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

4 Multiplane Stereo Particle Image Velocimetry
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FIGURE 4.14: Development Ñ of -vortices
along a flat plate (side and top view). The
rectangles represent the light-sheet position and
field of view.
λx
λ z
ment. For the illumination of the ÆÖÕ m olive oil droplets, generated by means of multi-hole
nozzles described in section 2.2, a four pulse Nd:YAG laser-system (Quantel Brillant B) in the
configuration shown in figure 4.2 with an output energy of 320 mJ per pulse at ×ÙØÚÈ¿Â0Û nm
was used. Due to the small boundary layer thickness at the beginning of the measurement
position (ÜÝØVÈ mm) a combination of four lenses (focal length in mm: -150 (bi-concave), 200
(bi-convex), 60 and 100 (both planar convex)) was necessary to form a sufficiently thin lightsheet
(¾ÁÌÈ mm) nearly constant in thickness for over more than Æ m. For the recording of the
particle images four Peltier cooled PCO cameras were used with ÆÛ¿Þ¾ÝÄFƾ0Û[Ë pixel resolution
and 12 Bit dynamic range at 12.5 MHz readout frequency. The read-out noise of the PCI interface
board is less than 8 electrons. The pixel size is ßÁÉÎ@Õ mÊ and the dimension of the chip is
Ûà¿Â?á á . For magnification and field of view adjustments (necessary for maximising the amount
of stereo information) all Scheimpflug-adapters were mounted on a two-axis linear translation
stage which allows high precision translations by thumb screws. The opening angle between
the cameras was âãØäË0¾ À and the magnification factor on the optical axis M=0.1 for all cameras.
For the imaging four 60 mm macro planar lenses (Carl Zeiss) were used with an aperture
of 8 to avoid optical aberrations introduced by the optical components in a non-collimated
beam according to section 4.7. After the beam-splitter and the ×màÛ plates had been properly
aligned, polarisation changes during the scattering process (at very large particles or particle
clusters) or the diffraction at the test-section window as outlined in section 4.6 could not be
observed under these conditions. Usually the intensity of ghost-images due to polarisation
changes during the scattering process is far below the minimum sensitivity level of the CCD
sensor (for small particles). However, when polarisation effects occur time separation can be
applied as well, as long as the separation between the first orthogonally polarised light pulse
pair is negligible, e.g. the position of the particle image remains constant in terms of digital
registration. In this case, which holds for a wide range of flow velocities, a difference between
instantaneous and time separated measurements cannot be observed. Technologically,
this can be done by transferring the first acquired images right behind the first illumination so
that the second illumination with differently polarised light cannot be recorded by means of
the previous system.
To trigger all necessary components quickly and easily, a 16 channel sequencer-board with
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