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

6.4 Properties of coherent velocity structures
300
250
Q4
z
+
200
150
Vortex pair
Q4
Vortex pair
Q4
Vortex pair
100
Q2
50
Q4
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 6.28: Velocity fluctuations measured simultaneously at Ó ® Ö¥Ø
(top) and Ó ® Ú¥Ø
.
x
+
??]
x [???]
directly associated with the low-speed streaks. As these flow structures appear frequently in
the near-wall region according to figure 6.21 to 6.30, the high detection rate in the hot wire
results is not surprising and also the strong variation of the pattern length can be explained.
The maximum length is given by the extension of the streaks, which can be longer than 1000
wall-units, and the lower limit of the length appears when only the cross-section of a narrow
streak convects along the probe. Since the streaks are only slightly twisted, the projection of
the width of the cross-section in stream-wise direction is the relevant parameter for the lower
limit of the pattern length. As the minimum width is approximately 30 wall units, according
to section 6.2.2, and the maximum angle between the stream-wise coordinate and the streak
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