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

As order can be always observed when the convection term in the equation of motion
is dominant over the production, diffusion and dissipation term, there is no doubt about the
existence of organised flow structures. However, their exact geometrical and kinematical properties
are widely unknown and there is still a general controversy about which structures are
fundamental and which ones are only secondary, which ones are dominant and which ones are
irrelevant [73, 86, 87, 94]. Based on the technological progress in laser, camera and computer
technology in the last decade of the 20th century, it became possible to link the qualitative
visualisations with the statistical results from the quantitative single-point measurements by
applying non-intrusive optical multi-point measurement techniques such as the conventional
and stereoscopic particle image velocimety, see [4, 38, 44, 74] for example, and it was partially
possible to prove the validity of the proposed vortical models. However, due to the measurement
noise and the limited spatial resolution, storage capacity and computer power at that
time, only the properties of the large-scale structure could be deduced from a small number of
samples. The information about the geometrical and kinematical properties of the small-scale
structures on the other hand is still uncertain and also a consistent physical picture, which
links the basic processes and explains their importance with regard to the turbulent transport
is missing [31, 76, 86, 89, 94]. There is no doubt that further sophisticated experimental or
numerical investigations are required to solve these questions and to add another piece to the
puzzle of near-wall turbulence. As a direct numerical integration of the conservation equations
is still out of reach, especially at high Reynolds numbers, it is generally believed that
deeper insight strongly relies on the capabilities of advanced measurement techniques. To
overcome the limitations of the existing measuring and visualisation techniques applied in
fluid mechanics a stereo-scopic particle image velocimetry based measurement system was
developed and applied for the investigation of the phenomena of near-wall turbulence. By using
this technique it is possible to determine at any flow velocity all three velocity components
in spatially-separated planes simultaneously or separated in time. It will be shown in the first
part of this thesis that this technique is very reliable, robust and well suited for measuring with
high accuracy and spatial resolution a variety of fundamentally important fluid-mechanical
quantities, which are at present not available by any other known technique. In the second
part of the thesis the experimental results obtained with this technique in a turbulent boundary
layer flow will be outlined in order to answer some of the previously mentioned questions
about the geometrical and kinematical properties of the coherent flow structures and their relation
relative to the proposed vortex models.
Beside the difficulties associated with the measurement techniques nearly all experimental
boundary layer investigations reported in the literature are faced with the problem that the
generation of a fully-developed, turbulent boundary layer flow of large extent and range of
scales is difficult to achieve due to the limited wind-tunnel dimensions. By using tripping
devices right behind the leading edge of a flat plate, such as sand paper, wires or ribbon
elements for example, a well-defined transition to turbulence can be reached, accompanied by
a strongly-growing thickness boundary layer and range of scales. However, these devices
introduce artificial flow-structures which alter the natural turbulence level and flow structure
in the wake [21, 83, 111]. Without tripping the boundary layer, only natural flow structures are
developing, but due to the gradual, non-uniform transition process, the onset of turbulence is
delayed and strongly varies in time and spatial location. As a consequence, quantities like the
thickness boundary layer are not well defined and the range of scales is rather small¥"!$#) relative
to the disturbed flow. In addition, a direct interaction between the outer region of (
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