6,715 research outputs found
The Role of Projection in the Control of Bird Flocks
Swarming is a conspicuous behavioural trait observed in bird flocks, fish
shoals, insect swarms and mammal herds. It is thought to improve collective
awareness and offer protection from predators. Many current models involve the
hypothesis that information coordinating motion is exchanged between neighbors.
We argue that such local interactions alone are insufficient to explain the
organization of large flocks of birds and that the mechanism for the exchange
of long-ranged information necessary to control their density remains unknown.
We show that large flocks self-organize to the maximum density at which a
typical individual is still just able to see out of the flock in many
directions. Such flocks are marginally opaque - an external observer can also
just still see a substantial fraction of sky through the flock. Although
seemingly intuitive we show that this need not be the case; flocks could easily
be highly diffuse or entirely opaque. The emergence of marginal opacity
strongly constrains how individuals interact with each other within large
swarms. It also provides a mechanism for global interactions: An individual can
respond to the projection of the flock that it sees. This provides for faster
information transfer and hence rapid flock dynamics, another advantage over
local models. From a behavioural perspective it optimizes the information
available to each bird while maintaining the protection of a dense, coherent
flock.Comment: PNAS early edition published online at
http://www.pnas.org/cgi/doi/10.1073/pnas.140220211
Synchronization of dynamical hypernetworks: dimensionality reduction through simultaneous block-diagonalization of matrices
We present a general framework to study stability of the synchronous solution
for a hypernetwork of coupled dynamical systems. We are able to reduce the
dimensionality of the problem by using simultaneous block-diagonalization of
matrices. We obtain necessary and sufficient conditions for stability of the
synchronous solution in terms of a set of lower-dimensional problems and test
the predictions of our low-dimensional analysis through numerical simulations.
Under certain conditions, this technique may yield a substantial reduction of
the dimensionality of the problem. For example, for a class of dynamical
hypernetworks analyzed in the paper, we discover that arbitrarily large
networks can be reduced to a collection of subsystems of dimensionality no more
than 2. We apply our reduction techique to a number of different examples,
including a class of undirected unweighted hypermotifs of three nodes.Comment: 9 pages, 6 figures, accepted for publication in Phys. Rev.
Monitoring multicomponent transport using in-situ FTIR spectroscopy
The permeation of aqueous solutes through membranes is of critical importance in many applications, including fuel cells and solar-driven electrochemical cells. In these devices, the undesirable crossover of small organics through a membrane separating halves of the electrochemical cell reduces device efficiency. The techniques traditionally used to measure organic permeation through a membrane, such as gas chromatography, typically require aliquot sampling and are limited by the chromatographâs capability to resolve analytes and water. In-situ FTIR spectroscopy, is capable of quantitative measurement of organic permeation through membranes in a simple diffusion cell without aliquot sampling. Furthermore, in-situ FTIR is capable of the simultaneous measurement of multicomponent permeation, provided that each analyte has a distinct IR signature.
Within the context of devices for solar fuel production, membranes are desired that facilitate the ion transport necessary to feed the electrochemical reactions while meeting various additional selectivity and permeability demands. For example, in an artificial photosynthesis device, water is oxidized to O2 and CO2 is reduced to a liquid transportation fuel, such as methanol or ethanol. However, the catalysts responsible for CO2 reduction are non-specific, producing a library of small molecule products. Here, we demonstrate the use of in-situ FTIR spectroscopy to quantitatively determine the concentration of single and multicomponent mixtures of various CO2 reduction products including methanol, formate and acetate. We then apply this methodology to the in-situ monitoring of the permeation of single and multicomponent mixtures across commercially available membranes, such as Nafion 117 and Selemion AMV, using a diffusion cell. Membrane permeabilities are extracted from time-resolved half-cell concentration data using fee volume models. Membrane permeabilities and selectivities calculated from the single component permeation experiments are compared to those calculated for solutes in multicomponent permeation experiments. In some instances, the membrane permeability to an organic solute changes substantially, sometimes by orders of magnitude, when co-solutes are present. These results and their implications will be discussed in the context of next-generation energy devices.
