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

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7.1 Experimental set-up
For the evaluation of the stereo-scopic images the second order warping technique was
applied again, along with the calibration validation procedure described in section 3.3. This
ensures that the interrogation spots from each of a pair of stereo-scopic images correspond
to the same region of the flow. The interrogation of the data was performed with the FFTbased
free shape cross-correlation outlined in section 2.4, and for the determination of the
signal-peak with sub-pixel accuracy, the two-dimensional Gaussian fit using the Levenberg-
Marquardt method has been applied. This peak finding method is less sensitive to sub-pixel
displacements compared with the three point Gaussian peak fit, see figure 2.14 and table 2.1 for
h ¨ji h ¨
À ¤lknm hpo¥ÍeÏpo À k
oüÍrq8o Ç Í5Î Í5ÎEs > out Ç
details. For the calculation of the velocity vectors pixel interrogation windows were
used for both Reynolds number investigations. The bandwidth of particle images displacements
varies between zero and 8 pixel (zero and -8 for the left camera system in figure 7.1) for
the light pulse delay listed in table 7.2, and the number of spurious vectors was on average below
by applying the following set of band-pass and gradient filter ( pixel;
pixel and pixel). The basic details about the recording and
evaluation are summarised in table 7.2.
[1]
160000 [1]
[1]
0.37 0.34 [m]
3000 5980 [1]
ÿ 3 7 [ m/s]
| ¡ 0.121 0.263 [ m/s]
s %
field of
Þ h ijÆ É Þ h iNÆ É view mmá [ ]
field of À ¤}k È i À ¤ ¨ Þ À ¤lk Æji À ¤ ¨$Æ
view [ á ]
field of view % kk i È Æ Þ kk ÀÈ i k È ¨¨ [Írq
s i ÍlÏ
Ç
spatial À ¤:Þ i~¨ ¤}k h i~¨ ¤}k h ¨ ¤lk h i ¨ ¤lk h resolution [ mm ]
spatial resolution B ¤ À i k È ¤ h i k È ¤ h hÈ ¤ t i hÈ ¤ t [Í5Î
Ç
]
]
€ s‚
À h ¤ k ¤ É À ¤:Þ À kk¤ É
pulse separation 200 100
dynamic range to to [ pixel]
vectors per sample 13113 13113
number of samples 2975 2100
Írq
ÍeÏ
TABLE 7.2: Relevant parameters for the characterisation of the experiment performed 18 m behind the
leading edge of the flat plate in the Óc -plane of the turbulent boundary layer flow.
Two similar experiments have been performed independently. In the first experiment the
spatial location of all light-sheet planes was identical and the time separation between a pair
of velocity fields being acquired was varied (see left plot of figure 7.2). This allows to study
the changes of the velocity structures with time. In the second experiment the time-separation
between a pair of measured velocity fields was altered as before but, in addition, the measurement
planes were spatially shifted in stream-wise direction by tÀ mm (Í5Î
indicated in figure 7.1 and in the right plot of figure 7.2. Thus it was possible to select a flow
pattern at an upstream position and to measure the structural changes as a function of the spatial
distance between the light-sheet pairs and of the time interval between the two acquisitions
s Ê h¢À¢À ) as
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