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

Problem with Convergence (Over flow)

Register Blogs Community New Posts Updated Threads Search

Like Tree2Likes
  • 1 Post By ghorrocks
  • 1 Post By ghorrocks

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   July 12, 2014, 11:43
Post Problem with Convergence (Over flow)
  #1
New Member
 
Join Date: Jul 2014
Location: BW, Germany
Posts: 9
Rep Power: 12
armlic is on a distinguished road
Hello everyone.
I´m trying to simulation the flow in a turbine but I get only over flow problems in the solver.

-Inlet stationary domain (A) then to a rotor (rotary domain B) and finally to the outlet which form part of the same domain (A).
-Multiphase transient. 2D analysis (its 3d but only 1 layer depth)
-65,000 nodes

Inlet 1 - velocity (air)
Opening 1 - Opening Pres and Dirn. (0 Pas.) only for air
Outlet 1 - average static pressure

-I have tried countless times to finish a simulation but I always get this Overflow error. I´m simulating with very small time steps equivalent to every 5 degrees of rotation of the rotor. (timestep: .0029 s) and with a total time of 0.21 s which is the time that takes for only 1 revolution.
max 10 iterations, transient scheme: second order backward euler, Advection scheme: High resolution, convergence criteria RMS 1E-4.

- Since the problem requires to have a water level inside the turbine (below the runner) I incorporated an expression for the volume of fraction for the stationay doman:
Waterlevel=-235mm, WaterVolumefraction= if(y<Waterlevel,1,0)

-I have tried with Double Precision/Normal, Parallel(2,4,8,12)/serial, but always around the same time step (65-75) i get the overflow problem.

Any idea what could be the problem??
(I tried the same mesh before with steady state and worked, and before introducing the volume fraction equations; but i don’t see any problem with them)

Thank you very much in advance!
armlic is offline   Reply With Quote

Old   July 13, 2014, 08:36
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,854
Rep Power: 144
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
Have you read the FAQ? http://www.cfd-online.com/Wiki/Ansys...do_about_it.3F

What makes you think a 5 degree rotation is a small timestep? It sounds like a huge one to me. Your time step will need to be much smaller than that.
armlic likes this.
ghorrocks is offline   Reply With Quote

Old   July 14, 2014, 04:26
Default
  #3
New Member
 
Join Date: Jul 2014
Location: BW, Germany
Posts: 9
Rep Power: 12
armlic is on a distinguished road
Hello ghorrocks,

Thank you! I reduced the time step duration from 5degree/interval (70 intervals) to 4 degree/interval (90 intervals) and it converged in every simulation since.
Regards!
armlic is offline   Reply With Quote

Old   July 14, 2014, 04:30
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,854
Rep Power: 144
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
You have reduced your time step size from huge to big.

Have you done a sensitivity study of your time step size? You need to make the time step smaller until your results do not change. I would expect this would give you a time step of more like 0.5 degrees or maybe less if you have a fine mesh.
armlic likes this.
ghorrocks is offline   Reply With Quote

Old   July 14, 2014, 05:48
Default
  #5
New Member
 
Join Date: Jul 2014
Location: BW, Germany
Posts: 9
Rep Power: 12
armlic is on a distinguished road
Hello ghorrocks
My only further plan was to make a Mesh quality study but since you mentioned it, now it makes total sense to include the timestep duration into it. Thank you for your advice.
armlic is offline   Reply With Quote

Reply


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
Convergence Problem icoFoam steady flow over an airfoil Lucas OpenFOAM Running, Solving & CFD 6 July 12, 2018 16:06
Swirl flow convergence problem with simpleFoam iqbalsk8 OpenFOAM Running, Solving & CFD 7 November 28, 2012 01:54
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
Problem with Convergence at high flow rates Syed Siemens 1 April 10, 2007 16:18


All times are GMT -4. The time now is 19:37.