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Radio Properties of z>4 Optically-Selected Quasars
We report on two programs to address differential evolution between the
radio-loud and radio-quiet quasar populations at high (z>4) redshift. Both
programs entail studying the radio properties of optically-selected quasars.
First, we have observed 32 optically-selected, high-redshift (z>4) quasars with
the VLA at 6 cm (5 GHz). These sources comprise a statistically complete and
well-understood sample. We detect four quasars above our 3-sigma limit of ~0.15
mJy, which is sufficiently sensitive to detect all radio-loud quasars at the
probed redshift range. Second, we have correlated 134 z>4 quasars, comprising
all such sources that we are aware of as of mid-1999, with FIRST and NVSS.
These two recent 1.4 GHz VLA sky surveys reach 3-sigma limits of approximately
0.6 mJy and 1.4 mJy respectively. We identify a total of 15 z>4 quasars, of
which six were not previously known to be radio-loud. The depth of these
surveys does not reach the radio-loud/radio-quiet demarcation luminosity
density (L(1.4 GHz) = 10^32.5 h(50)^(-2) ergs/s/Hz) at the redshift range
considered; this correlation therefore only provides a lower limit to the
radio-loud fraction of quasars at high-redshift. The two programs together
identify eight new radio-loud quasars at z>4, a significant increase over the
seven currently in the published literature. We find no evidence for radio-loud
fraction depending on optical luminosity for -25 > M_B > -28 at z~2, or for
-26>M_B>-28 at z>4. Our results also show no evolution in the radio-loud
fraction between z~2 and z>4 (-26>M_B>-28).Comment: 19 pages, 7 figures; to appear in The Astronomical Journal (April
2000
Catalytic Ignition and Upstream Reaction Propagation in Monolith Reactors
Using numerical simulations, this work demonstrates a concept called back-end ignition for lighting-off and pre-heating a catalytic monolith in a power generation system. In this concept, a downstream heat source (e.g. a flame) or resistive heating in the downstream portion of the monolith initiates a localized catalytic reaction which subsequently propagates upstream and heats the entire monolith. The simulations used a transient numerical model of a single catalytic channel which characterizes the behavior of the entire monolith. The model treats both the gas and solid phases and includes detailed homogeneous and heterogeneous reactions. An important parameter in the model for back-end ignition is upstream heat conduction along the solid. The simulations used both dry and wet CO chemistry as a model fuel for the proof-of-concept calculations; the presence of water vapor can trigger homogenous reactions, provided that gas-phase temperatures are adequately high and there is sufficient fuel remaining after surface reactions. With sufficiently high inlet equivalence ratio, back-end ignition occurs using the thermophysical properties of both a ceramic and metal monolith (coated with platinum in both cases), with the heat-up times significantly faster for the metal monolith. For lower equivalence ratios, back-end ignition occurs without upstream propagation. Once light-off and propagation occur, the inlet equivalence ratio could be reduced significantly while still maintaining an ignited monolith as demonstrated by calculations using complete monolith heating
Approximating Tverberg Points in Linear Time for Any Fixed Dimension
Let P be a d-dimensional n-point set. A Tverberg-partition of P is a
partition of P into r sets P_1, ..., P_r such that the convex hulls conv(P_1),
..., conv(P_r) have non-empty intersection. A point in the intersection of the
conv(P_i)'s is called a Tverberg point of depth r for P. A classic result by
Tverberg implies that there always exists a Tverberg partition of size n/(d+1),
but it is not known how to find such a partition in polynomial time. Therefore,
approximate solutions are of interest.
We describe a deterministic algorithm that finds a Tverberg partition of size
n/4(d+1)^3 in time d^{O(log d)} n. This means that for every fixed dimension we
can compute an approximate Tverberg point (and hence also an approximate
centerpoint) in linear time. Our algorithm is obtained by combining a novel
lifting approach with a recent result by Miller and Sheehy (2010).Comment: 14 pages, 2 figures. A preliminary version appeared in SoCG 2012.
This version removes an incorrect example at the end of Section 3.
