BSA Flow Software Installation and User's Guide - CSI

Zero padding
Often the sampling period T is much larger than the integral timescale τI of
the flow, meaning that the true correlation will quickly drop off to “almost
zero” as τ increases. Consequently the error will be small for small lag times,
but it is impossible to obtain reliable correlation estimates for τ>T/2, where
the circular contribution will dominate the estimate.
As depicted in Figure 7-90 errors can however be considerable if the
sampling period T is of the same order of magnitude as the integral time
scale τI, and in general the problem of circular correlation cannot be ignored.
The errors from circular correlation can be avoided completely by the use of
a technique called zero padding: Prior to the FFT-analysis, the sampling
period is artificially doubled by adding a zero signal of duration T
immediately after the sampled signal. The FFT-analysis still assume the
signal to be cyclic, but now the assumed period is 2T, and the signal is zero
half the time as shown in Figure 7-91.
τT
τT) will produce circular correlation due to the assumed cyclic
nature of the sampled signal. This is illustrated in Figure 7-92.
BSA Flow Software:Reference guide 7-135