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

FFT on flow field to find peaks?

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

Reply
 
LinkBack Thread Tools Display Modes
Old   October 9, 2006, 10:57
Default FFT on flow field to find peaks?
  #1
CFDtoy
Guest
 
Posts: n/a
I have an oscillating fluid flow field and at some distance X from the inlet, I would like to see if the imposed frequency field is still the primary forcing factor. I was thinking to do a FFT on a section downstream. But I do not know the procedure or easy way to carry out this approximation? What do I need to do the FFT?

Can someone comment?

CFDtoy
  Reply With Quote

Old   October 9, 2006, 12:25
Default Re: FFT on flow field to find peaks?
  #2
simon
Guest
 
Posts: n/a
To do the FFT you could do as I do, and use a software based spectrum analyser such as Sigview ( see www.sigview.com ) . Sigview is very low cost ( about $50 I think ) and is well worth it considering it's capabilities. FWIW if you are tempted to use Excel I wouldn't bother - I tried using Excel initially ( creaating an FFT function is very simple ) but quickly found that the restriction on the number of rows in Excel made for a very low number of samples at my frequencies and poor FFT resolution.

I have written a 2D Euler time marching code and like you need to be able to examine the frequency content of the transient flow in my domain due to a periodic forcing function. Basically I write the thermodynamic pressure from my code to file every nth timestep as a text file ( this can be at a given cell, or averaged over a line of constant J say ) to create a pressure time history text file. I can then use Sigview's "import ASCII file" function to read in the time history data and produce a graphical output of the time waveform, then use the FFT function to create a graphical visualiation of the frequency spectrum. Very simple, but useful !

You can also compare multiple waveforms, or spectra as a waterfall plot to examine the transient freqency content say, or to be more useful do a 2 channel comparison in Sigview, so if you had say the time history for your primary forcing factor, and time history for another location downstream you could compare the amplitude attenuation, frequency content, cross phase etc. I have had to do this on rectangular 2D ducts to explore the effect of grid dependency on frequency content, attenuation etc to be sure that the effects I am seeing in my domain ( a gas turbine intake silencer ) are not an artefact of a poor grid resolution - 50 cells/wavelength seems about right.

If you go down the Sigview route and get stuck, I'll be more than happy to help you via this board.

Hope this helps,

Simon.
  Reply With Quote

Old   October 9, 2006, 12:25
Default Re: FFT on flow field to find peaks?
  #3
Joe
Guest
 
Posts: n/a
Matlab has good FFT functions.
  Reply With Quote

Old   October 9, 2006, 17:14
Default Re: FFT on flow field to find peaks?
  #4
sk
Guest
 
Posts: n/a
If we take the pressure-time trace and perform an FFT on it, what are the units for the X axis and Y axis?

Thanks
  Reply With Quote

Old   October 10, 2006, 03:26
Default Re: FFT on flow field to find peaks?
  #5
Steve
Guest
 
Posts: n/a
Nobody yet has told you what you need to be measuring. A straight FFT will tell you nothing about the relationship between the input (imposed frequency field) and the output (section downstream). You need to measure the coherence between them. Simply put, this is a measure of how much of the output signal was created by the input signal.

Any DSP text will describe how to measure this.
  Reply With Quote

Old   October 10, 2006, 08:59
Default Re: FFT on flow field to find peaks? on a profile
  #6
CFDtoy
Guest
 
Posts: n/a
Thanks for your comments. I guess I ll be little bit more specific.

Consider a downstream distance X where I have Ux, Uy, Uz components along radial direction Y. (fixed X, Z)

I have this information at different time intervals t1,t2..etc..upto tN.

I have imposed a frequency in the flow field and expect the Ux, Uy, Uz to fluctuate in this frequency with some noise ofcourse.

Now, I am not sure If i should do an FFt in that location for each point ? or ..

do some kind of FFt for the point say each Uy1, Uy2, Uy3..UyN at different time in that line segment

or should I do an FFt, say ..take a time t1 and use all the information along the y direction from Uy1 to UyN and do some analysis ?

Y direction is taken since it is the profile I want to see how it fluctuates to the imposed oscillation.

Could give me some insight to what I should do to get the frequency hidden in the transient profile across this section ?

Thanks

CFDtoy
  Reply With Quote

Old   October 11, 2006, 08:41
Default Re: FFT on flow field to find peaks? on a profile
  #7
jojo
Guest
 
Posts: n/a
Google 'numerical recipies' on the Internet. They have a nice section on FT.
  Reply With Quote

Old   October 11, 2006, 16:17
Default Re: FFT on flow field to find peaks? on a profile
  #8
CFDtoy
Guest
 
Posts: n/a
Hi Jojo:

Thanks for the reply. I am looking for the procedure to perform FFT's. Like I explained, I have a fixed x,z location and I have say axial velocity component Ux varying in y direction at different time intervals. Should I do a FFT for each point along y or ..take all the points and do FFT..and what is the procedure if either of them is adopted. Thanks for your kind help. Please let me know what you think about it.

