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

4 Multiplane Stereo Particle Image Velocimetry
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Streamwise direction [mm]
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Streamwise direction [mm]
FIGURE 4.16: Instantaneous velocity (left) and vorticity (right) field with Ñ -vortices recorded in two
planes parallel to the wall but at different heights (lower field closer to wall) and separated in time.
behind the span-wise slot and the distance between the orthogonally polarised light-sheets is
¾ÁÌÈ mm. The measurement position of the lower picture was at a height of ¾¥ÁÌÞ mm from the
wall (the velocity fluctuations reach their maximum at this position) and the upper field was
recorded ÆÁÌÂ mm above the wall. This investigation is well suited to determine experimentally
the orientation of the counter rotating vortex pair axis and its variation within the lifetime of
the structure and the intensity of the acoustic excitation. As similar structures plays a dominant
role in turbulent boundary layers according to chapter 1, the results are also instructive
from this point of view, see [44] for example. Figure 4.16 shows velocity and vorticity fields
in the same representation as figure 4.15 but the lower field was recorded 0.0013 s earlier and
the intensity of the acoustic excitation was slightly increased.
This experiment demonstrates clearly the reliability and robustness of the measurement
technique and numerical procedures that were developed to overcome the basic problems of
existing PIV methods for the investigation of turbulent flows. In the following chapters the
main turbulent boundary layer results measured with this technique will be outlined in detail.
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