The significance of coherent flow structures for the turbulent mixing ...

were not considered in detail in the literature [2, 29, 85, 104]. Starting point is the basic problem
associated with the reproducible generation of sufficiently monodisperse liquid particles
with an appropriate size, shape and density such that they follow the macroscopic flow motion
faithfully without disturbing the flow or fluid properties. In the past, very little attention has
been paid to the observation that the particle size distribution generated may strongly deviate
from the desired distribution if the liquid level in the aerosol generator changes, the pressure
varies, or the nozzle holes are contaminated for any reason. This may cause serious problems
as the accuracy of all tracer-based velocity measurement techniques, such as laser Doppler
anemometry (LDA), particle image velocimetry (PIV) or Doppler global velocimetry (DGV),
is ultimately determined by the particle dynamics [3]. Beside the fundamental problems associated
with the generation of appropriate tracer particles, the registration, the storage and
the read-out of the particle images will be investigated in detail in section 2.3. This process is
another key element in PIV as the accuracy of the technique strongly depends on the precision
with which the image displacement can be related to particle locations and their respective
particle displacements [3]. Parts of the results have been already published in [39, 42, 47].
Chapter 3 focuses on the stereoscopic PIV techniques because the conventional PIV system
outlined in chapter 2 is not well suited to examine turbulent flows for two reasons. First,
only the projection of the three-dimensional flow velocity inside the measurement plane can
be determined with the conventional PIV technique. Second, the two velocity components
that can be measured are biased when the flow is three-dimensional because of the perspective
error. In section 3.1 the advantages and drawbacks of various stereoscopic recording configurations
will be compared and the basic principles of different evaluation schemes known
as image warping, vector field warping and interrogation window warping will be discussed
in section 3.2. Of primary interest is the investigation of the different image interpolation
schemes which are required for the analysis of the image pairs, because the measurement error
depends strongly on the interpolation algorithm applied. Another systematic measurement
error that was discovered by applying the stereoscopic PIV is investigated in section 3.3. As
this error can be large relative to all other possible measurement errors a calibration validation
technique is examined which has been developed to detect and compensate this error
completely. The general material of this chapter is mainly based on [40, 42] and the section
about the calibration validation method and the different evaluation schemes can be found
partially in [39, 44].
Chapter 4 describes the principles and properties of the multiplane stereo PIV technique.
This technique was developed to obtain with high accuracy and spatial resolution information
about the spatio-temporal flow unsteadiness and to determine fundamentally important
fluid-mechanical quantities which are presently not accessible with any other known measurement
technique technique. First of all the principles and properties of the techniques will be
outlined along with the required four pulse laser system. Thereafter, the synchronisation and
alignment of the system component will be discussed and different modes of operation will
be outlined which have been applied for the fluid-mechanical investigations in chapter 6 and
7. In section 4.6 and 4.7 polarisation effects and the effect of optical aberrations on the measurement
precision will be studied. In particular, it will be shown how to reduce undesirable
optical aberrations below the resolution of the recording medium and how to eliminate certain
aberrations completely by accepting aberrations of other types which are of no harm in
PIV. To examine the performance and reliability of the multiplane stereo PIV technique, the
acoustic receptivity of a laminar boundary layer along a flat plate with zero pressure gradient
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