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

2.1 Principles
It is of fundamental importance to realize that the particle displacement must be small relative
to the finest flow scales, as only phenomena that occur over a time interval which is longer
than
mand that have a spatial extent larger than the absolute displacement can
be resolved, but the particle image displacement , on the other hand, must be large for
accurate measurements.
This brief overview already implies that the complexity of the technique arises basically
from the technical components involved and their mutual dependence on each other and less on
the principles of the technique itself. In terms of accuracy, for example, the particles should
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be sufficiently small and their density should exactly match the density of the surrounding
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fluid. Unfortunately, this is not often feasible for a desired field of view and a given laser
power, light sheet thickness, transparency of the fluid, imaging optics and sensitivity of the
digital camera, as the scattering intensity decreases rapidly with decreasing particle diameter
as shown in figure 2.2. Decreasing the light-sheet width or thickness may partially help but
Light
3 5 Light
10 10 10 10 7
0 o 180 o 0 o 180 o
FIGURE 2.2: Light intensity scattered by spherical oil particles of different size in air (left:
ƒ…„m, ƒa†„m), right: illuminated from left with a plane monochromatic wave front, after [85].
The complex spatial intensity distribution, with a maximum in forward direction, results from the
interference between the reflected, refracted and diffracted wave front.
p‚ p‚
the size of the largest resolvable scales will decrease as well and the three dimensionality of
the flow may cause further problems as will be explained later. To use a powerful laser seems
to be the appropriate solution but beside the costs, strong reflections from model surfaces
or undesirable disturbances of the flow due to acoustic excitation or thermal response of the
flow have to be taken into account [48]. An intensified camera could be applied as well but
a reduced spatial resolution and an increased noise level must be accepted. Alternatively, the
evaluation procedure could be adapted but then the accuracy of the velocity estimation and the
validity of the well established principles may become questionable. Furthermore, it cannot
be recommended to analyse data where the desired fluid mechanical information, hidden in
the particle image displacement, cannot be uniquely determined. So increasing the particle
size may be the appropriate solution at the end, but how this can be achieved is unknown.
Anyway, in order to make the right decision while setting up and aligning the experiment, a
clear understanding of the basic components is essential. For this reason the production of
desired particles, the problems associated with the recording of the particle images and the
determination of the image displacement will be treated in chapter 2. In chapter 3 and 4 the
more sophisticated recording techniques are considered which have been applied for the fluid
mechanical investigations presented in the second part of the thesis.
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