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Integral length of turbulence

  1. Aug 22, 2016 #1

    ATY

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    ##\require{color} \color{white} \colorbox{crimson} {Mod Note: to see images in this post displayed correctly, click on Reply and view this in editor}##

    Hey guys, I need your help. It is about the integral length of turbulence flow.
    So we got data from some wind measurement, recorded at 60000Hz.
    My tutor already told me that we should get something like 0.18 m as result.
    We got the formula L = 1/<u^2(x)>*sum(autocorrelation(Data))
    I tried to use
    Code (Text):
    L=sum(xcorr(Data)/max(xcorr(Data)));
    L=L*(1/freq)*meanu/mean(Data.^2);
    (I used (1/freq)*mean) to give "L" a unit)

    I get the correct result of around 0.18 m, but the autocorrelation function is a triangle e8cdd4fbba.png

    So this does not seem to be correct. I tried it also with some other formula which I found around the Internet.

    Code (Text):
    x = [ Data];
    x_pad = [x zeros(size(x))];
    X     = fft(x_pad);
    X_psd = abs(X).^2;
    r_xx = ifft(X_psd);
    r_xxx=r_xx(:,1)
    I still get the same result for the integral length, but this time the function is different.
    1c9e6aa5f5.png

    So...I am not sure about this one. Maybe it is correct, just the way the function is shown is weird.


    I also tried the regular autcorr thing it matlab, which gave me wrong results (or I have simply no clue how to use it correctly) for the length but a good image
    Code (Text):
    sum((autocorr(Data,length(Data)-1))/freq*meanu)
    a7d388be2e.png


    I hope that somebody can give me an advice or a hint where the problem is.
     
  2. jcsd
  3. Aug 27, 2016 #2
    Thanks for the thread! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post? The more details the better.
     
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