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

4.8 Feasibility study
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Streamwise direction [mm]
Streamwise direction [mm]
FIGURE 4.15: Instantaneous velocity vector field (left) and vorticity (right) with Ñ -vortices recorded
simultaneously in two planes parallel to the wall but at different heights (lower field closer to wall).
The flow direction is from left to right.
an output frequency between 0.01 Hz and 1 MHz was used. The width of each pulse is freely
adjustable according to the specifications of the equipment used. The delay of all output
trigger pulses with respect to the input trigger can be selected in the range from 50 ns up to
140 s with a resolution of 50 ns (jitter 12.5 ns).
The mean displacement of corresponding particle-images was 8 pixel for ³æå”Ø©Þ¿¾çÕ s and
the height of the correlation coefficient was 0.6 for ÂÛ)ÄèÂ0Û pixelÊ interrogation windows. The
evaluation technique of the stereoscopic images is based on the second order warping to ensure
that the interrogation spots from each of a pair of stereoscopic images correspond to the same
region of flow and the magnification is constant for all image positions [108]. For the evaluation
of the data the FFT-based free shape cross-correlation was employed, which combines
the advantages of the direct correlation (free-sized and free-shaped windows, high accuracy)
and the simple FFT-based correlation (high speed evaluation), see section 2.4 for details. For
sub-pixel accuracy the two dimensional Gaussian fit using the Levenberg-Marquardt method
has been applied to find the position of the correlation peak and the Fisher transformation
to determine the weight for each value, see section 2.4.2 for details. Figure 4.15 shows the
in-plane velocity (left) and out-of-plane vorticity distribution (right) for two distances from
the wall measured simultaneously. The flow direction is from left to right and a constant
convection velocity of ÈOÁÉÎÈ m/s has been subtracted from both vector fields in order to show
the distribution of the velocity structures more clearly. The observation area started 265 mm
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