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

Need help with a compressible flow problem

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

LinkBack Thread Tools Display Modes
Old   August 19, 2008, 13:11
Default Need help with a compressible flow problem
Posts: n/a
Hello Everyone,

I need to model flow / temperature / pressure in a long, sealed off (closed from everywhere), air-containing cylinder. There is a propeller fixed inside the cylinder, which stirs up the air to subsonic (but close to 330 m/s) speeds. Pressure and temperature effects are important, and the flow is modeled as compressible.

The system was meshed in GAMBIT; mesh quality is high ( << 0.7 skewness).

In FLUENT, I tried both Segregated and Coupled solvers in implicit formulation. Enabled Energy equation and then tried Viscous --> Spalart-Allmaras (standard) with default values and vorticity-based production OR Viscous --> k-epsilon (standard) with standard wall functions both times viscous heating was enabled.

To produce compressibility, the fluid was modeled with ideal-gas law.

Problems start when I have to set operating pressure. I am unsure how to define it in a closed, turbulent system. Trying to set it to zero brings many problems - the solution is unsteady (or diverges), even though I tried decreasing the Courant number to 1 as well as decreasing all the URFs and choosing 'First order upwind' in all options. My limits are quite reasonable too.

If I set the operating pressure to 1atm, the solution converges with solution parameters at their default values and 'second order upwind' enabled. That's OK, but I want to be sure in my results... and the 1atm operating pressure makes me uncomfortable. Besides, most residuals converge to below 1e-3, but keep oscillating (for many, many cycles) between 1.5e-3 and say, 9e-4.

Please let me know what do you think; I really appreciate your help.

  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
Problem with compressible flow in Star CD 3.26 suresh Siemens 0 February 17, 2009 02:02
convergence problem with compressible flow S.B.J. Siemens 2 February 2, 2009 05:05
Convergence problem with compressible flow S.B.J. Siemens 2 January 7, 2009 04:29
Convergence problem for compressible flow Saad Main CFD Forum 2 June 5, 2005 15:24
Compressible flow problem Lewis FLUENT 2 July 11, 2004 07:29

All times are GMT -4. The time now is 14:54.