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

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3.3 Calibration validation
by means of fluid-mechanical considerations. This is almost impossible for small ,
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especially
when the relation between the components possesses some obvious symmetry which
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may focus the attention in the wrong direction. This is clearly visible in figure 3.9 where a
Rankine vortex with a tangential particle image displacement ¢ ø of pixel is displayed.
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The
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pixel interrogation windows, each of
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them containing approximately 25 particles generated at random positions within the Gaus-
pixel images have been analysed with § ú
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sian shaped light-sheet of finite thickness. The curvature of the particle trajectory is taken into
account, such that particle images close to the core of the vortex actually orbit the core at the
same radius but centrifugal forces are not considered. Before the analysis has been performed
the simulated particle image pattern has been duplicated and properly shifted in opposite directions
by choosing the values of the ¨ ç©
coefficient ¨ î©
and and the sub-pixel increment
properly. The opening angle between the assumed observation directions is û óaü
again and the
object-distance
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pixel (20 times the field of view) to avoid projection effects.
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FIGURE 3.9: Spatial distribution of the out-of-plane displacement introduced by non properly aligned
calibration grid or poorly performed evaluation (average over 20 fields). The line spacing in the contour
plots is incremented in steps of pixel and different line styles indicate different out-of-plane
directions.
The range of the out-of-plane displacements as a function of the in-plane shift depends on
both, magnitude and direction and can be quite large with respect to all other error sources in
PIV according to table 3.1. Figure 3.9 shows the spatial distribution of the error. The shift is
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