CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > OpenFOAM > OpenFOAM Post-Processing

A problem on the Energy spectrum plot by using FFT

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

Like Tree1Likes
  • 1 Post By rkc.cfd

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   August 21, 2013, 06:55
Default A problem on the Energy spectrum plot by using FFT
  #1
New Member
 
Join Date: Aug 2011
Posts: 2
Rep Power: 0
Martin duan is on a distinguished road
Hi, guys

I am using the FFT to plot the energy spectrum of the turbulent velocity at the axial direction. but the figure shows that the sidelobes decrease very fast at the high frequencies, just as shown in black rectangular in the attached figure. I spend a lot of time to find out the reason. If you come across such problem before, please give me some clues.

P.S. The precision of the original singular is double precision.

Thanks in advance.
Attached Images
File Type: jpg Picture1.jpg (41.9 KB, 513 views)
Martin duan is offline   Reply With Quote

Old   October 23, 2013, 14:13
Default
  #2
Member
 
Krishna
Join Date: Jun 2012
Posts: 33
Blog Entries: 1
Rep Power: 13
rkc.cfd is on a distinguished road
I have used the Spectrum Function for LES data from the following link and it worked out fine.

Python Scripts for Turbulent Statistics
cfdonline2mohsen likes this.
rkc.cfd is offline   Reply With Quote

Old   February 23, 2014, 05:23
Default
  #3
Member
 
Join Date: Apr 2012
Location: Trivandrum
Posts: 37
Rep Power: 13
toolpost is on a distinguished road
Hello guys!

I am simulating flow through a constricted pipe at Re=1000 using LES. DNS studies available in the literature predicts it as fully turbulent, and I am also getting similar results. I probed velocity at a suitable location and wrote a code in octave to create the spectrum. But I am getting the same trend similar to Martin's case. Towards the higher frequencies, the oscillations in energy are gone, and the curve looks like a straight line. Is this correct? Is there any problem with the simulation? or the fft code is the one to blame? Somebody please give some advice..

Thanks a lot for your time and consideration.
Attached Images
File Type: png sectraW.png (26.8 KB, 264 views)
toolpost is offline   Reply With Quote

Old   April 20, 2014, 11:24
Default
  #4
Senior Member
 
Huang Xianbei
Join Date: Sep 2013
Location: Yangzhou,China
Posts: 302
Rep Power: 13
huangxianbei is on a distinguished road
Quote:
Originally Posted by toolpost View Post
Hello guys!

I am simulating flow through a constricted pipe at Re=1000 using LES. DNS studies available in the literature predicts it as fully turbulent, and I am also getting similar results. I probed velocity at a suitable location and wrote a code in octave to create the spectrum. But I am getting the same trend similar to Martin's case. Towards the higher frequencies, the oscillations in energy are gone, and the curve looks like a straight line. Is this correct? Is there any problem with the simulation? or the fft code is the one to blame? Somebody please give some advice..

Thanks a lot for your time and consideration.
I think this comes from the temporal spectrum, in fact, the smooth curve appears in books and articles is averaged in time. BTW, how is the spectrum obtain? I'm disoriented now
huangxianbei is offline   Reply With Quote

Old   February 22, 2016, 11:11
Default
  #5
New Member
 
Mr.liu
Join Date: Sep 2012
Posts: 27
Rep Power: 13
lx882211 is on a distinguished road
Quote:
Originally Posted by toolpost View Post
Hello guys!

I am simulating flow through a constricted pipe at Re=1000 using LES. DNS studies available in the literature predicts it as fully turbulent, and I am also getting similar results. I probed velocity at a suitable location and wrote a code in octave to create the spectrum. But I am getting the same trend similar to Martin's case. Towards the higher frequencies, the oscillations in energy are gone, and the curve looks like a straight line. Is this correct? Is there any problem with the simulation? or the fft code is the one to blame? Somebody please give some advice..

Thanks a lot for your time and consideration.
Hi, could you tell me how to get the TKE, which parameter should i monitor? Thanks.
lx882211 is offline   Reply With Quote

Old   February 22, 2016, 11:12
Default
  #6
New Member
 
Mr.liu
Join Date: Sep 2012
Posts: 27
Rep Power: 13
lx882211 is on a distinguished road
Quote:
Originally Posted by Martin duan View Post
Hi, guys

I am using the FFT to plot the energy spectrum of the turbulent velocity at the axial direction. but the figure shows that the sidelobes decrease very fast at the high frequencies, just as shown in black rectangular in the attached figure. I spend a lot of time to find out the reason. If you come across such problem before, please give me some clues.

P.S. The precision of the original singular is double precision.

Thanks in advance.
Hi, could you tell me how to get the TKE, which parameter should i monitor? Thanks.
lx882211 is offline   Reply With Quote

Reply

Tags
double precision, energy spectrum, fft, sidelobes

Thread Tools Search this Thread
Search this Thread:

Advanced Search
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 Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
energy problem dldl FLUENT 2 February 6, 2011 08:02
LES: Energy spectrum Hummi Fidelity CFD 0 June 16, 2008 17:21
Energy spectrum LES Hummi Phoenics 0 June 16, 2008 17:18
Energy Spectrum Emad Khalifa Main CFD Forum 3 June 30, 2003 17:03
Why FVM for high-Re flows? Zhong Lei Main CFD Forum 23 May 14, 1999 14:22


All times are GMT -4. The time now is 07:01.