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

4.5 Simplified recording system
4.5 Simplified recording system
For some experiments it might be sufficient to replace the complex recording unit outlined in
figure 4.1 by a simple stereoscopic PIV system. When the first two illuminations are stored
onto the first fields of the progressive scan CCD camera pair in stereoscopic arrangement
and the second two illuminations onto the last field of both cameras, each of the four double
exposed images can be analysed using the conventional evaluation method based on the autocorrelation
technique [109]. By combining the corresponding displacement fields, two independent
three-component displacement fields can be obtained. The time separation between
these displacement fields can be chosen arbitrarily depending on the quantity in question (acceleration,
space-time correlation or proper orthogonal decomposition are relevant values for
example). For constant pulse separation and overlapping light-sheets, a third displacement
field can be calculated as well by cross-correlating the two double exposed images from each
camera. For overlapping light-sheet arrangements there are almost no drawbacks with respect
to the polarising beam-splitter based system except the reduced spatial resolution due to
the evaluation technique and the direction ambiguity [51]. In the case of spatially separated
light-sheet pairs there are two drawbacks. Firstly, the simultaneous measurement in time is
excluded, so the vorticity and the velocity gradient tensor based values cannot be determined.
Secondly, the spatial separation between the light-sheets is limited by the depth-of-focus of the
imaging system as the same lens has to be used for the measurement in both planes. The latter
restriction can be influenced by the aperture and focal length of the imaging system. Although
the power of this four-pulse technique is limited with respect to the previously described arrangement,
it provides the user with much more fluid-mechanical information compared to a
conventional two pulse stereoscopic PIV system.
4.6 Polarisation effects
When the Multiplane Stereo PIV technique is applied it is important to keep various optical
effects into account in order to maximise the performance of the measurement technique. The
first two undesired effects the experimentalist has to deal with results from the multiple reflections
between optical components, because they reduce the contrast in the captured image and
the light scattered in the object direction is lost. To avoid these effects, the optical elements
are coated with a thin layer of material that reflections from the outer surface of the film and
the outer surface of the substrate cancel each other by destructive interference or interfere constructively
in order to produce a highly efficient reflector. The requirements to create an exact
cancellation of the reflected beams with a single layer coating are that the path difference of
the reflections is exactly an integral number of half wavelength (
÷aóaü
out of phase) and of the
same intensity. The first requirement is automatically fulfilled as during external reflection
the light waves undergo a ¤ ÷aóaü
phase shift (no such phase shift occurs for internal reflection
except in the case of total internal reflection). To fulfil the last requirement, the coating material
must be chosen in such a way that the refractive index
¤ air and Z film are equal for a given
Z
wavelength Z air
Z film èsZ film
Z substrate. For Z air è
¤
follows Z film è—– Z substrate. Assuming the
last condition cannot be fulfilled by any known material or the reflectance by a single layer
is not sufficient enough for the application, a multi-layer coating is necessary. V-coatings are
alternate high and low refractive index films, each one having an optical thickness of 1/4 of
a wave (at the design wavelength), which reduce the reflection of a component to near-zero.
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