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

7 Investigation of the yz-plane
light-sheet by turning the cylindrical lens in the light-sheet optic. The observation angle for
each camera in angular imaging configuration with Scheimpflug correction is shown in the
following table along with the exact positions of the master cameras (1 and 2 in figure 7.1)
with respect to the centre of the field of view (all asymmetries have been taken into account
for the calculation of the three velocity components). Four 180 mm lenses (Carl Zeiss)
camera Î [mm] Ï [mm] f@g [mm]
¯
[deg]
1 -1570 1464 2143 42.9
2 -1560 -1463 2140 43.2
TABLE 7.1: Position and observation distance of the master cameras with respect to the centre of each
field of view (meeting point of optical axis) and corresponding observation angles.
a
b
40 mm separation
~86°
main flow-direction
6
5
4
7
8
test-section 3
x
2
side window
z
1
FIGURE 7.1: Schematic set-up of the recording system and light-sheet position for both experiments
(different scales). 1-4 digital cameras, 5 lens, 6 mirror, 7 polarising beam-splitter cube, 8 absorbing
material, a measurement location for 1st investigation (40 mm separation between both measurement
planes in stream-wise direction), b measurement location for 2nd investigation (all measurement positions
at the same location). Different light ray colours indicate different states of polarisation.
were used for the measurements with an aperture of 8 and a magnification of 1/6 along the
principal axis of the lens. Due to the strong out-of-plane velocity component, as a result
of the light-sheet orientation relative to the main flow direction, the light-sheet pairs with
equal polarisation have been shifted in stream-wise direction as indicated in figure 7.2. This
improves the signal to noise ratio, as the loss-of-correlation due to unpaired particle images is
minimised without reducing the dynamic range and the spatial resolution. Decreasing Í_] or
increasing the magnification of the imaging system would reduce the dynamic range or spatial
resolution.
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