BSA Flow Software Installation and User's Guide - CSI

To evaluate the new algorithm, comparisons were made with spectra
obtained by constant temperature anemometry (CTA), generally accepted as
the reference measurement method for turbulence spectra, and with the
traditional sample-and-hold algorithm used in the Spectrum Add-on
currently available for Dantec's BSA Flow Software. The measurements
were taken at the same location in the wake of a cylinder in a wind tunnel
producing vortex shedding at a frequency of 1 kHz.
Effect of data rate
Figure 6-58 Data rate 4.6 kHz, 100k samples
Figure 6-59 Data rate 82 kHz, 100k samples
Figure 6-58 and Figure 6-59 show how data rate affects the results. The red
curve is the "correct spectrum" obtained by CTA. The green and blue curves
are the spectra obtained from LDA data using sample and hold and refined
sample and hold, respectively.
Figure 6-58 shows that sample and hold overestimates low frequencies and
falls off with a slope of 1 decade per decade from a frequency of approx. 1
kHz. Adrian an Yao (1987) reported that the cut-off frequency is equal to the
data rate divided by 2π, which corresponds well with the graphs. They
recommended that data rates of 20 times the highest frequency of interest
should be used for S&H, or more than 100 kHz for the present example.
The figure also shows that the refined sample and hold spectrum corresponds
well with the CTA spectrum up to 3 kHz, at which point noise becomes
dominant.
BSA Flow Software:Options and Add-ons 6-63