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

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6 Investigation of the xz-plane
velocity fluctuations measured at «¢ÀÁ À and ª¡À (top to bottom). Clearly noticeable is
the characteristic elliptical shape of both correlations, with the principal axis in stream-wise
direction, as well as the strong dependence of the correlation width from the wall distances
­.®zÊ
in accordance with [44] and the results presented in section 5.3.2. However, the most striking
feature, which could not be established before, is the variation of the size and span-wise
location of the weak negative correlation regions ÃËÌË of , when approaching the wall. This
is clearly visible in the upper right plot of figure 6.7 where the span-wise dependence of this
correlation Í5Î ® ¿ À at is shown in wall-units. The location of the minima is highlighted
with symbols and the legend reveals the corresponding displacement of the symbols from the
maximum of correlation, located ÍeÏ ® ¿ À at . The range of correlation values indicates a large
range of scales and the minima imply a span-wise periodicity whose wavelength strongly increases
with increasing wall distance. From the negative correlation regions, the existence of
stream-wise vortices can be concluded. However, it should be noted that their length is not
+ +
R uu (y =30)
R (y =30)
vv
∆ z +
∆ z +
∆ z
+
+
R uu (y =20)
+
R uu (y =10)
+
R vv (y =20)
+
R vv (y =10)
∆x + ∆x +
FIGURE 6.6: Two-dimensional spatial correlation function of fluctuating stream-wise ÐÒÑ˜Ñ velocity
(left) and wall-normal Ð ËÌË velocity (right) measured Ó ®ÕÔ×֥آÙÛÚ¥Ø
at Ü Ø
and (top to bottom).
106