Deformation of the Pacific/North America plate boundary at Queen Charlotte Fault : the possible role of rheology
Published 2018. This article is a U.S. Government work and is in the public domain in the USA. The definitive version was published in Journal of Geophysical Research: Solid Earth 123 (2018): 4223-4242, doi:10.1002/2017JB014770.The Pacific/North America (PA/NA) plate boundary between Vancouver Island and Alaska is similar to the PA/NA boundary in California in its kinematic history and the rate and azimuth of current relative motion, yet their deformation styles are distinct. The California plate boundary shows a broad zone of parallel strike slip and thrust faults and folds, whereas the 49âmm/yr PA/NA relative plate motion in Canada and Alaska is centered on a single, narrow, continuous ~900âkmâlong fault, the Queen Charlotte Fault (QCF). Using gravity analysis, we propose that this plate boundary is centered on the continent/ocean boundary (COB), an unusual location for continental transform faults because plate boundaries typically localize within the continental lithosphere, which is weaker. Because the COB is a boundary between materials of contrasting elastic properties, once a fault is established there, it will probably remain stable. We propose that deformation progressively shifted to the COB in the wake of Yakutat terrane's northward motion along the margin. Minor convergence across the plate boundary is probably accommodated by fault reactivation on Pacific crust and by an eastward dipping QCF. Underthrusting of Pacific slab under Haida Gwaii occurs at convergence angles >14°â15° and may have been responsible for the emergence of the archipelago. The calculated slab entry dip (5°â8°) suggests that the slab probably does not extend into the asthenosphere. The PA/NA plate boundary at the QCF can serve as a structurally simple site to investigate the impact of rheology and composition on crustal deformation and the initiation of slab underthrusting
A Hard Look at NGC 5347: Revealing a Nearby Compton-thick AGN
Current measurements show that the observed fraction of Compton-thick (CT) active galactic nuclei (AGN) is smaller than the expected values needed to explain the cosmic X-ray background. Prior fits to the X-ray spectrum of the nearby Seyfert-2 galaxy NGC 5347 (z = 0.00792, D = 35.5 Mpc ) have alternately suggested a CT and Compton-thin source. Combining archival data from Suzaku, Chandra, andâmost importantlyânew data from NuSTAR, ... See full text for complete abstrac
Predilection for developing a hematogenous orthopaedic implant-associated infection in older versus younger mice
BACKGROUND: The pathogenesis of hematogenous orthopaedic implant-associated infections (HOIAI) remains largely unknown, with little understanding of the influence of the physis on bacterial seeding. Since the growth velocity in the physis of long bones decreases during aging, we sought to evaluate the role of the physis on influencing the development of Staphylococcus aureus HOIAI in a mouse model comparing younger versus older mice.
METHODS: In a mouse model of HOIAI, a sterile Kirschner wire was inserted retrograde into the distal femur of younger (5-8-week-old) and older (14-21-week-old) mice. After a 3-week convalescent period, a bioluminescent Staphylococcus aureus strain was inoculated intravenously. Bacterial dissemination to operative and non-operative legs was monitored longitudinally in vivo for 4 weeks, followed by ex vivo bacterial enumeration and X-ray analysis.
RESULTS: In vivo bioluminescence imaging and ex vivo CFU enumeration of the bone/joint tissue demonstrated that older mice had a strong predilection for developing a hematogenous infection in the operative legs but not the non-operative legs. In contrast, this predilection was less apparent in younger mice as the infection occurred at a similar rate in both the operative and non-operative legs. X-ray imaging revealed that the operative legs of younger mice had decreased femoral length, likely due to the surgical and/or infectious insult to the more active physis, which was not observed in older mice. Both age groups demonstrated substantial reactive bone changes in the operative leg due to infection.
CONCLUSIONS: The presence of an implant was an important determinant for developing a hematogenous orthopaedic infection in older but not younger mice, whereas younger mice had a similar predilection for developing periarticular infection whether or not an implant was present. On a clinical scale, diagnosing HOIAI may be difficult particularly in at-risk patients with limited examination or other data points. Understanding the influence of age on developing HOIAI may guide clinical surveillance and decision-making in at-risk patients
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