10,747 research outputs found
Experimental evidence of shock mitigation in a Hertzian tapered chain
We present an experimental study of the mechanical impulse propagation
through a horizontal alignment of elastic spheres of progressively decreasing
diameter , namely a tapered chain. Experimentally, the diameters of
spheres which interact via the Hertz potential are selected to keep as close as
possible to an exponential decrease, , where the
experimental tapering factor is either ~% or ~%.
In agreement with recent numerical results, an impulse initiated in a
monodisperse chain (a chain of identical beads) propagates without shape
changes, and progressively transfer its energy and momentum to a propagating
tail when it further travels in a tapered chain. As a result, the front pulse
of this wave decreases in amplitude and accelerates. Both effects are
satisfactorily described by the hard spheres approximation, and basically, the
shock mitigation is due to partial transmissions, from one bead to the next, of
momentum and energy of the front pulse. In addition when small dissipation is
included, a better agreement with experiments is found. A close analysis of the
loading part of the experimental pulses demonstrates that the front wave adopts
itself a self similar solution as it propagates in the tapered chain. Finally,
our results corroborate the capability of these chains to thermalize
propagating impulses and thereby act as shock absorbing devices.Comment: ReVTeX, 7 pages with 6 eps, accepted for Phys. Rev. E (Related papers
on http://www.supmeca.fr/perso/jobs/
A USB3.0 FPGA Event-based Filtering and Tracking Framework for Dynamic Vision Sensors
Dynamic vision sensors (DVS) are frame-free sensors
with an asynchronous variable-rate output that is ideal for hard
real-time dynamic vision applications under power and latency
constraints. Post-processing of the digital sensor output can
reduce sensor noise, extract low level features, and track objects
using simple algorithms that have previously been implemented
in software. In this paper we present an FPGA-based framework
for event-based processing that allows uncorrelated-event noise
removal and real-time tracking of multiple objects, with dynamic
capabilities to adapt itself to fast or slow and large or small
objects. This framework uses a new hardware platform based on
a Lattice FPGA which filters the sensor output and which then
transmits the results through a super-speed Cypress FX3 USB
microcontroller interface to a host computer. The packets of
events and timestamps are transmitted to the host computer at
rates of 10 Mega events per second. Experimental results are
presented that demonstrate a low latency of 10us for tracking
and computing the center of mass of a detected object.Ministerio de EconomÃa y Competitividad TEC2012-37868-C04-0
Experimental evidence of solitary wave interaction in Hertzian chains
We study experimentally the interaction between two solitary waves that
approach one to another in a linear chain of spheres interacting via the Hertz
potential. When these counter propagating waves collide, they cross each other
and a phase shift respect to the noninteracting waves is introduced, as a
result of the nonlinear interaction potential. This observation is well
reproduced by our numerical simulations and it is shown to be independent of
viscoelastic dissipation at the beads contact. In addition, when the collision
of equal amplitude and synchronized counter propagating waves takes place, we
observe that two secondary solitary waves emerge from the interacting region.
The amplitude of secondary solitary waves is proportional to the amplitude of
incident waves. However, secondary solitary waves are stronger when the
collision occurs at the middle contact in chains with even number of beads.
Although numerical simulations correctly predict the existence of these waves,
experiments show that their respective amplitude are significantly larger than
predicted. We attribute this discrepancy to the rolling friction at the beads
contacts during solitary wave propagation
EVALUATION OF ALTERNATIVE CONFIGURATIONS OF A WATER-OIL HEAT EXCHANGER SYSTEM
The success in performance of equipment at industrial facilities is associated with its capacity of normal operation. When some equipment is subject to maintenance at large cost, the operation is uninteresting or even impracticable. The heat exchanger is an equipment widely employed, which performs transfer of heat between two fluids at different temperatures, separated by a solid wall. In this work, we evaluate alternative configurations of a system of shell-and-tube heat exchangers, aiming to improve the performance of this system in economical point of view, and to mitigate the maintenance cost. We use the effectiveness method to obtain outlet temperature values, and compare such results with values measured in situ. We conclude that alternative configurations are feasible in order to improve the thermal performance of the system of heat exchangers, reducing installation cost
A causal model for a closed universe
We study a closed model of a universe filled with viscous fluid and
quintessence matter components. The dynamical equations imply that the universe
might look like an accelerated flat Friedmann-Robertson-Walker (FRW) universe
at low redshift. We consider here dissipative processes which obey a causal
thermodynamics. Here, we account for the entropy production via causal
dissipative inflation.Comment: 9 pages. Accepted for publication in IJMP
Chaos around a H\'enon-Heiles-inspired exact perturbation of a black hole
A solution of the Einstein's equations that represents the superposition of a
Schwarszchild black hole with both quadrupolar and octopolar terms describing a
halo is exhibited. We show that this solution, in the Newtonian limit, is an
analog to the well known H\'enon-Heiles potential. The integrability of orbits
of test particles moving around a black hole representing the galactic center
is studied and bounded zones of chaotic behavior are found.Comment: 7 pages Revte
Neutrino helicity asymmetries in leptogenesis
It is pointed out that the heavy singlet neutrinos characteristic of
leptogenesis develop asymmetries in the abundances of the two helicity states
as a result of the same mechanism that generates asymmetries in the standard
lepton sector. Neutrinos and standard leptons interchange asymmetries in
collisions with each other. It is shown that an appropriate quantum number,
B-L', combining baryon, lepton and neutrino asymmetries, is not violated as
fast as the standard B-L. This suppresses the washout effects relevant for the
derivation of the final baryon asymmetry. One presents detailed calculations
for the period of neutrino thermal production in the framework of the singlet
seesaw mechanism.Comment: 11 pages, 1 figure, revtex, matches PRD versio
How Hertzian solitary waves interact with boundaries in a 1-D granular medium
We perform measurements, numerical simulations, and quantitative comparisons
with available theory on solitary wave propagation in a linear chain of beads
without static preconstrain. By designing a nonintrusive force sensor to
measure the impulse as it propagates along the chain, we study the solitary
wave reflection at a wall. We show that the main features of solitary wave
reflection depend on wall mechanical properties. Since previous studies on
solitary waves have been performed at walls without these considerations, our
experiment provides a more reliable tool to characterize solitary wave
propagation. We find, for the first time, precise quantitative agreements.Comment: Proof corrections, ReVTeX, 11 pages, 3 eps (Focus and related papers
on http://www.supmeca.fr/perso/jobs/
Causal amplitudes in the Schwinger model at finite temperature
We show, in the imaginary time formalism, that the temperature dependent
parts of all the retarded (advanced) amplitudes vanish in the Schwinger model.
We trace this behavior to the CPT invariance of the theory and give a physical
interpretation of this result in terms of forward scattering amplitudes of
on-shell thermal particles.Comment: 4 pages with 5 figures, two minor typos corrected, to appear in
Physical Review
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