298 research outputs found
Constraints and vibrations in static packings of ellipsoidal particles
We numerically investigate the mechanical properties of static packings of
ellipsoidal particles in 2D and 3D over a range of aspect ratio and compression
. While amorphous packings of spherical particles at jamming onset
() are isostatic and possess the minimum contact number required for them to be collectively jammed, amorphous packings of
ellipsoidal particles generally possess fewer contacts than expected for
collective jamming () from naive counting arguments, which
assume that all contacts give rise to linearly independent constraints on
interparticle separations. To understand this behavior, we decompose the
dynamical matrix for static packings of ellipsoidal particles into two
important components: the stiffness and stress matrices. We find that
the stiffness matrix possesses eigenmodes
with zero eigenvalues even at finite compression, where is the number of
particles. In addition, these modes are nearly eigenvectors of the
dynamical matrix with eigenvalues that scale as , and thus finite
compression stabilizes packings of ellipsoidal particles. At jamming onset, the
harmonic response of static packings of ellipsoidal particles vanishes, and the
total potential energy scales as for perturbations by amplitude
along these `quartic' modes, . These findings illustrate
the significant differences between static packings of spherical and
ellipsoidal particles.Comment: 18 pages, 21 figure
An Analysis of the Impact of Local Drought Conditions on Gross Sales in the Lake Hartwell Region
Proceedings of the 2011 Georgia Water Resources Conference, April 11, 12, and 13, 2011, Athens, Georgia.Lake Hartwell is a United States Army Corps of Engineers (USACE) impoundment of the Savannah River constructed between 1955 and 1963 as a part of a flood control, navigation and hydropower project on the border of South Carolina and Georgia. In addition to the original reasons for its creation, the lake is increasingly used today for tourism and recreational purposes, as well as issues related to water quality, water supply, and fish and wildlife management. As economic activity on and around the lake has intensified, the perceived importance of the lake as a driver of economic activity has correspondingly strengthened. Increased interest in Lake Hartwell has resulted in the creation of a number of stakeholder associations with the objective of protecting the economic interests of lake property owners and lake-oriented businesses. These stakeholders have increasingly brought their economic concerns into discussions of lake management. The multiyear regional droughts of 1999 to 2003 and 2006 to 2009 escalated stakeholder concerns about the economic impact of prolonged low lake levels on lake-oriented real estate and businesses. As drought conditions worsened throughout 2007 and 2008, stakeholders increasingly called for policy changes that would take greater account of the economic and tourism concerns of lake stakeholders. This research documents the unique relationship between consumer spending activity and Lake Hartwell lake levels for the six county region bordering the lake. While this research is portion of a much larger analysis documenting the economic impacts of drought conditions on the region, this piece of the project is informative for understanding the complexities of the relationship between general economic activity and lake levels. This analysis covers the six counties that border Lake Hartwell: Franklin, Hart, and Stephens counties in Georgia, and Anderson, Oconee, and Pickens counties in South Carolina.Sponsored by:
Georgia Environmental Protection Division
U.S. Geological Survey, Georgia Water Science Center
U.S. Department of Agriculture, Natural Resources Conservation Service
Georgia Institute of Technology, Georgia Water Resources Institute
The University of Georgia, Water Resources FacultyThis book was published by Warnell School of Forestry and Natural Resources, The University of Georgia, Athens, Georgia 30602-2152. The views and statements advanced in this publication are solely those of the authors and do not represent official views or policies of The University of Georgia, the U.S. Geological Survey, the Georgia Water Research Institute as authorized by the Water Research Institutes Authorization Act of 1990 (P.L. 101-307) or the other conference sponsors
N,N′-Bis[(E)-2-fluorobenzylidene]-1-(2-fluorophenyl)methanediamine
In the title compound, C21H15F3N2, the benzene ring bonded to the central C atom forms dihedral angles of 77.5 (7) and 89.0 (5)°, respectively, with the remaining two benzene rings. Weak intermolecular C—H⋯F hydrogen bonds link the molecules into chains propagated in [101]. The crystal packing exhibits weak π–π interactions as evidenced by relatively short distances between the centroids of the aromatic rings [3.820 (7) and 3.971 (5) Å]. A MOPAC PM3 optimization of the molecular geometry in vacuo supports a suggestion that intermolecular forces have a significnt influence on the molecular conformation in the crystal
Multi-phonon Resonant Raman Scattering Predicted in LaMnO3 from the Franck-Condon Process via Self-Trapped Excitons
Resonant behavior of the Raman process is predicted when the laser frequency
is close to the orbital excitation energy of LaMnO3 at 2 eV. The incident
photon creates a vibrationally excited self-trapped ``orbiton'' state from the
orbitally-ordered Jahn-Teller (JT) ground state. Trapping occurs by local
oxygen rearrangement. Then the Franck-Condon mechanism activates multiphonon
Raman scattering. The amplitude of the -phonon process is first order in the
electron-phonon coupling . The resonance occurs {\it via} a dipole forbidden
to transition. We previously suggested that this transition (also seen
in optical reflectivity) becomes allowed because of asymmetric oxygen
fluctuations. Here we calculate the magnitude of the corresponding matrix
element using local spin-density functional theory. This calculation agrees to
better than a factor of two with our previous value extracted from experiment.
