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

high frequency errors

Register Blogs Community New Posts Updated Threads Search

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   May 28, 2008, 00:51
Default high frequency errors
  #1
sudhakar
Guest
 
Posts: n/a
hi, I have simulated the flow field over oscillating circular cylinder in free stream flow. The problem is I am getting high frequency oscillations in my cd plot. Can any one tell me how to eliminate those errors? Thank you very much..
  Reply With Quote

Old   May 28, 2008, 15:40
Default Re: high frequency errors
  #2
Harish
Guest
 
Posts: n/a
I am not sure of the numerical discretization that you use but since the oscillations are high frequency parasite waves I assume its some form of central differencing. There are two ways to remove the oscillations. The first way is to add dissipation/damping terms(which you can find in any CFD book, jameson dissipation , papers of Tam) to the equations and the second way is to use numerical filtering of the solution( Look for paper by Lele on compact scheme, papers of visbal and gaitonde).
  Reply With Quote

Old   May 29, 2008, 01:30
Default Re: high frequency errors
  #3
sudhakar
Guest
 
Posts: n/a
hi harish, Thanks for your kind reply. I am discretizing convective terms in conservative form using central diff scheme; because if I use upwind schemes, I am getting higher drag value. I will try use numerical filtering. thanks once again..
  Reply With Quote

Old   May 29, 2008, 04:29
Default Re: high frequency errors
  #4
skg
Guest
 
Posts: n/a
Hi sudhakar, You hav'nt written the in which Reynolds number,grid quality(fine or coarse?),frequency etc. you are using. In case of oscillating cylinder in free stream flow the two main parameters influence the flow are i) forcing frequncy of oscillation ii)amplitude of oscillation. The extra frequencies may not come from discretization error as you are using central differencing(CD)+Upwinding(Order?), which helps to dampen the oscillation property of CD. On the other hand this hybrid scheme accuracy of the discretization may bring down to first order. Presence of extra frequency component in lift spectrum carries another meaning of flow instability.
  Reply With Quote

Old   May 29, 2008, 08:49
Default Re: high frequency errors
  #5
sudhakar
Guest
 
Posts: n/a
hi skg, Thanks for your reply. The parameters describing the flow conditions are: oscillation frequency=0.154, amplitude=0.2*diameter and Re=185. Also I am not using hybrid scheme. I use CDS only. In literature, a smooth cd curve is presented, but my Cd vs time curve oscillates with high freq, eventhough the avg cd is predicted with good accuracy. Any suggestions?
  Reply With Quote

Old   May 30, 2008, 01:58
Default Re: high frequency errors
  #6
skg
Guest
 
Posts: n/a
Hi, sudhakar I can't understand ur amplitude parameter. I have assumed that the cylinder is oscillating in sinusoidally. In that case u have to give ur amplitude parameter in terms of angular displacement i.e. in terms of PI(PI=3.14159...). I think u have run ur programme as flow past stationary circular cylinder. Compare ur Cl,Cd values with the standard bench mark results. Also compare ur results varying the grid parameters. CDS may produce oscillation if the grid is too coarse. At the same time CDS in fine uniform grid exibits second order accurate results. How u are calculating the frequencies of Cd curve and how long in non-dimensional time parameter ur programme have been excuted? If every thing is OK, short time execution of programme may furnish some undesired high frquencies.
  Reply With Quote

Old   May 30, 2008, 03:34
Default Re: high frequency errors
  #7
sudhakar
Guest
 
Posts: n/a
hi skg, My cylinder is oscillating in vertical direction sinusoidally (it does not rotate).I run for 20 oscillation cycles. I think I have to reduce my time step. Thanks for your kind reply..
  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
Building OpenFOAM1.7.0 from source ata OpenFOAM Installation 46 March 6, 2022 14:21
Upgraded from Karmic Koala 9.10 to Lucid Lynx10.04.3 bookie56 OpenFOAM Installation 8 August 13, 2011 05:03
IcoFoam parallel woes msrinath80 OpenFOAM Running, Solving & CFD 9 July 22, 2007 03:58
Could anybody help me see this error and give help liugx212 OpenFOAM Running, Solving & CFD 3 January 4, 2006 19:07
Multicomponent fluid Andrea CFX 2 October 11, 2004 06:12


All times are GMT -4. The time now is 23:50.