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

6 Investigation of the xz-plane
300
250
z
+
200
150
Q4
Q4
Q4
Q4
100
50
300
0
0 50 100 150 200 250 300 350 400 450 500 550
250
??]
x [???]
200
z
+
150
100
50
0
0 50 100 150 200 250 300 350 400 450 500 550
FIGURE 6.26: Velocity fluctuations measured simultaneously at Ó ® Ú¥Ø
(top) and Ó ® óÜ Ø
.
x
+
??]
x [???]
effect, the streaks move away from the wall, indicated by the blue contours, and it is visible
from the lower image that this is associated with the production of Reynolds shear-stress
(dashed lines § · ºñÀ indicate ). Though, it should be noted that the production of Reynolds
shear stress induced by the sweeps is quite large as can be estimated from regions denoted
by Q4 in the lower contour plot. Based on this experimental result it can be concluded that,
in general, the lifting of low-speed streaks can be considered as a secondary motion which is
induced by an interaction between a low-speed streak with a sweep, and the size and strength
of the region, which moves away from the wall, is related to the momentum of the sweep. In
addition it can be stated that the lifting of low-speed fluid into higher momentum flow regions
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