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

˜
E
˜
T
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T
2.4 Particle image analysis
is identical but the noise uncorrelated [27]. Once this has been done, the signal peak can
be clearly detected by calculating a correlation of the correlation (the uncorrelated noise will
vanish), but again it should be taken into account that evaluation schemes based on few particle
images are highly sensitive to noise that is introduced by the characteristics of digital cameras.
In order to enhance the signal quality when strong in-plane gradients are present, Huang et al
[33] has proposed to transform the image in such a way that the deformation vanishes after
the mapping, see also [22]. This approach works fine as long as the flow
^¡
under
^¡
consideration
possesses (˜
only ˜
in-plane gradients ˜sš
, ˜sš
and , ˜ñ). In case of
^¡
strong outof-plane
gradients ) this and most of the other sophisticated methods will fail, as can
(˜
be easily realized by considering a turbulent boundary layer experiment with a light-sheet
parallel to the flat plate. Under these conditions the apparent particle image gradients caused
by particles from different layers cannot be compensated because of arising instabilities in the
image analysis.
˜ñ
35