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

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6 Investigation of the xz-plane
One of the well established and widely accepted results of the turbulent boundary layer research
is the characteristic mean velocity profile and the maximum of the stream-wise velocity
fluctuation in the buffer layer at ¢¡¤£¦¥¨§ , see [21, 55]. This is shown in the lower left graph
of figure 6.1 measured at © with the stereoscopic PIV arrangement described in
chapter 5. However, when the stream-wise fluctuations are subdivided according to their sign
before calculating the rms values, it turns out that the negative fluctuations have a maximum
at ¡£ , while those with a positive component reach the maximum at ¡£¥ , com-
1
20
0.8
15
∞
⎺U / U
0.6
0.4
0.2
0
0
0.05 0.1 0.15
y /
δ
4
3
u + rms
v + rms
+
⎺U
10
5
0
4
3
10 100
1
y +
u + rms (u+ 0)
⎺U + (Re θ =7800)
⎺U + = y +
2.44 ln(y + ) +5.1
xz experiment
+
+
u +
rms v +
rms
2
1
u +
rms
2
1
0
y +
10 100
1
0
1
10 100
y +
FIGURE 6.1: Results of high-resolution stereoscopic PIV boundary layer measurements performed
"$#
at
. Top: Linear and semi-logarithmic representations of the normalised mean velocity profile
&%(')*+*
(average over 2500 velocity fields). Bottom: Root-mean-square values of the stream-wise velocity
, fluctuations (left) and conditional averaged representation (right).
97