CFD Online URL
[Sponsors]
Home > Forums > CFX

overflow problem

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

Reply
 
LinkBack Thread Tools Display Modes
Old   June 15, 2009, 16:16
Default overflow problem
  #1
New Member
 
Join Date: May 2009
Posts: 10
Rep Power: 7
Marteusz is on a distinguished road
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 16:46.
Marteusz is offline   Reply With Quote

Old   June 15, 2009, 21:23
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 9,881
Rep Power: 78
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
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
ghorrocks is online now   Reply With Quote

Old   June 16, 2009, 04:22
Default
  #3
Member
 
Join Date: Mar 2009
Posts: 44
Rep Power: 7
Timon is on a distinguished road
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."
Timon is offline   Reply With Quote

Old   June 16, 2009, 09:01
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 9,881
Rep Power: 78
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
Thanks for the correction Timon, it's been a while since I used that option so I forgot the details!

Glenn Horrocks
ghorrocks is online now   Reply With Quote

Old   June 16, 2009, 14:20
Default
  #5
New Member
 
Join Date: May 2009
Posts: 10
Rep Power: 7
Marteusz is on a distinguished road
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
Marteusz is offline   Reply With Quote

Old   June 17, 2009, 04:21
Default
  #6
Member
 
Join Date: Mar 2009
Posts: 44
Rep Power: 7
Timon is on a distinguished road
Have you tried to initialize your solution with lower velocities, ie. gradually increasing your boundary conditions until you reach the desired values?
Timon is offline   Reply With Quote

Old   June 17, 2009, 06:49
Default geometry
  #7
fab
New Member
 
Join Date: Jun 2009
Posts: 13
Rep Power: 7
fab is on a distinguished road
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
fab is offline   Reply With Quote

Old   June 17, 2009, 09:14
Default
  #8
Senior Member
 
ckleanth's Avatar
 
George
Join Date: Mar 2009
Location: Birmingham, UK
Posts: 257
Rep Power: 8
ckleanth is on a distinguished road
Quote:
Originally Posted by fab View Post
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
ckleanth is offline   Reply With Quote

Old   June 19, 2009, 01:41
Default
  #9
LSC
Member
 
LSC
Join Date: May 2009
Posts: 58
Rep Power: 7
LSC is on a distinguished road
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?
LSC is offline   Reply With Quote

Old   June 19, 2009, 08:02
Default
  #10
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 9,881
Rep Power: 78
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
http://www.cfd-online.com/Wiki/Ansys...gence_criteria
ghorrocks is online now   Reply With Quote

Old   June 19, 2009, 08:42
Default
  #11
LSC
Member
 
LSC
Join Date: May 2009
Posts: 58
Rep Power: 7
LSC is on a distinguished road
Hi Glenn,

many thanks for the link!
LSC is offline   Reply With Quote

Old   June 20, 2009, 07:57
Default
  #12
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 9,881
Rep Power: 78
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
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.
ghorrocks is online now   Reply With Quote

Old   June 20, 2009, 08:05
Default
  #13
LSC
Member
 
LSC
Join Date: May 2009
Posts: 58
Rep Power: 7
LSC is on a distinguished road
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.
LSC is offline   Reply With Quote

Reply

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
Incoherent problem table in hollow-fiber spinning Gianni FLUENT 0 April 5, 2008 11:33
Adiabatic and Rotating wall (Convection problem) ParodDav CFX 5 April 29, 2007 20:13
Problem in Tutorial problem of fluent Phanindra FLUENT 5 April 17, 2007 10:57
overflow problem bruno CFX 2 November 26, 2006 17:28
Is this problem well posed? Thomas P. Abraham Main CFD Forum 5 September 8, 1999 15:52


All times are GMT -4. The time now is 05:39.