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FFT Of Selection Over Time (Fourier Transform) in ParaView 3.14.1 

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June 6, 2014, 08:27 
FFT Of Selection Over Time (Fourier Transform) in ParaView 3.14.1

#1 
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Mehrez
Join Date: Nov 2012
Location: Bordeaux, France
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Hello,
I'm running a transient simulation of flow around cylinder and storing the pressure and velocity fields in files corresponding to different times. I apply then on the pressure the ParaView filter "FFT Of Selection Over Time". I'm not sure what is this filter doing exactly !? I didn't find deep documentation on the Fourier transform with ParaView. Do you have an idea on how does this filter choose the point on which it applies the FFT ? Any docs are welcome. Thanks Mehrez 

December 16, 2014, 10:39 

#2 
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Bobi
Join Date: Oct 2012
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Greetings Mehrez
I have been through this question as well. At first I thought maybe this filter plots energy spectrum along a specified line. I tried to search some links to find out what does this filter do exactly: http://www.itk.org/Wiki/ParaView/Use...ters#Table_FFT http://comments.gmane.org/gmane.comp...iew.user/22835 The above links are a bit vague. However some interesting points are mentioned. I would be very pleased if somebody describe this filter with an example. Since I have some doubts on the way of using this filter. For instance should we use it on a slice or line or why it does not accept velocity components ( I mean a separated component not a vector array) ? Best, Bobi 

December 16, 2014, 11:40 

#3  
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Mehrez
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Quote:
Hi dear Bobi, I have tried to undertand what is really done in the FFT filter but didn't get any convincing answer. I have written my own python script that reads the OpenFoam files and plots the FFT. Please tell me if you are interested, I can share it. Best regards, Mehrez 

December 16, 2014, 11:52 

#4 
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Bobi
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Greetings Mehrez
Thanks for your reply. In deed, I am interested in deriving Energy Spectrum (1D or 3D) from LES simulations in OpenFOAM. I hope that your python script do that. It would be my very pleasure if you share it with me. Best, Bobi 

December 16, 2014, 12:15 

#5 
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Mehrez
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Here is the python script:
# * coding:Latin1 * from math import * import numpy import re import pylab import math import os import glob # Script to read files of a given variable from an transient OpenFoam case # It performs an fft on the selected variable from the different time fields (variable may be "p" or "U"...) # WARNING ! time steps must be equidistant # MUST BE GIVEN : path and a reference point through which the analysis will be done path='/home/agnaou/Bureau/Square/porosity75%/Re27000/' # path of the OpenFoam case ref_point=5000 # an integer comprised between 0 and the number of mesh cells  1 variable="p" # variable on which the FFT will be performed ################################################## ################################################## ######################################## # Read the pressure fields ################################################## ################################################## ######################################## folders=[x[0] for x in os.walk(path)] # search the different folders contained in the path directory files_number=0 # number of variable fields to be treated or number of time steps min_t=1e+50 # minimum time of the simulation max_t=0 # maximum time of the simulation for i in range (len(folders)): os.chdir(folders[i]) for file in glob.glob(variable): files_number=files_number+1 t=float(folders[i][len(folders[0]):len(folders[i])]) if (t<min_t): min_t=t if (t>max_t): max_t=t time_step=(float(max_tmin_t))/(float(files_number1)) # time step of the simulation VARIABLE=numpy.empty((files_number),dtype='object' ) # fields of the selected variable stored and ordered as a function of time for i in range (len(folders)): os.chdir(folders[i]) for file in glob.glob(variable): TakeValue=open(folders[i]+"/"+variable) Lines=TakeValue.readlines() mesh=int(Lines[20]) t=float(folders[i][len(folders[0]):len(folders[i])]) VARIABLE[int((tmin_t)/time_step)]=numpy.empty((mesh),dtype='object') for j in range (mesh): VARIABLE[int((tmin_t)/time_step)][j]=float(Lines[j+22]) point=numpy.empty((files_number),dtype='float64') for i in range (files_number): point[i]=VARIABLE[i][ref_point] ################################################## ################################################## ######################################## # perform the ffts ################################################## ################################################## ######################################## fft=numpy.fft.fft(point) frequency= numpy.fft.fftfreq(files_number, d=time_step) fftr=abs(fft.real) ffti=abs(fft.imag) fftb=numpy.sqrt(fft.imag**2+fft.real**2) frequency= numpy.fft.fftfreq(files_number, d=time_step) ################################################## ################################################## ######################################## # Plot ################################################## ################################################## ######################################## pylab.subplot(221) pylab.title("Original Data") pylab.grid() pylab.plot(numpy.arange(files_number)*time_step,po int,'r',alpha=1) pylab.xlabel("Time") pylab.ylabel(variable) pylab.subplot(222) pylab.title("Real FFT") pylab.xlabel("Frequency") pylab.ylabel("Power") pylab.grid() #pylab.plot(frequency,fftr,'b',alpha=1) pylab.scatter(frequency,fftr, c='b') pylab.subplot(223) pylab.title("Imaginary FFT") pylab.xlabel("Frequency") pylab.ylabel("Power") pylab.grid() pylab.plot(frequency,ffti,'g',alpha=1) pylab.subplot(224) pylab.title("Real+Imaginary FFT") pylab.xlabel("Frequency") pylab.ylabel("Power") pylab.grid() pylab.plot(frequency,fftb,'k',alpha=1) pylab.show() 

December 16, 2014, 12:25 

#6 
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Bobi
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Greetings Mehrez
Many thanks for sharing your work. I will try it and may contact you in case of questions. Best, Bobi 

December 17, 2014, 03:44 

#7 
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Bobi
Join Date: Oct 2012
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Greetings Mehrez
I have activated Programmable filter on my paraview 3.12.0. Then I copied your script in the empty script space. I modified the path and cell number in your script. However, after applying your Filter nothing happens. Actually, I am interested in energy spectrum along the axis of my case. Is it possible to hint me to efficiently use your Filter? I can send you my case so that probably it would be easier to get help. Best, Bobi Last edited by babakflame; December 18, 2014 at 06:56. 

December 19, 2014, 06:25 

#8  
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Mehrez
Join Date: Nov 2012
Location: Bordeaux, France
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Hi dear Bobi,
You can use the script directly on a python shell without using Paraview. You should have installed the loaded modules (matplotlib...) if it is not done yet. You are right, you have to change the path, put a cell number and put a variable (replace 'p' in the original script by 'k' or 'eps' ...) Please send me your case to take a look. Best regards, Mehrez. Quote:


December 21, 2014, 12:12 

#9 
Senior Member
Bobi
Join Date: Oct 2012
Posts: 286
Rep Power: 5 
Greetings Mehrez
I have installed matplotlib via the instructions in the following link: http://matplotlib.org/users/installing.html However, I have still my problem with using it. Is it possible to give step by step hints to use matplotlib to draw power spectrum with your code? PS: I sent you the case with file2send, For instance I need Power Spectrum for a point in the shear layer. PS: How do you determine cell number in your script? Best, Bobi 

March 6, 2015, 12:01 

#10  
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Mehrez
Join Date: Nov 2012
Location: Bordeaux, France
Posts: 52
Rep Power: 4 
Hi dear Bobi,
I'm sorry to respond so late. The link you sent me by email didn't work (may be because I opened it late :/). I you are still interested, please resend me your files. Hope that my python script helped you. Best regards, Mehrez Quote:


Tags 
fft, filters, fourier transform, paraview 
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