963 research outputs found
Lagrangian temperature, velocity and local heat flux measurement in Rayleigh-Benard convection
We have developed a small, neutrally buoyant, wireless temperature sensor.
Using a camera for optical tracking, we obtain simultaneous measurements of
position and temperature of the sensor as it is carried along by the flow in
Rayleigh-B\'enard convection, at . We report on statistics of
temperature, velocity, and heat transport in turbulent thermal convection. The
motion of the sensor particle exhibits dynamics close to that of Lagrangian
tracers in hydrodynamic turbulence. We also quantify heat transport in plumes,
revealing self-similarity and extreme variations from plume to plume.Comment: 4 page
Ergodic and non-ergodic clustering of inertial particles
We compute the fractal dimension of clusters of inertial particles in mixing
flows at finite values of Kubo (Ku) and Stokes (St) numbers, by a new series
expansion in Ku. At small St, the theory includes clustering by Maxey's
non-ergodic 'centrifuge' effect. In the limit of St to infinity and Ku to zero
(so that Ku^2 St remains finite) it explains clustering in terms of ergodic
'multiplicative amplification'. In this limit, the theory is consistent with
the asymptotic perturbation series in [Duncan et al., Phys. Rev. Lett. 95
(2005) 240602]. The new theory allows to analyse how the two clustering
mechanisms compete at finite values of St and Ku. For particles suspended in
two-dimensional random Gaussian incompressible flows, the theory yields
excellent results for Ku < 0.2 for arbitrary values of St; the ergodic
mechanism is found to contribute significantly unless St is very small. For
higher values of Ku the new series is likely to require resummation. But
numerical simulations show that for Ku ~ St ~ 1 too, ergodic 'multiplicative
amplification' makes a substantial contribution to the observed clustering.Comment: 4 pages, 2 figure
Optically switched magnetism in photovoltaic perovskite CHNH(Mn:Pb)I
The demand for ever-increasing density of information storage and speed of
manipulation boosts an intense search for new magnetic materials and novel ways
of controlling the magnetic bit. Here, we report the synthesis of a
ferromagnetic photovoltaic CHNH(Mn:Pb)I material in which the
photo-excited electrons rapidly melt the local magnetic order through the
Ruderman-Kittel-Kasuya-Yosida interactions without heating up the spin system.
Our finding offers an alternative, very simple and efficient way of optical
spin control, and opens an avenue for applications in low power, light
controlling magnetic devices
Probing quantum and classical turbulence analogy through global bifurcations in a von K\'arm\'an liquid Helium experiment
We report measurements of the dissipation in the Superfluid Helium high
REynold number von Karman flow (SHREK) experiment for different forcing
conditions, through a regime of global hysteretic bifurcation. Our
macroscopical measurements indicate no noticeable difference between the
classical fluid and the superfluid regimes, thereby providing evidence of the
same dissipative anomaly and response to asymmetry in fluid and superfluid
regime. %In the latter case, A detailed study of the variations of the
hysteretic cycle with Reynolds number supports the idea that (i) the stability
of the bifurcated states of classical turbulence in this closed flow is partly
governed by the dissipative scales and (ii) the normal and the superfluid
component at these temperatures (1.6K) are locked down to the dissipative
length scale.Comment: 5 pages, 5 figure
Thoracic trident pigmentation in Drosophila melanogaster: latitudinal and altitudinal clines in Indian populations
International audienc
Design and operation of a field telescope for cosmic ray geophysical tomography
International audienceThe cosmic ray muon tomography gives an access to the density structure of geological targets. In the present article we describe a muon telescope adapted to harsh environmental conditions. In particular the design optimizes the total weight and power consumption to ease the deployment and increase the autonomy of the detector. The muon telescopes consist of at least two scintillator detection matrices readout by photosensors via optical fibres. Two photosensor options have been studied. The baseline option foresees one multianode photomultiplier (MAPM) per matrix. A second option using one multipixel photon counter (MPPC) per bar is under development. The readout electronics and data acquisition system developed for both options are detailed. We present a first data set acquired in open-sky conditions compared with the muon flux detected across geological objects
Is the Earth's magnetic field a constant ? a legacy of Poisson
In the report he submitted to the Acad\'emie des Sciences, Poisson imagined a
set of concentric spheres at the origin of the Earth's magnetic field. It may
come as a surprise to many that Poisson as well as Gauss both considered the
magnetic field to be constant. We propose in this study to test this surprising
assertion for the first time evoked by Poisson (1826). First, we will present a
development of Maxwell's equations in the framework of a static electric field
and a static magnetic field in order to draw the necessary consequences for the
Poisson hypothesis. In a second step, we will see if the observations can be in
agreement with Poisson (1826). To do so, we have chosen to compare 1) the polar
motion drift and the secular variation of the Earth's magnetic field, 2) the
seasonal pseudo-cycles of day length together with those of the sea level
recorded by different tide gauges around the globe and those of the Earth's
magnetic field recorded in different magnetic observatories. We then propose a
mechanism, in the spirit of Poisson, to explain the presence of the 11-year in
the magnetic field. We test this mechanism with observations and finally we
study closely the evolution of the g10 coefficient of the IGFR over time
On variations of global mean surface temperature: When Laplace meets Milankovi\'c
In his mathematical theory, Milankovic finds a link between the heat received
by the Earth surface per unit time as a function of the solar ephemerids and
derives a model of climate changes at periods longer than a few thousand years
and more. In this paper, we investigate the potential connections of global
temperature and Earth rotation at much shorter periods, in the complementary
range of one to a few hundred years. For temperature, we select the HadCrut05.
For Earth rotation, defined by pole coordinates and length of day, we use the
IERS data sets. Using iterative Singular Spectrum Analysis (iSSA), we extract
the trend and quasi-periodic components of these time series. The first
quasi-periodic components (period ~80-90 years) are expressions of the
Gleissberg cycle and are identical (at the level of uncertainty of the data).
Taken together, the trend and Gleissberg components allow one to reconstruct
87% of the variance of the data for lod and 48% for temperature. The next four
iSSA components, with periods ~40, 22, 15 and 9 years. The Lagrange and Laplace
theories imply that the derivative of pole motion should be identical to lod
variations: this strong check is passed by the trend + Gleissberg
reconstructions. The annual oscillations of pole motion and lod are linked to
annual variations in Sun-Earth distance, in agreement with an astronomical, but
not a climatic origin. The results obtained in this paper for the observed
temperature/rotation couple add to the growing list of evidence of solar and
planetary forcings of gravitational nature on a number of geophysical processes
(including sea-level, sea-level pressure, sea-ice extent, oceanic climate
indices).Comment: 13 pages, 13 figure
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