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

6 Investigation of the xz-plane
cally accessible. Figure 6.10 shows the two-dimensional spatial cross-correlation function of
fluctuating stream-wise with span-wise velocity components Ã Ñ ¹ Í5Î ® Á ­.® Á–Í ® Ï ) (left) and
correlation of wall-normal with span-wise components ÃË ¹ Í5Î ® Á ­.® Á ÍlÏ ® ¼ (right) measured
at Ã‚Ä¥Å Ê ÈÉ¢ÀÀ and ­ ® Ê «¢ÀÁ ¨ ÀÁ¥ª¡À (top to bottom). First of all, it can be concluded from the
general size, shape and height of both correlations, that a very well organised span-wise motion
exists, especially at ­.®pÊ
where the height of ÃË is above the ÃË Ñ correlation. This
ª¡À
organised motion implies that the concept of local isotropy, which links the turbulent-energy
dissipation in shear flows with the dissipation of isotropic turbulence, is inadequate in nearwall
turbulence. Secondly, it should be noted that the degree of organisation increases with
decreasing wall distance while the opposite holds for ÃË Ñ the correlation as can be estimated
from the intensity of the maximum. Thirdly, the spacing between the extrema is increasing
with larger wall distances but also, it can be seen that the location of these particular points
+
R (-u)(w) (y =30)
+
R (+u)(w) (y =30)
∆ z +
∆ z +
∆ z
+
+
R (-u)(w) (y =20)
+
R (-u)(w) (y =10)
+
R (+u)(w) (y =20)
+
R (+u)(w) (y =10)
∆x + ∆x +
FIGURE 6.11: Conditional cross-correlation of negative stream-wise fluctuations with the span-wise
velocity component Ð ê Ñ¡ë3ìÌí ê í (left) and correlation positive stream-wise fluctuations with span-wise
components Ð ê Ñ3ìÌí ê í (right) measured at БŠû§¥Ø¡Ø
and Ó ® ֥آÙÛÚ¥Ø¢Ù Ü Ø
(top to bottom).
112