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

6 Investigation of the xz-plane
300
250
200
Q4
z
+
150
Q4
secondary flow
100
50
300
0
0 50 100 150 200 250 300 350 400 450 500 550
250
??]
x [???]
z
+
200
150
Q2
Q4
Q2
100
Q2
50
0
0 50 100 150 200 250 300 350 400 450 500 550
FIGURE 6.24: Same as figure 6.23 but measured at Ó ® Ú¥Ø
(red: CG Ø
; blue: C_! Ø
).
x
+
??]
x [???]
less elongated and broader than the low-speed regions because these flow structures originate
statistically from flow regions which are further away from the wall, see section 5.4. This can
be estimated from the sign of the wall-normal velocity component which is represented by the
contours in figure 6.22. At ® Ê ª¥À these high momentum structures look sometimes similar
to the streaks with regard to size and shape according to figure 6.21 but more frequently they
resemble small elliptically shaped islands, approximately 200 wall units in length and roughly
­
50 to 100 wall-units in width according to figure 6.23. It can be seen from the out-of-plane velocity
component, displayed in figure 6.24, that these structures, which are labelled as sweeps,
transfer momentum towards the wall. Thus, these structures are statistically represented by
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