14,901 research outputs found
Acoustic, thermal and flow processes in a water filled nanoporous glasses by time-resolved optical spectroscopy
We present heterodyne detected transient grating measurements on water filled
Vycor 7930 in the range of temperature 20 - 90 degrees C. This experimental
investigation enables to measure the acoustic propagation, the average density
variation due the liquid flow and the thermal diffusion in this water filled
nano-porous material. The data have been analyzed with the model of Pecker and
Deresiewicz which is an extension of Biot model to account for the thermal
effects. In the whole temperature range the data are qualitatively described by
this hydrodynamic model that enables a meaningful insight of the different
dynamic phenomena. The data analysis proves that the signal in the intermediate
and long time-scale can be mainly addressed to the water dynamics inside the
pores. We proved the existence of a peculiar interplay between the mass and the
heat transport that produces a flow and back-flow process inside the
nano-pores. During this process the solid and liquid dynamics have opposite
phase as predicted by the Biot theory for the slow diffusive wave.
Nevertheless, our experimental results confirm that transport of elastic energy
(i.e. acoustic propagation), heat (i.e. thermal diffusion) and mass (i.e.
liquid flow) in a liquid filled porous glass can be described according to
hydrodynamic laws in spite of nanometric dimension of the pores. The data
fitting, based on the hydrodynamic model, enables the extraction of several
parameters of the water-Vycor system, even if some discrepancies appear when
they are compared with values reported in the literature.Comment: 32 pages, 11 figure
Pulsar Wind Nebulae as a source of the observed electron and positron excess at high energy: the case of Vela-X
We investigate, in terms of production from pulsars and their nebulae, the
cosmic ray positron and electron fluxes above GeV, observed by the
AMS-02 experiment up to 1 TeV. We concentrate on the Vela-X case. Starting from
the gamma-ray photon spectrum of the source, generated via synchrotron and
inverse Compton processes, we estimated the electron and positron injection
spectra. Several features are fixed from observations of Vela-X and unknown
parameters are borrowed from the Crab nebula. The particle spectra produced in
the pulsar wind nebula are then propagated up to the Solar System, using a
diffusion model. Differently from previous works, the omnidirectional intensity
excess for electrons and positrons is obtained as a difference between the
AMS-02 data and the corresponding local interstellar spectrum. An equal amount
of electron and positron excess is observed and we interpreted this excess
(above 100 GeV in the AMS-02 data) as a supply coming from Vela-X. The
particle contribution is consistent with models predicting the gamma-ray
emission at the source. The input of a few more young pulsars is also allowed,
while below 100 GeV more aged pulsars could be the main contributors.Comment: Accepted for publication in Journal of High Energy Astrophysics
(2015
Slow dynamics in a turbulent von K\'arm\'an swirling flow
We present an experimental study of a turbulent von K\'arm\'an flow produced
in a cylindrical container using two propellers. The mean flow is stationary up
to , where a bifurcation takes place. The new regime breaks some
symmetries of the problem, and is time-dependent. The axisymmetry is broken by
the presence of equatorial vortices with a precession movement, being the
velocity of the vortices proportional to the Reynolds number. The reflection
symmetry through the equatorial plane is broken, and the shear layer of the
mean flow appears displaced from the equator. These two facts appear
simultaneously. In the exact counterrotating case, a bistable regime appears
between both mirrored solutions and spontaneous reversals of the azimuthal
velocity are registered. This evolution can be explained using a three-well
potential model with additive noise. A regime of forced periodic response is
observed when a very weak input signal is applied.Comment: Improved model, additional results and figures, accepted in PR
Agricultura de precisão: fundamentos, aplicações e perspectivas para a cultura do arroz.
O uso da agricultura de precisão vem se tornando cada vez mais frequente nos países em desenvolvimento e desenvolvidos onde, como nos Estados Unidos e Japão, os governos vêm enfatizando a pesquisa e o desenvolvimento nesta área como uma estratégia nacional (NRC, 1997; Japanese Government, 1998).bitstream/CNPDIA/9498/1/CT30_99.pd
Uso de microcontroladores e realidade virtual na agricultura.
bitstream/CNPDIA/10059/1/CT22_97.pd
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