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

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tures keep their spatial organisation to a large extent while travelling down-stream by a few
hundred wall units. The experimental results indicate that the maximum of the ªS·· correlation
reach values above 0.8 for wall locations of the fixed point larger 100 wall units when the
flow moves 300 wall units in stream-wise direction, see page 152. For ¹ » %[Â the maximum
reaches values above 0.6. This implies that the structural features of the velocity pattern conserve
their identity to a large extent. This justifies the assumption made for the interpretation
of the results in chapter 6. Furthermore, this result implies that the complex turbulent motion
at a single point is a result of relatively simple coherent flow structures which are convecting
downstream in form of a frozen pattern. In this sense, the complexity is a result of the spatial
distribution of the structures and their orientation relative to the main flow direction and not a
result of a strong structural changes of the flow field itself. To examine the interaction of the
coherent flow structure below ¹&» ¬
see page 144 and 145. These functions imply that a vertical motion towards the wall induces
a horizontal motion away from the centreline due to continuity, and a vertical motion at the
fixed point location ã with induces a horizontal motion ×pû ¬\ towards in the near-wall
region and away from the centre at higher wall locations. This correlated motion indicates
that a motion away from the wall is associated with the generation of a stream-wise vortex
pair in accordance with the interpretation in chapter 6. To examine the characteristic features
of the stream-wise vortices in detail and their significance for the turbulent mixing, instantaneous
velocity fields were analysed, see page 156 to 160. It is shown that stream-wise vortices
can be frequently observed in the near-wall region and also vortex pairs which transfer lowmomentum
fluid away from the wall could be detected. However, the analysis shows that the
stream-wise length of these vortices is not several thousand wall units in length, in agreement
with the results in chapter 6, and the number of vortices which could be detected was relatively
small relative to the number of low-speed streaks present in the near-wall region. This implies
that the streaks are not generally flanked by stream-wise vortices as assumed in the literature.
the cross-correlation ªSÔÙ and ªSÙ¥Ô was calculated,
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