.0). I've attached some screenshots with an auto timescale factor of 0.1.

Could anyone provide insight on where I should begin to look for issues? Thank you very much for your time.
Attached Images
File Type: jpg Capture.jpg (45.9 KB, 10 views)
File Type: jpg Capture2.jpg (34.3 KB, 6 views)
File Type: jpg Capture3.jpg (29.1 KB, 6 views)
ferocity is offline   Reply With Quote

Old   November 7, 2013, 17:21
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 14,309
Rep Power: 110
ghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the rough
This type of effect is usually caused by some weirdness in the flow which convects with the flow and when it hits the exit of the domain goes bezerk. So I would produce a series of results files from before the convergence problem and after it and see if you can spot the weirdness.

And using a laminar flow model does not simplify the simulation, it makes it harder. You need the dissipation of a turbulence model.
ghorrocks 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
Convergence Centurion2011 FLUENT 44 June 20, 2017 09:57
2D Hypersonic Inlet in FLUENT - Convergence Issues Fraisdegout FLUENT 6 December 15, 2016 03:07
Convergence Issues 2D Airfoil AeroEngiNerd STAR-CCM+ 1 February 28, 2012 10:34
Force can not converge colopolo CFX 13 October 4, 2011 22:03
convergence issues on steady state solver icemaniac178 CFX 1 March 30, 2011 19:11

All times are GMT -4. The time now is 03:09.