# Cumulative energy spectrum

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April 3, 2020, 03:53
Cumulative energy spectrum
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Ashish bhigah
Join Date: Jan 2019
Posts: 31
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Hi all,
I am reading the book "Turbulence: The Legacy of A. N. Kolmogorov" and I am facing a doubt. (you can find it here https://epdf.pub/turbulence-the-lega...olmogorov.html)

In this book Energy Spectrum E(f) is introduced as:

E(k)=dε(f)/df

where ε(f) is called "cumulative energy spectrum".

I did not understand why ε(f) is called "cumulative" and how ε(f) can be calculated. At page 53 (see image) of this book there is a definition of this parameter. Is this definition equal to the Fourier transform of the turbulent kinetic specific energy? If not how can I calculate ε(f)??

Attached Images
 page53.PNG (49.0 KB, 20 views)

 April 3, 2020, 13:09 #2 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,685 Rep Power: 66 The velocity has been filtered using a low-pass filter. Therefore, when you take the average of this filterered velocity, it contains only the cumulative energy of the frequency scales up to the cut-off frequency and isn't the infinite-time average kinetic energy at 0 Hz of the unfiltered velocity. Taking this average using a bunch of different cutoff frequecies you then build a distribution at different frequencies which they call a spectrum.

 April 3, 2020, 16:54 #3 Member   Ashish bhigah Join Date: Jan 2019 Posts: 31 Rep Power: 7 But, for istance, if we take the value of the cumulative energy spectrum at the frequency of 50 Hz, the value contains the cumulative energy from 0 up to 50 Hz or just the value at 50 Hz?

 April 3, 2020, 17:29 #4 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,685 Rep Power: 66 ε(50 Hz) contains the cumulative energy from 0 to 50 Hz. E(50 Hz) is the energy density at 50 Hz.

 April 11, 2020, 09:11 #5 Member   Ashish bhigah Join Date: Jan 2019 Posts: 31 Rep Power: 7 Ok thank you Last edited by bhigahAshish; April 14, 2020 at 13:45.

 Tags cumulative, spectrum, turbulence