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FFT analysis to detect time-averaged and oscillating components of velocity for sinusoidal waveform

Hi everyone,

 

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Hi everyone,

 

I study on time-dependent (sinusoidal waveform) pipe flow and measure the velocity and pressure. The velocity of sinusoidal waveform is composed of two components such as time-averaged velocity, uta and oscillating velocity, uos and is represented with the mathematical formula given as follows:

 

u=uta+uos*(coswt+phaselag) where u is the velocity, uta is the time-averaged component of this velocity, uos is the oscillating component of this velocity, w is the angular frequency and t is the time.

 

So, in order to detect the time-averaged and oscillating components of any velocity for sinusoidal waveform, the waveform is fitted into the above equation. So, uta and uos are detected by using the waveform output and the equation above. This analysis is known as FFT (fast Fourier transform).

 

I want to detct these components using the FFT vis. However, FFT vis in the LabView 2009SP1 only detects the frequency of signal and amplitude. Moreover, I try to detect the uta and uos using the "statistics.vi" express vi. In this express vi, the arithmetic mean value and amplitude can be determined. And, as I understood, the arithmetic mean is equal to uta and the half of the amplitude is uos. But, actually, I do not know that my approach is correct or not. How does this express "statistics.vi" work? using FFT on its base or what?

 

Have anyone known any FFT.vi which determines directly uta and uos using the above equation?

 

I'm looking forward to hearing from you (forum members and application engineers in NI) as soon as possible.

 

Regards,

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Hi,

 

  FFT gives you the frequency the phase and a DC component, so in your case it will give you only:

u=uta+uos*(coswt+phaselag) = (uta+ uos*phaselag)  + uos*coswt so you get:

1) DC =  (uta+ uos*phaselag)

2) w freq. component = uos

 

So to estimate correctly th uta and phaselog you need some more information, or additional analysis technique.

 

MIchael

_________________________________________________________________________________________________
LV 8.2 at Windows & Linux


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Hi mishklyar,

Thanks for your reply. So, what should I do?

 

As you said, I can only determine uos and (uta+uos*phaselag). Is it correct?

 

I try to estimate uta using arithmetic mean of "statistics.vi" . Is it correct?

 

What do you mean for  "2) w freq. component =uos" ?

As I understood, FFT.vi gives the frequency, phase and DC component and you say DC=(uta+uos*phaselag) and frequency*phase=uos. I think the second one is wrong idea.

 

What should I do?

 

Is there anyone who deals with time-dependent signals to estimate the uta and uos components using FFT analysis?

 

Regards,

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Hi,

 

I have a time-dependent signal which is the sinusoidal waveform. I want to detect the time-averaged component of the velocity signal,   uta,    and the oscillating component,    uos.   As can be known, the sinusoidal waveform is modelled by FFT as

 

u=uta + uos*(coswt+phaselag)

 

 

Is there anyone who deals with time-dependent signals to determine the uta and uos components using FFT analysis?

 

Is there any prepared FFT.vi except original FFT.vi of LabView in its library? The FFT.vi in the library does not satisfy my requirements. Because it only gives the frequency, phase and DC of the sinusoidal waveform. However, I want to detect the time-averaged component of the signal, uta, and its oscillating component, uos.

 

What should I do?

 

Please help...

 

 

 

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