366 research outputs found
Frequency-dependent attenuation and elasticity in unconsolidated earth materials: effect of damping
We use the Discrete Element Method (DEM) to understand the underlying
attenuation mechanism in granular media, with special applicability to the
measurements of the so-called effective mass developed earlier. We consider
that the particles interact via Hertz-Mindlin elastic contact forces and that
the damping is describable as a force proportional to the velocity difference
of contacting grains. We determine the behavior of the complex-valued normal
mode frequencies using 1) DEM, 2) direct diagonalization of the relevant
matrix, and 3) a numerical search for the zeros of the relevant determinant.
All three methods are in strong agreement with each other. The real and the
imaginary parts of each normal mode frequency characterize the elastic and the
dissipative properties, respectively, of the granular medium. We demonstrate
that, as the interparticle damping, , increases, the normal modes exhibit
nearly circular trajectories in the complex frequency plane and that for a
given value of they all lie on or near a circle of radius centered on
the point in the complex plane, where . We show that each
normal mode becomes critically damped at a value of the damping parameter , where is the (real-valued) frequency when
there is no damping. The strong indication is that these conclusions carry over
to the properties of real granular media whose dissipation is dominated by the
relative motion of contacting grains. For example, compressional or shear waves
in unconsolidated dry sediments can be expected to become overdamped beyond a
critical frequency, depending upon the strength of the intergranular damping
constant.Comment: 28 pages, 7 figure
Measuring Significance of Community Structure in Complex Networks
Many complex systems can be represented as networks and separating a network
into communities could simplify the functional analysis considerably. Recently,
many approaches have been proposed for finding communities, but none of them
can evaluate the communities found are significant or trivial definitely. In
this paper, we propose an index to evaluate the significance of communities in
networks. The index is based on comparing the similarity between the original
community structure in network and the community structure of the network after
perturbed, and is defined by integrating all the similarities. Many artificial
networks and real-world networks are tested. The results show that the index is
independent from the size of network and the number of communities. Moreover,
we find the clear communities always exist in social networks, but don't find
significative communities in proteins interaction networks and metabolic
networks.Comment: 6 pages, 4 figures, 1 tabl
A framework for the successful implementation of food traceability systems in China
Implementation of food traceability systems in China faces many challenges due to the scale, diversity and complexity of China’s food supply chains. This study aims to identify critical success factors specific to the implementation of traceability systems in China. Twenty-seven critical success factors were identified in the literature. Interviews with managers at four food enterprises in a pre-study helped identify success criteria
and five additional critical success factors. These critical success factors were tested through a survey of managers in eighty-three food companies. This study identifies six dimensions for critical success factors: laws, regulations and standards; government support; consumer knowledge and support; effective management and communication; top management and vendor support; and information and system quality
Avoiding catastrophic failure in correlated networks of networks
Networks in nature do not act in isolation but instead exchange information,
and depend on each other to function properly. An incipient theory of Networks
of Networks have shown that connected random networks may very easily result in
abrupt failures. This theoretical finding bares an intrinsic paradox: If
natural systems organize in interconnected networks, how can they be so stable?
Here we provide a solution to this conundrum, showing that the stability of a
system of networks relies on the relation between the internal structure of a
network and its pattern of connections to other networks. Specifically, we
demonstrate that if network inter-connections are provided by hubs of the
network and if there is a moderate degree of convergence of inter-network
connection the systems of network are stable and robust to failure. We test
this theoretical prediction in two independent experiments of functional brain
networks (in task- and resting states) which show that brain networks are
connected with a topology that maximizes stability according to the theory.Comment: 40 pages, 7 figure
Granulocyte-colony stimulating factor in experimental stroke and its effects on infarct size and functional outcome: a systematic review
Background
Granulocyte-colony stimulating factor (G-CSF) shows promise as a treatment for stroke. This systematic review assesses G-CSF in experimental ischaemic stroke.
Methods
Relevant studies were identified with searches of Medline, Embase and PubMed. Data were extracted on stroke lesion size, neurological outcome and quality, and analysed using Cochrane Review Manager using random effects models; results are expressed as standardised mean difference (SMD) and odds ratio (OR).
Results
Data were included from 19 publications incorporating 666 animals. G-CSF reduced lesion size significantly in transient (SMD -1.63, p4 weeks post ischaemia) was not (SMD 0.76, p=0.35). Death (OR 0.27, p<0.0001) was reduced with G-CSF. Median study quality was 4 (range 0-7/8); Egger’s test suggested significant publication bias (p=0.001).
