127,162 research outputs found
Experimental evidence of ageing and slow restoration of the weak-contact configuration in tilted 3D granular packings
Granular packings slowly driven towards their instability threshold are
studied using a digital imaging technique as well as a nonlinear acoustic
method. The former method allows us to study grain rearrangements on the
surface during the tilting and the latter enables to selectively probe the
modifications of the weak-contact fraction in the material bulk. Gradual ageing
of both the surface activity and the weak-contact reconfigurations is observed
as a result of repeated tilt cycles up to a given angle smaller than the angle
of avalanche. For an aged configuration reached after several consecutive tilt
cycles, abrupt resumption of the on-surface activity and of the weak-contact
rearrangements occurs when the packing is subsequently inclined beyond the
previous maximal tilting angle. This behavior is compared with literature
results from numerical simulations of inclined 2D packings. It is also found
that the aged weak-contact configurations exhibit spontaneous restoration
towards the initial state if the packing remains at rest for tens of minutes.
When the packing is titled forth and back between zero and near-critical
angles, instead of ageing, the weak-contact configuration exhibits "internal
weak-contact avalanches" in the vicinity of both the near-critical and zero
angles. By contrast, the stronger-contact skeleton remains stable
Understanding Android Obfuscation Techniques: A Large-Scale Investigation in the Wild
In this paper, we seek to better understand Android obfuscation and depict a
holistic view of the usage of obfuscation through a large-scale investigation
in the wild. In particular, we focus on four popular obfuscation approaches:
identifier renaming, string encryption, Java reflection, and packing. To obtain
the meaningful statistical results, we designed efficient and lightweight
detection models for each obfuscation technique and applied them to our massive
APK datasets (collected from Google Play, multiple third-party markets, and
malware databases). We have learned several interesting facts from the result.
For example, malware authors use string encryption more frequently, and more
apps on third-party markets than Google Play are packed. We are also interested
in the explanation of each finding. Therefore we carry out in-depth code
analysis on some Android apps after sampling. We believe our study will help
developers select the most suitable obfuscation approach, and in the meantime
help researchers improve code analysis systems in the right direction
Certified Computation from Unreliable Datasets
A wide range of learning tasks require human input in labeling massive data.
The collected data though are usually low quality and contain inaccuracies and
errors. As a result, modern science and business face the problem of learning
from unreliable data sets.
In this work, we provide a generic approach that is based on
\textit{verification} of only few records of the data set to guarantee high
quality learning outcomes for various optimization objectives. Our method,
identifies small sets of critical records and verifies their validity. We show
that many problems only need verifications, to
ensure that the output of the computation is at most a factor of away from the truth. For any given instance, we provide an
\textit{instance optimal} solution that verifies the minimum possible number of
records to approximately certify correctness. Then using this instance optimal
formulation of the problem we prove our main result: "every function that
satisfies some Lipschitz continuity condition can be certified with a small
number of verifications". We show that the required Lipschitz continuity
condition is satisfied even by some NP-complete problems, which illustrates the
generality and importance of this theorem.
In case this certification step fails, an invalid record will be identified.
Removing these records and repeating until success, guarantees that the result
will be accurate and will depend only on the verified records. Surprisingly, as
we show, for several computation tasks more efficient methods are possible.
These methods always guarantee that the produced result is not affected by the
invalid records, since any invalid record that affects the output will be
detected and verified
Rain water transport and storage in a model sandy soil with hydrogel particle additives
We study rain water infiltration and drainage in a dry model sandy soil with
superabsorbent hydrogel particle additives by measuring the mass of retained
water for non-ponding rainfall using a self-built 3D laboratory set-up. In the
pure model sandy soil, the retained water curve measurements indicate that
instead of a stable horizontal wetting front that grows downward uniformly, a
narrow fingered flow forms under the top layer of water-saturated soil. This
rain water channelization phenomenon not only further reduces the available
rain water in the plant root zone, but also affects the efficiency of soil
additives, such as superabsorbent hydrogel particles. Our studies show that the
shape of the retained water curve for a soil packing with hydrogel particle
additives strongly depends on the location and the concentration of the
hydrogel particles in the model sandy soil. By carefully choosing the particle
size and distribution methods, we may use the swollen hydrogel particles to
modify the soil pore structure, to clog or extend the water channels in sandy
soils, or to build water reservoirs in the plant root zone
Effects of grain shape on packing and dilatancy of sheared granular materials
Granular material exposed to shear shows a variety of unique phenomena:
Reynolds dilatancy, positional order and orientational order effects may
compete in the shear zone. We study granular packings consisting of macroscopic
prolate, oblate and spherical grains and compare their behaviour. X-ray
tomography is used to determine the particle positions and orientations in a
cylindrical split bottom shear cell. Packing densities and the arrangements of
individual particles in the shear zone are evaluated. For anisometric
particles, we observe the competition of two opposite effects. One the one
hand, the sheared granulate is dilated, but on the other hand the particles
reorient and align with respect to the streamlines. Even though aligned
cylinders in principle may achieve higher packing densities, this alignment
compensates for the effect of dilatancy only partially. The complex
rearrangements lead to a depression of the surface above the well oriented
region while neigbouring parts still show the effect of dilation in the form of
heaps. For grains with isotropic shapes, the surface remains rather flat.
Perfect monodisperse spheres crystallize in the shear zone, whereby positional
order partially overcompensates dilatancy effects. However, already slight
deviations from the ideal monodisperse sphere shape inhibit crystallization.Comment: 12 pages, 13 figures, accepted in Soft Matte
Unjamming due to local perturbations in granular packings with and without gravity
We investigate the unjamming response of disordered packings of frictional
hard disks with the help of computer simulations. First, we generate jammed
configurations of the disks and then force them to move again by local
perturbations. We study the spatial distribution of the stress and displacement
response and find long range effects of the perturbation in both cases. We
record the penetration depth of the displacements and the critical force that
is needed to make the system yield. These quantities are tested in two types of
systems: in ideal homogeneous packings in zero gravity and in packings settled
under gravity. The penetration depth and the critical force are sensitive to
the interparticle friction coefficient. Qualitatively, the same nonmonotonic
friction dependence is found both with and without gravity, however the
location of the extrema are at different friction values. We discuss the role
of the connectivity of the contact network and of the pressure gradient in the
unjamming response.Comment: 12 pages, 13 figure
Causes of accidents on construction sites: the case of a large construction contractor in Great Britain
In the construction industry in Great Britain, it is estimated that workplace accidents and work-related ill-health cost society £3 billion – this is equivalent to 4% of the construction industry revenue of about £75 billion. Thus, the need to study, understand and effectively manage health and safety (H&S) on construction sites cannot be overemphasised. This paper presents an analysis of accident data recorded by a large construction contractor in Great Britain. The data cover a period of 36 months from April 2004 to March 2007. Pareto analysis was used to determine the relative importance of the causes of accidents on the basis of number of workdays lost. Differences between the four sectors (highways, infrastructure, rail and utilities)
in which the company operates were investigated. The case study suggests that the main causes of accidents on construction sites relate to individual attitudes towards
H&S. Ability and willingness to implement safe approaches to working and an awareness of their own and others’ H&S can contribute to safe performances. It is suggested that the company could increase awareness of H&S issues among the workforce. This should be done on a regular basis through effective training, briefing
and debriefing
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