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

6.4 Properties of coherent velocity structures
300
250
Q2
Q4
z
+
200
150
Q2
Q2
Q4
Q4
secondary
secondary
Q4
100
50
Streak
Q2
0
0 50 100 150 200 250 300 350 400 450 500 550
300
??]
250
Q2
Q4
x [???]
200
Q4
z
+
150
100
Q2
Q4
Sweep
secondary motion
Q4
50
Q2
0
0 50 100 150 200 250 300 350 400 450 500 550
x
+
y [???]
x [???]
FIGURE 6.25: Velocity fluctuations measured at Ó ® Ú¥Ø
(top) and Reynolds stress component &C
(bottom).
the correlations shown in figure 6.8. If such a structure moves towards a low-speed streak, as
visible in figure 6.24 for example, an interaction takes place and parts of the streaks, which
are directly affected by the sweeps, are forced to move away from the wall due to continuity
(see blue region in ellipse). On average, this process is represented by the spatial correlation
functions shown in figure 6.10 to 6.12. However, it can be seen from the samples shown in figure
6.24 that the effect on the streaks is quite small in the present case. This can be explained
by the fact that the momentum is insufficient to create a strong lifting of the streak. Figure 6.25
shows the same interaction but the momentum transferred by the sweeps is much larger. In
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