Geoscience Reference
In-Depth Information
exploited. For instance, the versatility of 3D PTV, the ulti-
mate temporal resolution of ELDV, the possibility to per-
form large-scale atmospheric in situ measurements with
acoustic tracking, the capacity to directly access Fourier
modes of vorticity using acoustic scattering, the capacity
of instrumented particles to measure not only kinematic
quantities but also heat transport, salinity, etc. show the
many possible developments and advances that can still be
expected from these techniques in addressing geophysical
fluid dynamics issues.
Let us briefly summarize the advantages and draw-
backs of the different techniques. High-resolution optical
tracking has become one of the most accurate tech-
niques in experimental fluid mechanics. It allows to track
simultaneously hundreds of particles in 3D and thus the
ability to address central questions related, for instance,
to mixing and transport properties of flows. Its main
drawback is the current cost of high-speed cameras.
Acoustic tracking and ELDV, both based on Doppler
velocimetry, are more affordable techniques, though they
are limited to the tracking of essentially one particle at
a time and are therefore not adapted to multiparticle
studies. Their main advantage lies in the fact that they
give a direct measurement of particle velocity (and not
particle position as in optical tracking), hence limiting
the increase of noise induced by the differentiation of
position to access velocity. Similarly, measuring accelera-
tion of the particles requires only one differentiation step
while second derivatives must be estimated from optical
tracking. These techniques are therefore very accurate
to investigate the Lagrangian dynamics of individual
particles. Instrumented particles have ported further the
capacity of investigation of Lagrangian properties of
flows by giving access not only to kinematic properties
(as velocity or acceleration) but also to a Lagrangian
description of almost any physical quantity for which
a relevant sensor can be embedded in the particle. The
main drawback for the moment concerns the size of the
particle, which does not allow to probe scales smaller
than about 1 cm. Finally, the acoustic vorticity mea-
surement is unique as it gives a simple and accurate way
to characterize the enstrophy spectrum of a flow, with
intrinsic spectral resolution at a selected scale (including
the smallest scales of the flow, which are hardly accessible
with such classical techniques as PIV).
Note that we have not reported here all the possi-
ble extensions and add-ons of these methods, as, for
instance, the use of digital holography [
Salazar et al.
,
2008;
Chareyron
, 2009], which allows to track particles
in 3D with one single camera, or the tracking of parti-
cle rotational dynamics [
Zimmermannetal.
, 2011a, 2011b;
Klein et al.
, 2012], which allows to simultaneously inves-
tigate the translation and rotation of finite objects trans-
ported in a flow.
Experimental techniques in fluid mechanics are being
constantly improved as new ideas combined with techno-
logical advances increase the resolution and the range of
existing methods: For instance, cameras are ever faster
and sensors better resolved; miniaturization and reduc-
tion of power consumption of electronic components will
allow to reduce the size of instrumented particles; an
important breakthrough in high resolution optical track-
ing is expected in the coming years due to FPGA (field
programmable gate array) technology, which allows to
process images on-board and hence to increase the effec-
tive data rate (for instance, particle detection could be
done on-board and only the particle positions would be
recorded). In this rapidly evolving context it is essential to
promote an efficient interaction between fluid mechanics
experimentalists and other communities, e.g., geophysi-
cists, in order to develop the appropriate instrumentation
for laboratory or field investigations.
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