Conclusions
G-CSF significantly reduced lesion size in transient but not permanent models of ischaemic stroke. Motor impairment and death were also reduced. Further studies assessing dose-response, administration time, length of ischaemia and long-term functional recovery are needed
Manipulating infrared photons using plasmons in transparent graphene superlattices
Superlattices are artificial periodic nanostructures which can control the
flow of electrons. Their operation typically relies on the periodic modulation
of the electric potential in the direction of electron wave propagation. Here
we demonstrate transparent graphene superlattices which can manipulate infrared
photons utilizing the collective oscillations of carriers, i.e., plasmons of
the ensemble of multiple graphene layers. The superlattice is formed by
depositing alternating wafer-scale graphene sheets and thin insulating layers,
followed by patterning them all together into 3-dimensional
photonic-crystal-like structures. We demonstrate experimentally that the
collective oscillation of Dirac fermions in such graphene superlattices is
unambiguously nonclassical: compared to doping single layer graphene,
distributing carriers into multiple graphene layers strongly enhances the
plasmonic resonance frequency and magnitude, which is fundamentally different
from that in a conventional semiconductor superlattice. This property allows us
to construct widely tunable far-infrared notch filters with 8.2 dB rejection
ratio and terahertz linear polarizers with 9.5 dB extinction ratio, using a
superlattice with merely five graphene atomic layers. Moreover, an unpatterned
superlattice shields up to 97.5% of the electromagnetic radiations below 1.2
terahertz. This demonstration also opens an avenue for the realization of other
transparent mid- and far-infrared photonic devices such as detectors,
modulators, and 3-dimensional meta-material systems.Comment: under revie
Percolation in Interdependent and Interconnected Networks: Abrupt Change from Second to First Order Transition
Robustness of two coupled networks system has been studied only for
dependency coupling (S. Buldyrev et. al., Nature, 2010) and only for
connectivity coupling (E. A. Leicht and R. M. D'Souza, arxiv:09070894). Here we
study, using a percolation approach, a more realistic coupled networks system
where both interdependent and interconnected links exist. We find a rich and
unusual phase transition phenomena including hybrid transition of mixed first
and second order i.e., discontinuities like a first order transition of the
giant component followed by a continuous decrease to zero like a second order
transition. Moreover, we find unusual discontinuous changes from second order
to first order transition as a function of the dependency coupling between the
two networks.Comment: 4pages,6figure
Tuberous sclerosis complex exhibits a new renal cystogenic mechanism
Tuberous sclerosis complex (TSC) is a tumor predisposition syndrome with significant renal cystic and solid tumor disease. While the most common renal tumor in TSC, the angiomyolipoma, exhibits a loss of heterozygosity associated with disease, we have discovered that the renal cystic epithelium is composed of type A intercalated cells that have an intact Tsc gene that have been induced to exhibit Tsc‐mutant disease phenotype. This mechanism appears to be different than that for ADPKD. The murine models described here closely resemble the human disease and both appear to be mTORC1 inhibitor responsive. The induction signaling driving cystogenesis may be mediated by extracellular vesicle trafficking.TSC renal cystic disease develops in about half of the patients. The disease appears to caused by an induction mechanism such that a small population of mutant cells can cause significant renal cystic disease comprised of mostly genetically normal cells.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/147796/1/phy213983.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/147796/2/phy213983_am.pd
Use of bauxite residue (red mud) as a low cost sorbent for sulfide removal in polluted water remediation
Sulfide is an important pollutant in aqueous systems. Sulfide removal from polluted waters is required prior to discharge. Red mud RM) is a solid waste of bauxite processing that is rich in reactive iron oxides and consequently has the potential to be used to remove sulfide from aqueous systems. A series of experiments were undertaken using raw and sintered RM to remove sulfide from waters. RM was highly efficient at sulfide removal (average 75% sulfide removal at initial concentration of ~5 mg L-1, with 500 mg L-1 RM addition) due to both physical adsorption (high specific area) and chemical reaction (with amorphous Fe). Sintered RM, which has a lower surface area and lower mineral reactivity, was much less efficient at removing sulfide (~20% removal under equivalent experimental conditions). Furthermore, concomitant metal release from raw RM was lower than for sintered RM during the sulfide removal process. The results showed that raw RM is a potentially suitable material for sulfide removal from polluted waters and consequently could be used as a low cost alternative treatment in certain engineering applications
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