CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Main CFD Forum

Weird Convergence error

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

Reply
 
LinkBack Thread Tools Display Modes
Old   August 2, 2006, 23:35
Default Weird Convergence error
  #1
Harish
Guest
 
Posts: n/a
I have trying to model the scattering of an acoustic pulse from a cylinder( 2d) using Linearized Euler equation.I use a 6th order compact scheme in space and a 3rd Order runga kutta time stepping.The filtering is a 10th order pade type filter.I was doing the grid convergence studies and faced this weird problem.I'm testing a structured non uniform grid.The boundary conditions on the cylinder surface are non penetration and dp/dn=0.Non reflecting boundary conditions are used at the artificial boundaries.I have validated the numerical schemes and artificial boundary conditions by comparison with benchmark and can say for sure that they might not be the reason.

My results converge to a periodic oscillation after some time intervals but the amplitude of oscillation and phase does not converge.I got a peak pressure amplitude(non dimensional) of 0.7 for a 150*150 grid which changed to 0.9 for a 200*200 grid and decreased to 0.8 for a 250*250 grid and again increased to 0.85 for a 300*300 grid.For the current study using 200*200 grid was sufficient for a polar grid to achieve convergence.

Would like comments from anyone who faced similar problems before.
  Reply With Quote

Old   August 3, 2006, 08:09
Default Re: Weird Convergence error
  #2
Mani
Guest
 
Posts: n/a
amplitude 0.7 ... 0.9 ... 0.8 ... 0.85 ...

I wouldn't be so sure it's not converging based on these numbers. I'd at least try one finer grid. (it's not "monotonically" converging but it doesn't have to.)

This is an unsteady problem, right? What about temporal resolution/convergence? Are you reducing the time step as you increase the grid resolution?
  Reply With Quote

Old   August 3, 2006, 08:59
Default Re: Weird Convergence error
  #3
Harish
Guest
 
Posts: n/a
Yes this is an unsteady problem.The time step is reduced automatically based on the grid spacing.

Regarding the numbers, the solution at each point becomes sinusoidal after the transient pulses have left the computational domain.So if I observe the results at any point in the domain all I would have is the sinusoidal variation in time.The problem which I'm facing is the amplitude of the sinusoidal fluctuation is not converging with grid refinement.The one particular problem I have is a coarse cell near the front and back stagnation points of the cylinder.But I'm not sure if that could be a reason.I will try refining that cell and check if it makes any difference.

  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
Compile problem ivanyao OpenFOAM Running, Solving & CFD 1 October 12, 2012 09:31
How to install CGNS under windows xp? lzgwhy Main CFD Forum 1 January 11, 2011 19:44
Accessing phi from a fvPatchField at same patch johndeas OpenFOAM 1 September 13, 2010 20:23
checking the system setup and Qt version vivek070176 OpenFOAM Installation 22 June 1, 2010 12:34
OF 1.6 | Ubuntu 9.10 (64bit) | GLIBCXX_3.4.11 not found piprus OpenFOAM Installation 22 February 25, 2010 14:43


All times are GMT -4. The time now is 02:07.