129 research outputs found
Energy Dissipation and Trapping of Particles Moving on a Rough Surface
We report an experimental, numerical and theoretical study of the motion of a
ball on a rough inclined surface. The control parameters are , the diameter
of the ball, , the inclination angle of the rough surface and ,
the initial kinetic energy. When the angle of inclination is larger than some
critical value, , the ball moves at a constant average
velocity which is independent of the initial conditions. For an angle , the balls are trapped after moving a certain distance. The
dependence of the travelled distances on , and . is
analysed. The existence of two kinds of mechanisms of dissipation is thus
brought to light. We find that for high initial velocities the friction force
is constant. As the velocity decreases below a certain threshold the friction
becomes viscous.Comment: 8 pages RevTeX, 12 Postscript figure
TEMPO-oxidized cellulose nanofibre (TOCN) films and composites with PVOH as sensitive dielectrics for microwave humidity sensing
This paper investigates TEMPO oxidized cellulose nanofibers (so-called TOCN)films as sensitive dielectrics for humidity detection in microwave frequencies. TOCN is used either as a sensitive material or as a host-matrix enclosing polyvinyl alcohol (PVOH)to achieve highly sensitive humidity sensing. A resonator in coplanar waveguide grounded (CPWG)technology was designed and fabricated. TOCN and TOCN/PVOH gels were dropped in the area of the resonator where the analysis showed the electromagnetic field to be maximum at the resonance. Gels became thin films after drying. Experimental humidity tests were then conducted within the 21.9–89.3%RH range, using the resonant frequency and the transmission phase as measurement variables. The best sensitivity with TOCN was 2.67 MHz/%RH regarding the resonant frequency, and 0.523°/%RH regarding the transmission phase. The effects of PVOH were visible starting from 55%RH, where the sensitivity was raised to 6.000 MHz/%RH and 0.785°/%RH respectively. © 2019 Elsevier B.V
Printed microwave frequency humidity sensor operating with phase shifting scheme
This paper investigates a shifting sensing scheme combining slots, transmission lines, and printing technologies. This sensing scheme translates the electrical sensitivity of a transmission line conductor to the insertion phase as a measurement variable. A coplanar waveguide (CPW) based structure was designed, screen-printed, and tested on relative humidity (RH) conditions ranging from 22.8-75.3 %RH. For the first time, a composite material made of poly-pyrrole and TEMPO Oxidized Cellulose Nanofibers (TOCN/PPy) was integrated to the structure and studied as a humidity sensitive conductor in microwave frequencies. The measured sensitivity was 0.154°/%RH at 5.870 GHz, while insertion losses decreased by 1.26 dB. The effects of sensing layers thickness as well as trade-off considerations between phase sensitivity and signal attenuation were analyzed by simulation. © 2001-2012 IEEE
Self-Supervised Relative Depth Learning for Urban Scene Understanding
As an agent moves through the world, the apparent motion of scene elements is
(usually) inversely proportional to their depth. It is natural for a learning
agent to associate image patterns with the magnitude of their displacement over
time: as the agent moves, faraway mountains don't move much; nearby trees move
a lot. This natural relationship between the appearance of objects and their
motion is a rich source of information about the world. In this work, we start
by training a deep network, using fully automatic supervision, to predict
relative scene depth from single images. The relative depth training images are
automatically derived from simple videos of cars moving through a scene, using
recent motion segmentation techniques, and no human-provided labels. This proxy
task of predicting relative depth from a single image induces features in the
network that result in large improvements in a set of downstream tasks
including semantic segmentation, joint road segmentation and car detection, and
monocular (absolute) depth estimation, over a network trained from scratch. The
improvement on the semantic segmentation task is greater than those produced by
any other automatically supervised methods. Moreover, for monocular depth
estimation, our unsupervised pre-training method even outperforms supervised
pre-training with ImageNet. In addition, we demonstrate benefits from learning
to predict (unsupervised) relative depth in the specific videos associated with
various downstream tasks. We adapt to the specific scenes in those tasks in an
unsupervised manner to improve performance. In summary, for semantic
segmentation, we present state-of-the-art results among methods that do not use
supervised pre-training, and we even exceed the performance of supervised
ImageNet pre-trained models for monocular depth estimation, achieving results
that are comparable with state-of-the-art methods
Effect of boundary conditions on diffusion in two-dimensional granular gases
We analyze the influence of boundary conditions on numerical simulations of
the diffusive properties of a two dimensional granular gas. We show in
particular that periodic boundary conditions introduce unphysical correlations
in time which cause the coefficient of diffusion to be strongly dependent on
the system size. On the other hand, in large enough systems with hard walls at
the boundaries, diffusion is found to be independent of the system size. We
