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

varying signal can be transformed into a time dependent signal at a fixed point, whether a characteristic
velocity pattern, that was observed in the past by using hot-wire probes, is related to
the motion of particular coherent structures. The results of this chapter are partially published
in [41].
Chapter 7 focuses finally on the wall-normal span-wise plane ([-plane) investigation of
the turbulent boundary layer by means of multiplane stereo PIV and*1¥¥¥.
These investigations were mainly performed to examine the validity of the frozen pattern
concept, applied in chapter 6, and to prove the existence of stream-wise vortices in the nearwall
region and their significance for the turbulent mixing normal to the wall. Two independent
experiments are described with different spatial separation between the measurement planes
in stream-wise direction (
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and various temporal delays between a pair
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of measurements. Due to the widespread opinion that reliable experimental results are very
difficult to achieve when the orientation of the light-sheet is perpendicular to the main flow
direction, in section 7.2 the statistical results are validated against the results presented in the
chapter 5. In section 7.2.2 the structural features of the coherent flow structures are displayed
and the model assumption made for the interpretations of the results presented in chapter
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5
and 6 will be examined. In section 7.4 the decay of the coherent structures with increasing
time is investigated as a function of the wall distance to validate the frozen pattern hypothesis.
In section 7.5 the existence and relevance of the stream-wise vortex pairs for the turbulent
mixing in wall bounded flows is finally examined and the concept of hairpin or horseshoes
like structures is discussed as well as their connection with shear-layers and streaks. Parts of
this chapter are already published in [49]. As the fully-developed turbulent boundary layer
flow along a flat plate is considered as one of the basic academic flows which is accessible by
Direct Numerical Simulations (DNS), and a test case for the validation of turbulence models
required for Large Eddy (LES) and Reynolds Averaged Navier-Stokes Simulations (RANS),
the statistical results presented in chapter 5 to 7 are displayed in various representations to
simplify a comparison in the future.
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