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

divergence problem

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   August 3, 2006, 13:19
Default divergence problem
  #1
vincent
Guest
 
Posts: n/a
hello I have to study the flow around an aircraft at different Mach number, I have created the mesh with gridgen but the simulation with fluent diverge for all my cases!! my simulation is a compressible, inviscid, steady calculation, with pressure far field boundary conditions and wall condition for the aircraft. The mesh has 1.5 million cells with a max aspect ratio of 6.3 and a max volume skewness of 0.93. I have checked my parameters, and the simulation converges for a first order calculation but as soon as I try the second order the calculation diverges. I really need these results for my Msc thesis... thanks vincent
  Reply With Quote

Old   August 3, 2006, 15:35
Default Re: divergence problem
  #2
Jason
Guest
 
Posts: n/a
What Mach numbers are you running? What solver and solver settings are you running?

If the solution is diverging, then there is typically a region where the flow field is falling apart... look at the diverged solution and find that area (look at Max/Min pressures, temperatures, and Mach numbers). In that region is the mesh refined enough to capture gradients (either due to shocks and/or curvature)? Is it diverging in the same area as the maximum skewed cell?

A typical response is to check your under-relaxation factors. That may keep your model from diverging. Personally, I think it's extremely important to understand where and why you're having a problem before trying that, but more and more people seem to jump to playing around with URFs first.

Hope this helps, and good luck, Jason
  Reply With Quote

Old   August 3, 2006, 16:20
Default Re: divergence problem
  #3
vincent
Guest
 
Posts: n/a
thank you for a so quick answer I am using a coupled, implicit 3D solver for Mach number from 0.7 to 1.2 Actually when I look at the part where there is a divergence it seems taht this is always the same place but I have already tried to refine the mesh on that particular place (where there is a strong shock wave) and the result is the same. Furthermore this palce is not the place where there is the maximum skewness About the URFs I do not know how to use it, my divergence seems to be about temperature and pressure. Will it affect the accuracy of my results?
  Reply With Quote

Old   August 3, 2006, 16:44
Default Re: divergence problem
  #4
Jason
Guest
 
Posts: n/a
For a coupled solver, the next thing I would try (after finding the problem and refining the mesh, which you've done) is lowering the Courant Number (CFL number). You can find the CFL and the URFs in the Solution Control window (Solve->Controls->Solution). The default is 5, so I would try dropping it to 1, and if that still doesn't work, 0.1. The key is to slowly raise the CFL number again if the model is behaving (i.e. if the residuals are decreasing).

Another thing you can try is setting the solution limits, so that the pressure and temperature are bounded and don't go off to extremes (the default temperature will go all the way to 1K... I seriously doubt you're modeling anything that gets that low). You need to leave some "slop" in the available range, but bounding the problem will sometimes help keep the solution from diverging. In my experience this is hit-or-miss with the coupled solver (but always helps with the segregated solver) so it's worth a try at least.

Hope this helps, and good luck, Jason
  Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
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 Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Meshing and divergence problem ghost FLUENT 9 February 5, 2010 13:24
Adiabatic and Rotating wall (Convection problem) ParodDav CFX 5 April 29, 2007 20:13
strange divergence when solving multiphase problem tanghao FLUENT 2 July 27, 2006 20:47
divergence problem Ayyappan.T FLUENT 2 May 16, 2005 13:10
help:spectral methods & divergence free functionsn D. Puigjaner Main CFD Forum 1 August 28, 2000 11:06


All times are GMT -4. The time now is 22:18.