compare the results obtained in this case with Langevin theory for an elastic
gas. Good agreement is found. We then calculate the relaxation time and the
influence of the mass for a particle of radius in a sea of particles of
radius . As granular gases are dissipative, we also study the influence of
an external random force on the diffusion process in a forced dissipative
system. In particular, we analyze differences in the mean square velocity and
displacement between the elastic and inelastic cases.Comment: 15 figures eps figures, include
Environmentally-friendly cellulose nanofibre sheets for humidity sensing in microwave frequencies
This paper investigates environment friendly cellulose nanofibres (CNF) as new and sensitive material for humidity sensing in RF/microwave frequencies. CNF sheets were fabricated by sonocatalyzed TEMPO process and physically characterized. Humidity sensing investigation was performed with CNF sheets taped on the top of circuits in coplanar waveguide (CPW) technology. This investigation includes sensitivity and dynamic range analysis with reflected waves along the CPW circuit through resonant frequency shift, and transmitted waves through S21 phase shift. Moreover, sheets with various amounts of CNF were used to study the influence of CNF weight on humidity sensing performances. Regarding the resonant frequency shift, the best sensitivity was measured with the weightier CNF film (71 g/m2), that is 2.82 MHz/%RH from 55%RH to 100%RH. Regarding the phase shift, the same film sensitivity is 0.7°/%RH from 70%RH to 100%RH, with a figure of merit of 7.43°/dB as a phase shifter. © 2017 Elsevier B.V
Experimental and computational studies of jamming
Jamming is a common feature of out of equilibrium systems showing slow
relaxation dynamics. Here we review our efforts in understanding jamming in
granular materials using experiments and computer simulations. We first obtain
an estimation of an effective temperature for a slowly sheared granular
material very close to jamming. The measurement of the effective temperature is
realized in the laboratory by slowly shearing a closely-packed ensemble of
spherical beads confined by an external pressure in a Couette geometry. All the
probe particles, independent of their characteristic features, equilibrate at
the same temperature, given by the packing density of the system. This suggests
that the effective temperature is a state variable for the nearly jammed
system. Then we investigate numerically whether the effective temperature can
be obtained from a flat average over the jammed configuration at a given energy
in the granular packing, as postulated by the thermodynamic approach to grains.Comment: 20 pages, 9 figure
Numerical model for granular compaction under vertical tapping
A simple numerical model is used to simulate the effect of vertical taps on a
packing of monodisperse hard spheres. Our results are in agreement with an
experimantal work done in Chicago and with other previous models, especially
concerning the dynamics of the compaction, the influence of the excitation
strength on the compaction efficiency, and some ageing effects. The principal
asset of the model is that it allows a local analysis of the packings. Vertical
and transverse density profiles are used as well as size and volume
distributions of the pores. An interesting result concerns the appearance of a
vertical gradient in the density profiles during compaction. Furthermore, the
volume distribution of the pores suggests that the smallest pores, ranging in
size between a tetrahedral and an octahedral site, are not strongly affected by
the tapping process, in contrast to the largest pores which are more sensitive
to the compaction of the packing.Comment: 8 pages, 15 figures (eps), to be published in Phys. Rev. E. Some
corrections have been made, especially in paragraph IV
The jamming transition of Granular Media
We briefly review the basics ideas and results of a recently proposed
statistical mechanical approach to granular materials. Using lattice models
from standard Statistical Mechanics and results from a mean field replica
approach and Monte Carlo simulations we find a jamming transition in granular
media closely related to the glass transition in super-cooled liquids. These
models reproduce the logarithmic relaxation in granular compaction and
reversible-irreversible lines, in agreement with experimental data. The models
also exhibit aging effects and breakdown of the usual fluctuation dissipation
relation. It is shown that the glass transition may be responsible for the
logarithmic relaxation and may be related to the cooperative effects underlying
many phenomena of granular materials such as the Reynolds transition.Comment: 18 pages with 6 postscript figures. to appear in J.Phys: Cond. Ma
Cracking Piles of Brittle Grains
A model which accounts for cracking avalanches in piles of grains subject to
external load is introduced and numerically simulated. The stress is
stochastically transferred from higher layers to lower ones. Cracked areas
exhibit various morphologies, depending on the degree of randomness in the
packing and on the ductility of the grains. The external force necessary to
continue the cracking process is constant in wide range of values of the
fraction of already cracked grains. If the grains are very brittle, the force
fluctuations become periodic in early stages of cracking. Distribution of
cracking avalanches obeys a power law with exponent .Comment: RevTeX, 6 pages, 7 postscript figures, submitted to Phys. Rev.
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