5,348 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
The optical light curve of GRB 970228 refined
We present the R and V light curves of the optical counterpart of GRB 970228.
A critical analysis of all the available data is made in light of the results
achieved in the recent GRB Symposium held in Huntsville and by considering the
latest information from the HST images on the underlying nebulosity.Comment: 3 pages, 2 .ps figures, Nuclear Physics style file espcrc2.sty
included. To appear in the proceedings of the conference "The Active X-Ray
Sky: Results from BeppoSAX and Rossi-XTE", Rome, Italy, 21-24 October, 1997.
L. Scarsi, H. Bradt, P. Giommi and F. Fiore editors, Nuc. Phys. B Proc. Supp
Push & Pull: autonomous deployment of mobile sensors for a complete coverage
Mobile sensor networks are important for several strategic applications
devoted to monitoring critical areas. In such hostile scenarios, sensors cannot
be deployed manually and are either sent from a safe location or dropped from
an aircraft. Mobile devices permit a dynamic deployment reconfiguration that
improves the coverage in terms of completeness and uniformity.
In this paper we propose a distributed algorithm for the autonomous
deployment of mobile sensors called Push&Pull. According to our proposal,
movement decisions are made by each sensor on the basis of locally available
information and do not require any prior knowledge of the operating conditions
or any manual tuning of key parameters.
We formally prove that, when a sufficient number of sensors are available,
our approach guarantees a complete and uniform coverage. Furthermore, we
demonstrate that the algorithm execution always terminates preventing movement
oscillations.
Numerous simulations show that our algorithm reaches a complete coverage
within reasonable time with moderate energy consumption, even when the target
area has irregular shapes. Performance comparisons between Push&Pull and one of
the most acknowledged algorithms show how the former one can efficiently reach
a more uniform and complete coverage under a wide range of working scenarios.Comment: Technical Report. This paper has been published on Wireless Networks,
Springer. Animations and the complete code of the proposed algorithm are
available for download at the address:
http://www.dsi.uniroma1.it/~novella/mobile_sensors
GRBs with optical afterglow and known redshift: a statistical study
We present a correlation between two intrinsic parameters of GRB optical
afterglows. These are the isotropic luminosity at the maximum of the light
curve (Lpeak) and the time-integrated isotropic energy (Eiso) radiated after
the observed maximum. We test the correlation between the logarithms of (Eiso)
and (Lpeak) and finally we value the effect of the different samples of GRBs in
according with the first optical observation reduced to proper time.Comment: To be published in the proceedings of the conference "SWIFT and GRBs:
Unveiling the Relativistic Universe", Venice, June 5-9, 200
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