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

aperture, it is shown how to eliminate certain aberrations by accepting aberrations of other
types which are of no harm in PIV. This point is of primary importance because higher order
aberrations like distortion and curvature of the field just influence the position and form of
the image but do not lower the resolution. They can be completely eliminated numerically by
using the methods examined in chapter 3. Primary aberrations, on the other hand, like spherical
aberration, coma and especially astigmatism deteriorate the image and alter the shape in
a characteristic way. This leads to an increased measurement error because the performance
of the peak-fit routine for sub-pixel accuracy strongly decreases for particle image diameter
not equal 2-3 pixel. For completeness it should be mentioned at this point that the multiplane
stereo PIV technique is frequently applied in other laboratories by now, see [77, 32] for
example, and also commercially available.
The second part of this thesis reveals the main fluid mechanical results measured with
the Multiplane Stereo PIV techniques in the temperature-stabilised, closed circuit wind tunnel
at the Laboratoire de Mécanique de Lille (LML). In chapter 5 the stream-wise wall-normal
plane (ñŸ¹ -plane) of the turbulent boundary layer flow is examined at ª«§=@? í parameter and the ¸ ä í ½A=@? í ï B
turbulence-level
76 to 78. The shape, size and coherence of the turbulent flow structures and their dependence
on the Reynolds number was estimated from the primary spatial ªS·· correlations ªSÔÔ , and
of the velocity fluctuations and ªDC#EFC#E , see page 80 to 87. The similarity between the
ªSÙ&Ù
size ªS·· of ªSÙ&Ù and in stream-wise direction and the elliptical shape around the maximum,
accompanied with the general variation of the orientation with increasing wall distance of
the fixed point, implies a strong relation between both fluctuations over several hundred wall
units. However, the crestfallen shape of ªSÙ&Ù the correlation indicates that the physical mechanism
which connects ä the å and fluctuations depends on the wall distance. This can be
explained by the different shape of the coherent flow structures. While the near-wall region
¹¥»&% °
below is dominated by well organised low-speed streaks, the log-law region above
is dominated by shear-layers and large-scale eddy structures. It is shown that the pro-
°
¹&»G%
duction of turbulence ¹ »
¡ °
at is frequently associated with span-wise vortices which are
located on top of the shear-layers. These vortices pump low-speed fluid from the shear-layer
away from the wall. This process is associated with a relatively large velocity component in
wall normal direction. However, the spatial extent of the region äŸã where is negative is quite
small. Large areas äŸã Û with are frequently associated with relatively simple eddy structures
(no vortices) which transfer high momentum fluid towards the wall as assumed in the mixing
length theory by Prandtl, see page 91 to 94. As the stream-wise momentum transported with
this structure is quite large, their contribution to the production of turbulence is already significant
when the negative wall-normal velocity component is relatively small. However, as the
probability density function of the wall-normal velocity component is symmetrically within
the log-law region, this process is compensated on average by a motion of high-speed fluid
away from the wall. This changes in the near-wall region examined in chapter 6. The analysis
of the various double- and triple-correlations, which are associated with the production of tur-
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