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

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5 Investigation of the xy-plane
the log-law holds, is quite large for both cases so that a direct interaction between the nearwall
flow structure with the intermittent flow region can not take place. Thus the turbulent
flow state can be considered as fully developed. Fully developed flows are characterised by
a Kolmogorov cascade including an inertial range over at least one order of magnitude in the
wavenumber space [78]. This condition implies unsteadiness, rotational, three-dimensionality,
non-deterministic, diffusion and dissipation, e.g. attributes which are generally referred to
characterise turbulent flows. The dynamic of the velocity signal as a function of the wall distance
can be estimated from figure 5.3 which shows the normalised rms values of the three
velocity fluctuations for both flow cases. It can be seen that the maximum can be properly
resolved with state-of-the-art PIV systems, provided the magnification of the imaging system
is well adjusted relative to the flow structures and the problems associated with the strong wall
reflections are solved. The low Reynolds number results agree quite well with the HWA measurements
kindly provided by Prof. Dr. Fernholz. Only the first two measurement points near
the wall are overestimated. This is natural due to the finite size of the measurement volume.
When the high Reynolds number results are considered the deviation increases but the increase
of the second peak with increasing Reynolds number is clearly visible and in agreement with
the literature [21]. To validate the degree of anisotropy between different velocity components
in turbulent shear flows, the ratio between orthogonal velocity fluctuations is shown in
the right graphs of the same figure. As the energy from the mean motion is first transferred into
the stream-wise velocity fluctuation before the transfer into ³ the ´ and component takes place
by means of pressure fluctuations, the value of the anisotropy parameter is usually around 0.6
for large wall distances. In addition, it can be seen µ ³ ¬' ·E¬'¸ ±º¹ ¬ that increases gradually with
the wall distance. Figure 5.4 shows the turbulence-level defined µ · ¬ ¸ ±º¹ ¬ » as for both
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flow
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cases. The agreement with the hot-wire measurements performed ¼ at
is excellent. Only the first PIV measurement point in the left graph is far away from the HWA
result, which is à · 4‘
around Ä4Å