CFDtoy
  Reply With Quote

Old   October 13, 2006, 09:13
Default Re: FFT on flow field to find peaks? on a profile
  #9
jojo
Guest
 
Posts: n/a
Do you want to detect harmonics with respect to time and space (in short: a frequency or a wavelength)?
  Reply With Quote

Old   October 13, 2006, 10:27
Default Re: FFT on flow field to find peaks? on a profile
  #10
cfdtoy
Guest
 
Posts: n/a
Frequency
  Reply With Quote

Old   October 13, 2006, 13:48
Default Re: FFT on flow field to find peaks? on a profile
  #11
jojo
Guest
 
Posts: n/a
If you want a frequency, it means your FFT is in time. You can do an FFT by time series. For each point (x,y,z) in your domain, put your value along time in a 2-D array and perfom the FFT on it.
  Reply With Quote

Old   October 17, 2006, 02:09
Default Re: FFT on flow field to find peaks? on a profile
  #12
CFDtoy
Guest
 
Posts: n/a
Hello Jojo:

I think thts what I want to knw. I put each point (x,y,z) in the domain as a function of time right? so, the file can read

time f(X) f(Y) f(Z) t f1x f1y f1z t f2x f2y f2z . . . . . . . . t fnx fny fnz

or

t f(x1) . . . f(xn) . f(y1) . . t f(yn) . f(z1) . . t f(zn)

and do an FFT of this ? is that waht you mean?

CFDtoy
  Reply With Quote

Old   October 17, 2006, 13:15
Default Re: FFT on flow field to find peaks? on a profile
  #13
jojo
Guest
 
Posts: n/a
FFT is done on a function. A function is made of two series: an abscissa and an ordinate. In your case, the abscissa is time and your ordinate is the variable f evolution at the probe point you want. Your file must have the following shape:

time1 f(x,y,z,time1) time2 f(x,y,z,time2) time3 f(x,y,z,time3) ... ...
  Reply With Quote

Old   October 18, 2006, 01:21
Default Re: FFT on flow field to find peaks? on a profile
  #14
CFDtoy
Guest
 
Posts: n/a
Having that file format can I use programs like matlab to get what i want? Could you direct me to the procedure after gettin that file format?

CFDtoy

  Reply With Quote

Old   October 18, 2006, 12:11
Default Re: FFT on flow field to find peaks? on a profile
  #15
jojo
Guest
 
Posts: n/a
I do not know Matlab. I know it has an FFT module.

This file is the only thing you need as an input to any FFT module. Choose the one you want. The user's guide should inform you about the command to read this file, run the FFT operation and graph the result.
  Reply With Quote

Old   May 31, 2011, 05:41
Default
  #16
New Member
 
amir
Join Date: Jan 2011
Posts: 10
Rep Power: 6
amirmohebbi is on a distinguished road
Quote:
Originally Posted by jojo
;46085
FFT is done on a function. A function is made of two series: an abscissa and an ordinate. In your case, the abscissa is time and your ordinate is the variable f evolution at the probe point you want. Your file must have the following shape:

time1 f(x,y,z,time1) time2 f(x,y,z,time2) time3 f(x,y,z,time3) ... ...
dear jojo
how i can make this input file for fft to obtain frequency in one position versus time?
thanks
amirmohebbi is offline   Reply With Quote

Old   May 31, 2011, 05:44
Default create input file in FFT
  #17
New Member
 
amir
Join Date: Jan 2011
Posts: 10
Rep Power: 6
amirmohebbi is on a distinguished road
Quote:
Originally Posted by CFDtoy
;46070
Hello Jojo:

I think thts what I want to knw. I put each point (x,y,z) in the domain as a function of time right? so, the file can read

time f(X) f(Y) f(Z) t f1x f1y f1z t f2x f2y f2z . . . . . . . . t fnx fny fnz

or

t f(x1) . . . f(xn) . f(y1) . . t f(yn) . f(z1) . . t f(zn)

and do an FFT of this ? is that waht you mean?

CFDtoy
hi
how i can create this file?
thanks CFDtoy
amirmohebbi is offline   Reply With Quote

Old   June 7, 2011, 06:34
Default problem by monitor history of flow field ?
  #18
New Member
 
amir
Join Date: Jan 2011
Posts: 10
Rep Power: 6
amirmohebbi is on a distinguished road
Dear jojo and CFDtoy
I easily done this by going to
solver/monitor/surface
and create and select the point that i want time dependent variable(in my case is Y velocity) to minitor its history by time of flow field.
the file update after each time step

BUT the component of y velocity versus flow time is all zero!!!!!!
how i can solve it?

best regards
thanks
amirmohebbi 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
Nonphysical flow field while using coodles solver ankgupta8um OpenFOAM Running, Solving & CFD 5 January 26, 2008 17:54
Creating a mesh of the flow field in GAMBIT Vidya Raja FLUENT 0 October 3, 2005 09:53
External flow field. Moon FLUENT 3 March 5, 2003 11:32
Injection of partical into flow field M.Mahendran FLUENT 2 November 24, 2002 01:54
fluid flow fundas ram Main CFD Forum 5 June 17, 2000 21:31


All times are GMT -4. The time now is 11:13.