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

6.4 Properties of coherent velocity structures
is accompanied by two weak stream-wise vortices because any local motion away from the
wall is associated with a stream-wise vortex pair, but these vortices are produced locally and
can not be considered as primary vortex structures. The similar stream-wise extent of sweeps
and regions of significant outward motion supports this conjecture. The average length of
these vortices can be estimated from figure 6.10 to 6.12.
Another effect associated with the movement of the sweeps is the creation of counter rotating
vortex pairs, as indicated by the red and blue circles in figure 6.25. First of all, it should
be noted that these structures are different from the vortex models presented in the figure 1.3
as these vortical structures pump high-speed fluid towards the wall and not low-speed away
as implied by the vortex models on page 6. Besides, it seems likely that these vortices are
generated when the sweeps interact directly with the low-speed regions. Another remarkable
effect which can be observed quite frequently is the generation of hairpin like structures when
a sweep interacts directly with the back of a streak, see red circles in figure 6.26 for example.
However, it can be seen from the lower image of the same figure that these vortices do not
extend into the near-wall region down to ¿ ª¡À in the present case. It can be speculated that
this might be related to the strength of the sweep or that the sweep streak interaction did not
last long enough at the time the image was acquired to form a clear vortex pair.
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6.4.3 Ejection
Figure 6.27 does not reveal any extended streak pattern but the intensity of the Reynolds stress
component is very large as can be estimated from the lower image. However, by visual inspection
of the velocity field it can be seen that the regions of strong production are associated with
counter rotating vortex pairs, denoted by the circles, which indicate the presence of a hairpin
like structures, [44]. Moreover it seems that these structures are convecting as a package as
described in [112]. It should be kept in mind that according to figure 6.20 the occurrence
of such arrangements of hairpin like structures is very low in the near-wall region as only a
minority of the fluctuations exhibits an intensity which is typically associated with such structures.
Figure 6.28 shows another pair of velocity fields measured simultaneously at ­®
À
(top) and 10. Clearly visible are the different structures discussed above and their interaction.
In this representation the vortical flow structures are inclined in stream-wise direction as the
¿`¨
structures in the lower representation appear slightly upstream with respect to the results in
the upper image. Since the position and intensity may be affected by the particular convection
velocity subtracted, the wall-normal vorticity a - component is shown in figure 6.29 for both
cases, calculated from the instantaneous velocity fields. Whereas the shear layers beneath the
streaks appear clearly in this representation, the vortices are less pronounced. However, the
relatively sudden change in the vorticity component at the beginning of a streak might indicate
the presence of hairpin like structures.
In the last decades of the previous century, detailed hot wire investigations were performed
in order to obtain quantitative information about the bursting phenomena. By using different
pattern recognition techniques, mentioned on page 90, a quite regular normalised velocity
signal could be extracted and it was assumed that this pattern is the signature of a coherent
structure which is associated with the bursting phenomena [102]. In the near-wall region
below ¿ «¢À the pattern could be observed in nearly 65 % of the total samples, and by
analysing the individual velocity signal it was found that the length of the pattern varied over
quite a wide range (1:25). To identify the structures which are responsible for the characteris-
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