Aerospace & Defense Technology - September 2023 - 15

4-Mpx pixel resolution. According to
Schroeder, in a typical LPT experiment,
the entire measurement volume fits
inside the camera's field of view. " However,
the presence of the wing in the
tunnel always blocked the view for a
subset of cameras as the wing moved, "
he explains. To overcome this issue,
Schroeder and his colleagues lined up
the cameras in two groups of four, with
one group facing each side of the tunnel
and each unit linking back to a single
recording workstation via 10 GB Ethernet.
" With this setup, we always had
three or four cameras recording each
illuminated region in the volume,
enabling us to capture the particle distribution
around the entire wing. "
To determine the elastic and inertial
forces acting on the wing, the researchers
seeded the flow with 60-micrometer
(µm) spherical polyamide particles. They
also applied a random, white dotted pattern
to the wing's surface, allowing them
to take time-resolved measurements of
the wing's motion and shape within the
water volume. Over the course of the
experiments, the team captured 12,597
consecutive images per 6.3 second run,
for a total of six cycles.
How Shake the Box Works
Central to the success of these experiments
was STB Lagrangian particle
tracking which, according to the
researchers, has two main ingredients.
The first ingredient is called Iterative
Particle Reconstruction (IPR). The DLR
team's special tracking software detected
the particle peaks on the captured
images and then triangulated the particle
positions using images from the
multiple cameras.
" In particle reconstructions, the particle's
3D position is often slightly displaced
in relation to the true position, "
Schanz explains. " This can be the result
of noise, image overlap or the presence
of ghost particles. " To overcome this
issue, the position and intensity of each
reconstructed particle were optimized
by fitting the particle's 3D position and
intensity to the camera images. This
position optimization step, based on
image matching, is integral to IPR.
According to Schanz, " We can optimize
a particle's position by comparing the
The dynamic testing rig that was used in this evaluation.
original image of the particle to an
image we create synthetically. As a
result, we can see the mismatch between
the reprojected image and the original
image. " This process of moving the particles
around in space to optimize their
position is what is meant by " shaking
the box. "
Although STB unlocks particle tracking
at higher seeding densities, ghost
particles remain a challenge. This is
where the second main STB ingredient
comes into play: exploitation of the temporal
domain. Using available temporal
information, the tracking software
extended known particle tracks to the
Aerospace & Defense Technology, September 2023
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next time-step and then corrected prediction
errors by the shaking process.
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Aerospace & Defense Technology - September 2023

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