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

3 Stereo-scopic Particle Image
Velocimetry
The conventional PIV technique described in the previous sections yields reliable results as
long as the flow under investigation is two-dimensional and parallel to the light-sheet. In case
of turbulent flows with a strong velocity component being normal to the light-sheet, the outof-plane
velocity component remains unknown and the in-plane components are biased due to
the perspective error as indicated in figure 3.1 for a simulated flow with a pure out-of-plane
velocity component.
FIGURE 3.1: Magnitude
and direction of the
projection-error as a function
of the image location
for a constant out-of-plane
displacement.
As this error is directly proportional to the viewing angle, according to figure 3.1, the object
distance must be increased to minimise the error while keeping the field of view. As this
approach requires long focal length lenses it is obvious that this approach is not satisfactory.
To overcome this constraint completely and to obtain the out-of-plane velocity component, a
stereoscopic observation arrangement has to be applied, which will be outlined in the following
sections.
3.1 Principles
Using the stereoscopic recording technique, the images of tracer particles are recorded simultaneously
from two different viewing directions, and the correct displacement (without
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