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

1 Introduction
One of the fascinating phenomena of natural science is the turbulent state of the macroscopic
flow motion in fluid-mechanics. Beside the omnipresent variety and beauty of the turbulent
motion, the inherent fascination and attraction is strongly connected with the enormous difficulties
associated with a mathematical and physical understanding. The principal difficulty,
with respect to an inviscid fluid or a classical gas of point particles in equilibrium, results from
the strong non-linearity in the conservation equation and the dissipative character e.g. the
flux of energy from the large scales or eddies into progressively smaller and smaller ones. As
a general mathematical solution of non-linear, non-equilibrium systems is out of reach from
the present point of view, the properties of idealised flows with simple geometries are examined
experimentally and numerically. Of primary interest is the two-dimensional turbulent
boundary layer flow of an incompressible fluid along a flat plate with zero pressure gradient,
because this flow reveals simultaneously two characteristic phenomena of turbulence, namely
the effects of near-wall turbulence and the effects of intermittency, e.g the interaction of the
turbulent boundary layer with the laminar outer flow according to figure 1.1. This particular
flow evolves from a laminar boundary layer flow when the Reynolds number
£¡
is sufficiently high. In this case flow disturbances with a particular wavelength grow, become
unstable and share the energy from the mean motion over the degrees of freedom by non-linear
interaction.
rot U = 0
1.2 δ
U / U δ
y
0.4 δ
rot U = 0
δ
x
1
FIGURE 1.1: Instantaneous structure of a turbulent boundary layer and mean velocity profile after [37]
Beginning with the early channel and pipe flow measurements, published by Laufer [68,
69], and the zero pressure gradient boundary layer benchmark investigations along a flat plate
by Klebanoff [55], turbulent boundary layers have been examined extensively because of their
technological importance, their significance for the development of fundamental turbulence
models and for the validation of numerical flow simulations [89, 96]. The bulk of the quantitative
investigations has been performed with intrusive single-point measurement techniques
[21] such as pressure probes and hot-wire anemometer, but also non-intrusive flow visualisations
techniques have been frequently applied to examine qualitatively the global features of
5