overflow problem

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

 June 15, 2009, 15:16 overflow problem #1 New Member   Join Date: May 2009 Posts: 10 Rep Power: 10 Hello everyone, At the beginning I want to mention that I have read all history about this error and the article at CFD-Wiki and all advices didn't solve that problem Problem to solve: http://student.agh.edu.pl/~marteusz/problemtosolve.png simple pipe 6mm diameter, 1m long, inlet velocity 825m/s and static relative pressure 5bar here you can see run definition settings: http://student.agh.edu.pl/~marteusz/definition.txt Domain: Ideal Gas Heat Transfer: total energy Turbulence model: K-epsilon BC Inlet - supersonic 825m/s and relative pressure 5bar Outlet - supersonic Initial values, 825m/s and temperature 20C I had changed local timestep from 1E-3 to 1E7 to see if it help, but it doesn't. Residuals target: 1E-05 Can anyone help me to get convergence, I will be very, very glad. Problem can be cause because too small diameter and too big velocity. Thanks, Mateusz Kesek Last edited by Marteusz; June 15, 2009 at 15:46.

 June 15, 2009, 20:23 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,982 Rep Power: 107 Hi, Try using Local Timescale Factor to get the thing started. Once it has converged for a bit using that for a while go back to a physical timescale. Also consider using the high speed numerics option. It is an expert parameter which does a second continuity loop and that occasionally helps with high speed flows. Glenn Horrocks

 June 16, 2009, 03:22 #3 Member   Join Date: Mar 2009 Posts: 44 Rep Power: 10 Actually, the second continuity loop is activated by a separate expert parameter: max continuity loops = 2 High speed numerics (found under compressibility control in the advanced solver control panel) does three other things. Copy-paste from the help: "Firstly, it activates a special type of dissipation at shocks to avoid a transverse shock instability called the carbuncle effect (which may occur if the mesh is finer in the transverse direction than in the flow direction). Secondly, it activates the High Resolution Rhie Chow option to reduce pressure wiggles adjacent to shocks. Finally, for steady state flows, it modifies the default relaxation factors for the advection blend factor and gradients."

 June 16, 2009, 08:01 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,982 Rep Power: 107 Thanks for the correction Timon, it's been a while since I used that option so I forgot the details! Glenn Horrocks

 June 16, 2009, 13:20 #5 New Member   Join Date: May 2009 Posts: 10 Rep Power: 10 I still have this problem, I discovered that If I turn off the turbulance (laminar flow) or increase diameter of pipe the analysis gets convergence. But I need to get convergence to that small tube

 June 17, 2009, 03:21 #6 Member   Join Date: Mar 2009 Posts: 44 Rep Power: 10 Have you tried to initialize your solution with lower velocities, ie. gradually increasing your boundary conditions until you reach the desired values?

 June 17, 2009, 05:49 geometry #7 New Member   Join Date: Jun 2009 Posts: 13 Rep Power: 10 hi everybody,i am just a CFX-beginner,i have some questions about the Geometry,i have to design a Flowchanel!can some body help me please? tanks lot

June 17, 2009, 08:14
#8
Senior Member

George
Join Date: Mar 2009
Location: Birmingham, UK
Posts: 257
Rep Power: 11
Quote:
 Originally Posted by fab hi everybody,i am just a CFX-beginner,i have some questions about the Geometry,i have to design a Flowchanel!can some body help me please? tanks lot
do the tutorials before asking any questions
__________________
Top 4 tips
1. Knowledge is everything and Ignorance is dangerous.
2. Understand your limitations and try to eliminate them.
3. Get yerself a bike and hoon the chuffer. You will soon learn why dogs like to hang their heads out the car window.
4. Please before asking any questions on how to run simulations in CFX, go though all the tutorials

 June 19, 2009, 00:41 #9 Member   LSC Join Date: May 2009 Posts: 58 Rep Power: 10 Hi I do encountered convergence issue quite often (playing with the values for the past month). I realised that using Local timescale indeed helps a lot but for some of my simulations the residual graph is diving smoothly until in the 1e-4 to 1e-5 region it starts to oscillates. I tried to tune the solver fluid and mass relaxation in the expert parameters but it wont help much. Also noticed that physical time scale is much faster but oscillations are often encountered. Are there any best known methods to tackle this issue?

 June 19, 2009, 07:02 #10 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,982 Rep Power: 107

 June 19, 2009, 07:42 #11 Member   LSC Join Date: May 2009 Posts: 58 Rep Power: 10 Hi Glenn, many thanks for the link!

 June 20, 2009, 06:57 #12 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,982 Rep Power: 107 I wrote it too, years ago. Getting lots of questions on "my simulation is not accurate" lately, might write one about how to ensure your simulation is accurate someday soon.

 June 20, 2009, 07:05 #13 Member   LSC Join Date: May 2009 Posts: 58 Rep Power: 10 Hi Glenn, my guess is correct! By the way, I have the beta features for V11 enabled. A lot of nice features. I have tried out the transition Roughness height and I am able to get nice convergence..Understand that the Roughness height for Wall B.C is based on Equivalent Sand Roughness Height and I am wondering what is the relationship for the transition roughness height found in the transition model (with SST) with the equivalent sand roughness height.

 Thread Tools Display Modes Linear Mode

 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 OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post Gianni FLUENT 0 April 5, 2008 10:33 ParodDav CFX 5 April 29, 2007 19:13 Phanindra FLUENT 5 April 17, 2007 09:57 bruno CFX 2 November 26, 2006 17:28 Thomas P. Abraham Main CFD Forum 5 September 8, 1999 14:52

All times are GMT -4. The time now is 00:01.