BSA Flow Software Installation and User's Guide - CSI

7.7.11 Data analysis
You should be aware that in this manual there is a distinction between
processing and analysis. Processing is done by hardware, and take place in
the processor, which may be a BSA, a FVA or something else. In the context
of LDA-measurements, the processor receives an analog signal directly from
the photodetector(s), and calculates the corresponding particle velocity for
each Doppler-burst detected. The calculated velocity is sent to a computer
for further analysis along with accompanying information such as particle
arrival and transit time. If the processor used support burst-triggered
sampling of external signals, the data-package sent to the computer may also
include data from external sensors, such as pressure- or temperature-
transducers.
The processing thus focuses on individual particles passing the measuring
volume, while the data analysis work on the measured velocities of several -
usually thousands - of particles. The data analysis performed in the computer
yield mean velocity and other statistical properties, such as RMS-values. If
several velocity components and/or external signals are measured
simultaneously, the analysis may also yield cross moments, allowing for
example the calculation of Reynolds stresses. Finally the analysis may
include calculation of spectrum and correlation, and to obtain results quickly
this can be done using FFT-algorithms.
In LDA there are two major problems faced when making a statistical
analysis of the measurement data; velocity bias and the random arrival of
seeding particles to the measuring volume. While velocity bias is the
predominant problem for simple statistics, such as mean and rms values, the
random sampling is the main problem for statistical quantities that depend on
the timing of events, such as spectrum and correlation functions.
Figure 7-72 illustrates the calculation of moments, correlation and spectra on
the basis of measurements received from the processor.
The velocity data coming from the processor consists of N validated bursts,
collected during the time T, in a flow with the integral time scale τI. For each
burst the arrival time ai and the transit time ti of the seeding particle is
recorded along with the non-cartesian velocity components (ui, vi, wi).
The different topics involved in the analysis will be described in more detail
in the following sections of this manual.
7-94 BSA Flow Software: Reference guide