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

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4.3 Modes of Operation I – In-plane flows
for multiple-plane recording can be easily generated. Slider 4 to 7 from the top specify the
delay between the flash-lamps and the Pockels-cells in order to adjust the output energy of
each laser individually and with slider 8 the time for the read-out of the first camera images
can be adjusted with respect to the first laser pulse. In the centre menu in the lower half
of figure 4.4 the specifications of the laser-system (Quantel, BMI) can be chosen as well as
the mode of operation (high or low energy, flash-lamps only to keep the laser material at the
appropriate temperature, laser off). All other possibilities like cameras only (for field of view
adjustments) or cameras together with only one oscillator (for checking the separation by
means of the polarisation) can be selected in the lower left menu by a mouse click on the right
bottom. After selecting the appropriate set of parameters the sequence has to be sent to the
electronic board and can be activated using the start bottom at the lower right of figure 4.4 and
afterwards terminated with the neighbouring stop bottom. This user-friendly interface enables
the user to perform all alignments and calibrations within reasonable time.
4.3 Modes of Operation I – In-plane flows
Once installed the multiplane stereo system is well suited to determine different fluid-mechanical
quantities simply by changing the time sequence or light-sheet position. For
(*
constant
pulse î) ç è ©) îŠè © separation (
) and overlapping light-sheets, a time
sequence of three velocity fields can be measured at any repetition rate by cross-correlating
the first acquired grey-level distribution with the second, the second with the third and the
third with the grey-level distribution from the last illumination, see figure 4.5. By increasing
(*
the time delay between the second and third
î+ çäè ©-, ©. î
illumination (
)
the first order estimation of the acceleration field in its Lagrangian and Eulerian form can be
calculated in order to study the dynamic behaviour and the interaction processes of moving
flow structures [36]. The Lagrangian acceleration of a moving fluid-element is defined as the
temporal derivative of the velocity and can be simply implemented as a difference quotient
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The nonlinear term on the right hand side of equation 4.2 can be approximated 6 ( to:
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55