# FFT for Post Simulation Analysis

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 May 31, 2011, 04:59 FFT for Post Simulation Analysis #1 Member   Faisal Durrani Join Date: Nov 2009 Location: England Posts: 62 Rep Power: 15 Hello Guys, Im looking at using LES to look at plumes in a naturally ventilated building. I have set up statistical results for the velocity, temperature and pressure variables in cfx-pre and run the simulation for which I now have the results. I need to now run an FFT analysis on the simulation results I have got. I do have matlab to help out with the FFT. But I need the help of you experts out there to tell how to do this. It would be really great if you could tell me in steps how to take things from cfx-post (for example), to matlab and then run the FFT. Thank you so very much

 May 31, 2011, 16:20 @ faisal #2 New Member     Mayank Ojha Join Date: May 2011 Posts: 22 Rep Power: 14 I ran a FFT using MATLAB to find the frequency of vortex shedding for a flow over a bluff body. This is what I did. Once you get your values of your data wrt time, export everything to EXCEL and make MATLAB read the EXCEL file to get the FFT graph of Power spectrum vs Freq. You can use the readxlsx command in MATLAB to read all your data values. Matlab will read all the data values from the EXCEL file into an ARRAY and then use the FFT function in MATLAB to calculate the Frequency. Hope this helped. openfoam_aero likes this.

June 7, 2011, 07:03
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Faisal Durrani
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Quote:
 Originally Posted by ojha.mayank485 I ran a FFT using MATLAB to find the frequency of vortex shedding for a flow over a bluff body. This is what I did. Once you get your values of your data wrt time, export everything to EXCEL and make MATLAB read the EXCEL file to get the FFT graph of Power spectrum vs Freq. You can use the readxlsx command in MATLAB to read all your data values. Matlab will read all the data values from the EXCEL file into an ARRAY and then use the FFT function in MATLAB to calculate the Frequency. Hope this helped.
Hey buddy thanks for the reply. I am also looking at vortex shedding myself. I would love to see your work if its published or your theses if you have written one. I have some questions I would like you to answer please. I am thinking of plotting velocity vs. time to get the graph. This will be done at a specific point right with the help of monitors right? And one i plot this as a chart in post then I will export the chart to excel, am I correct? Thanks for the help

June 7, 2011, 17:57
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Mayank Ojha
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Quote:
 Originally Posted by faisal_durr Hey buddy thanks for the reply. I am also looking at vortex shedding myself. I would love to see your work if its published or your theses if you have written one. I have some questions I would like you to answer please. I am thinking of plotting velocity vs. time to get the graph. This will be done at a specific point right with the help of monitors right? And one i plot this as a chart in post then I will export the chart to excel, am I correct? Thanks for the help
Quote:
 I would love to see your work if its published or your theses if you have written one
I do not have anything published results yet. I have a few stuff remaining to do.

Quote:
 I am thinking of plotting velocity vs. time to get the graph. This will be done at a specific point right with the help of monitors right? And one i plot this as a chart in post then I will export the chart to excel, am I correct?
For me I plotted the forces over the surface but you can certainly do it for a point. YES !!!! Then right click on the plot and export it to EXCEL.

Then write a Matlab script to read values from the EXCEL file that has been created and use the FFT function to get your frequency.

Let me know if you need any more help.

 June 8, 2011, 07:11 #5 Senior Member   omid Join Date: Apr 2011 Posts: 102 Rep Power: 14 Hi. ojha.mayank485 How do you use FFT in MATLAB? can you explain the syntax of this function that you use it? I do it by Freqz function in MATLAB but my result is not reasonable. Best regards

June 8, 2011, 07:13
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omid
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Quote:
 Originally Posted by ojha.mayank485 I do not have anything published results yet. I have a few stuff remaining to do. For me I plotted the forces over the surface but you can certainly do it for a point. YES !!!! Then right click on the plot and export it to EXCEL. Then write a Matlab script to read values from the EXCEL file that has been created and use the FFT function to get your frequency. Let me know if you need any more help.

Hi. ojha.mayank485
How do you use FFT in MATLAB? can you explain the syntax of this function that you use it?
I do it by Freqz function in MATLAB but my result is not reasonable.

Best regards

 June 8, 2011, 12:22 #7 New Member     Mayank Ojha Join Date: May 2011 Posts: 22 Rep Power: 14 Send me an email at ojha.mayank485@gmail.com and I'll send you the MATLAB script and the xlsx file containing the Cl over my bluff body. It is a small code. Just looking at the MATLAB syntax will give you a clear idea of what is happening. I have not heard of the freq function. You should be using the FFT in-build function. Subrahmanyam likes this.

 June 8, 2011, 15:11 #8 Senior Member   omid Join Date: Apr 2011 Posts: 102 Rep Power: 14 Thanks for reply Mayank Ojha Best regards

 August 5, 2011, 19:37 #9 New Member   MG Join Date: Aug 2009 Posts: 7 Rep Power: 15 Hey Mayank, I'm interested in exactly the same issue. Do you think it would be possible to have a look at your Matlab script, as well. Would be great and appreciated. thanks inn advance. matt

September 3, 2011, 16:59
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Quote:
 Originally Posted by omidiut Hi. ojha.mayank485 How do you use FFT in MATLAB? can you explain the syntax of this function that you use it? I do it by Freqz function in MATLAB but my result is not reasonable. Best regards
Hey,

Could you also send me your matlab script? I'm working on the same thing right now. Thanks! tamubruce at gmail.com

 September 5, 2011, 06:38 #11 New Member   Join Date: Jun 2011 Posts: 15 Rep Power: 13 Just to remind you guys that in the CFD-Post you can insert a new Chart XY - Transient or Sequence read data from any monitor point control, file (.csv that you can also export from CFX-Solver Manager Monitor) or time-dependent expression. Then enable Fast Fourier Transform (FFT) and off you go. This is very handy. Good luck.

 September 14, 2011, 06:07 #12 New Member   ya Join Date: Apr 2011 Posts: 7 Rep Power: 14 hello everyone: there are 3 component for velocity field in my results. In matlab, which velocity is used for getting the energy spectrum. maybe, the turbulence kinetic energy should be calculated for the spectrum, isn't it?

 March 13, 2012, 03:35 fft in Matlab #13 New Member   Anthony Join Date: Jul 2011 Posts: 9 Rep Power: 13 Hello guys, I am at the moment writing a code for solving the fluid motion about a 2D airfoil via a Linear-Strength-Vortex Method. I am at the point in which I need to validate my unsteady code and therefore I would like to compare my results with some NACA papers in which the coefficients of the Fourier series are plotted. For example: Cp= Cp0+Cp1s*sin(wt)+Cp1c*cos(wt)+Cp2*sin(2wt+phi) My problem is I don't really understand how to obtain these coefficients from the fft function. What I have is the value of Cp on each panel at each time step: so let's say we have 60 panels and 100 time steps, I have a Cp matrix of Cp(60,100). Applying the fft to this I get again a 60x100 matrix (with complex numbers). I do not know how to use this data. I somewhere found a guid which says the coefficients (a0, a1, b1, a2, b2, ...) are to be calculated via division by N/2... but I still don't understand. Could you help me letting some light in the matter? Thank you very much. A.