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

6.4 Properties of coherent velocity structures
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FIGURE 6.21: Velocity fluctuations measured at Ó ® óÜ Ø
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streaks which appear separated in stream-wise direction at ¿\¨ À belong to the same streak
visible at ­.® ¿ ª¥À , the decreasing length of these structures can be considered as an artefact
related to the statistical variation of their height. When the dynamic of the streaks is investigated,
it turns out that these structures tend to move away from the wall, which can be deduced
­.®
from the out-of-plane motion (blue contours denote a motion away from the wall ½ À and
red towards the wall · · À ). It is clear that this process is associated with the production of
º
turbulence according to the Reynolds equation as ] turb ¿
^R § ·
becomes positive on average.
However it is important to note the extent of this vertical motion is usually significantly shorter
than the total length of the low-speed streaks at the same ­ -value. This is in agreement with the
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