Geoscience Reference
In-Depth Information
and Shew et al. [2007] for more details). The entire mobile
circuitry is powered with a coin cell battery which condi-
tions the duration of the measurement, about 3 h. Using
an antenna and RF receiver and amplifiers, it is possible
to acquire directly the demodulated signal oversampling
with a high-speed DACQ. The square-wave signal fre-
quency is then directly measured using the standard Lab-
view library, and time-varying frequency is converted to
temperature using frequency-temperature calibration of
the thermistors. The time resolution for this distant tem-
perature measurement is about 10 Hz, which was suitable
for instance, for studying turbulent thermal convection in
laboratory experiments.
tracking velocimetry, it becomes possible to study the
correlations between position and temperature as shown
in Figure 15.18 (bottom). As all kinematic quantities
(velocity or acceleration) can be obtained from PTV data,
it is then possible to have information about the turbulent
heat flux q =
(with v and T fluctuating velocity
and temperature) in the whole experiment volume with
insight form the role of plumes in heat transport [ Gasteuil
et al. , 2007; Shew et al. , 2007].
v T
15.4.2. Further Lagrangian Measurements
More recently the smart particle concept was extended
to measurements of acceleration using numerical mod-
ulation and demodulation with suitable electronics
[ Zimmermann et al. , 2012]. The apparatus was designed
from the earlier work by Gasteuil [2009] and built by
smartINST S.A.S., an offspring company of CNRS and
ENS de Lyon. It is a spherical particle with diameter
25 mm which embarks an autonomous circuit with
a 3D acceleration sensor, a coin cell, and a wireless
transmission system. It transfers the measured data to a
data acquisition center (smartCENTER) which decodes,
processes, and stores the signal delivered by the smart
particle. The smart particle and smartCENTER measure,
display, and store the three-dimensional acceleration vec-
tors acting on the particle as it is advected in the flow. The
acceleration sensor is an ADXL 330 (Analog Device).
The three axes of the ADXL 330 are decoupled and form
an orthogonal coordinate system attached to the chip
package. This arrangement yields a 3D measurement of
acceleration, including gravity, with a full scale of
Measurement in Rayleigh-Bénard Convection. The par-
ticle was first used to investigate natural convection in a
square tank with size 30 cm at high Rayleigh numbers,
Ra
10 10 . Figure 15.18 (top) shows the time evolution
of temperature along the particle trajectory with irreg-
ular oscillations caused by the motion of the particle
crossing cold and hot regions in the vessel. Combining
Lagrangian temperature measurement and particle
30
29.5
29
28.5
28
27.5
27 0
500
1000
1500 2000
Time (s)
2500
3000
3500
3 g .
The sensor has to be calibrated to compute the physical
acceleration from the voltages of the accelerometer.
±
0.4
30
0.35
29.5
0.3
Other Sensors the smart particle technology allows the
transmition of additional information originating from
other sensors. Current developments aim at conductivity
measurements which are of interest for salinity measure-
ments and mixing issues in stratified flows.
29
0.25
0.2
28.5
0.15
28
15.5. CONCLUSIONS AND DISCUSSION
0.1
27.5
0.05
We have reported here some of the latest developments
in Lagrangian characterization of flow dynamics in lab-
oratory experiments. Although the Lagrangian approach
has been already identified by Taylor and Richardson
as a relevant description of geophysical flows, it is
only recently that technological progress has allowed to
develop platforms with sufficient accuracy and resolution.
Some of these techniques, including 3D PTV and instru-
mented particles, have already been successfully used in
model experiments with atmospheric and oceanographic
motivations. But most of their advantages are still to be
27
0
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
Horizontal position (m)
Figure 15.18. Top: Temporal evolution of temperature along
a particle trajectory in turbulent Rayleigh-Bénard convection
with square aspect ratio. Bottom: Combined PTV-instrumented
particle measurement showing the 2 d position of the particle
(X(t) , Y(t)) with local temperature T(t) (see colorbar for values
of temperature in Celsius). From Shew et al. [2007].
 
Search WWH ::




Custom Search