BSA Flow Software Installation and User's Guide - CSI

6.4.2 Advanced Spectrum object
Load block of data
More
data?
Autocorrelation
Model based noise
reduction
Inverse FFT
The Advanced spectrum object is similar to the Spectrum object, but uses a
refined signal reconstruction method.
Comparisons with hot-wire measurements have shown that the Advanced
Spectrum processing method produces spectra which resemble spectra
measured with a hot-wire anemometer better than spectra calculated with the
Spectrum object.
Spectra from the Spectrum object fall off from a frequency of [mean data
rate/2π], due to the particle rate filter effect, as explained in the previous
section.
Spectra from Advanced Spectrum typically start falling off at a higher
frequency, and the spectral density is lower at low frequencies.
The Advanced Spectrum calculation produces spectra with more variance
than Spectrum. To get smooth looking results, more samples are required,
typically more than 100.000 samples. This requires a powerful PC with a
large memory, min. 128 MB, preferably more.
The algorithm consists of four parts: resampling of the randomly distributed
data, spectral processing, block averaging, and noise reduction. The routine
is illustrated in more detail below:
Prefiltering
Autocorrelation
Refinement
Inverse FFT
Block averaging
R
( M )
k
= ae
−b
k
R
k
c =
c
⎧
= ⎨
⎩
[ ( 2c
+ 1)
R − c(
R + R ) ]
e
( M ) ⎧R
k = 0
0
Rk
= ⎨
⎩ R k ≠ 0
k
K 1
c
−bk
∑ Rke
k = 1 k
a = K 1
c
−2bk
∑ e
k = 1 k
−n&
∆τ
( ) 2
−n&
∆τ
e −1
BSA Flow Software:Options and Add-ons 6-61
k
R
k
k −1
k + 1
k = 0
k ≠ 0