BSA Flow Software Installation and User's Guide - CSI

The calculation of ε fails if Umean is 0, so this criterion should only be used in
experiments where 0 mean velocity is not possible. In case of 2D
measurements with the LDA1 component aligned with the main flow
direction, the LDA2 component can easily have 0 mean velocity. In that
case, LDA1 should be chosen as Type.
A safer choice of Type is the RMS velocity, which would fail only in case
the RMS velocity is 0. This is hardly ever the case in practice, because the
data will have some scatter.
For the RMS velocity, the equations are:
σ est σ
ε = =
Urms
σ
σu
σ est =
2N
Examples:
est
u
⎫
⎪
⎬
⎪
⎭
ε = 1%: N = 5000
ε = 5%: N = 200
1
⇒ ε = ⇒ N =
2N
1
ε
2 2
In the past, the measurement time in each position had to be chosen such that
sufficient samples were collected in case of high turbulence. In positions
with low turbulence, the measurement time could be far longer than
necessary.
Figure 5-70 Properties of the Confidence stop criteria
Type can be Mean or RMS velocity for any of the LDA channels. Only one
type can be selected at a time.
For PDA systems, the Type can be the above-mentioned, or the Mean or
RMS particle diameter.
If None is chosen, acquisition stops when either the Elapsed time or Number
of samples stop criterion has been satisfied.
If LDA1-Mean is chosen, acquisition stops when the relative uncertainty of
the LDA1-Mean is smaller than the Convergence Limit.
Similarly, if LDA1-RMS is chosen, acquisition stops when the relative
uncertainty of the LDA1-RMS is smaller than the Convergence Limit.
For further details, please refer to Chapter 7 – Data Analysis – Moments.
BSA Flow Software: Project explorer objects 5-77