This allows us to calculate the absolute value of the Raman tensor for
multiphonon scattering. Observation of this effect would be a direct
confirmation of the importance of the JT electron-phonon term and the presence
of self-trapped orbital excitons, or ``orbitons''.Comment: 8 pages and 3 embedded figures. The earlier short version is now
replaced by a more complete paper with a slightly different title. This
version includes a caculation by density-functional theory of the dipole
matrix element for exciting the self-trapped orbital exciton which activates
the multiphonon Raman signal
Force distributions near the jamming and glass transitions
We calculate the distribution of interparticle normal forces near the
glass and jamming transitions in model supercooled liquids and foams,
respectively. develops a peak that appears near the glass or jamming
transitions, whose height increases with decreasing temperature, decreasing
shear stress and increasing packing density. A similar shape of was
observed in experiments on static granular packings. We propose that the
appearance of this peak signals the development of a yield stress. The
sensitivity of the peak to temperature, shear stress and density lends credence
to the recently proposed generalized jamming phase diagram.Comment: 4 pages, 3 postscript figures;Version 3 replaces figure 1 and removes
figure 2 from version 1. Significant rewording of version 1 to emphasize the
formation of peak in P(F) when these systems jam along five different routes
of the recently proposed jamming phase diagram. Version 2 displayed the
incorrect abstrac
Random Packings of Frictionless Particles
We study random packings of frictionless particles at T=0.
The packing fraction where the pressure becomes nonzero is the same as the
jamming threshold, where the static shear modulus becomes nonzero. The
distribution of threshold packing fractions narrows and its peak approaches
random close-packing as the system size increases. For packing fractions within
the peak, there is no self-averaging, leading to exponential decay of the
interparticle force distribution.Comment: 4 pages, 3 figure
Regional Differences in Prevalence of HIV-1 Discordance in Africa and Enrollment of HIV-1 Discordant Couples into an HIV-1 Prevention Trial
Background: Most HIV-1 transmission in Africa occurs among HIV-1-discordant couples (one partner HIV-1 infected and one uninfected) who are unaware of their discordant HIV-1 serostatus. Given the high HIV-1 incidence among HIV-1 discordant couples and to assess efficacy of interventions for reducing HIV-1 transmission, HIV-1 discordant couples represent a critical target population for HIV-1 prevention interventions and prevention trials. Substantial regional differences exist in HIV-1 prevalence in Africa, but regional differences in HIV-1 discordance among African couples, has not previously been reported. Methodology/Principal Findings: The Partners in Prevention HSV-2/HIV-1 Transmission Trial (“Partners HSV-2 Study”), the first large HIV-1 prevention trial in Africa involving HIV-1 discordant couples, completed enrollment in May 2007. Partners HSV-2 Study recruitment data from 12 sites from East and Southern Africa were used to assess HIV-1 discordance among couples accessing couples HIV-1 counseling and testing, and to correlate with enrollment of HIV-1 discordant couples. HIV-1 discordance at Partners HSV-2 Study sites ranged from 8–31% of couples tested from the community. Across all study sites and, among all couples with one HIV-1 infected partner, almost half (49%) of couples were HIV-1 discordant. Site-specific monthly enrollment of HIV-1 discordant couples into the clinical trial was not directly associated with prevalence of HIV-1 discordance, but was modestly correlated with national HIV-1 counseling and testing rates and access to palliative care/basic health care (r = 0.74, p = 0.09). Conclusions/Significance: HIV-1 discordant couples are a critical target for HIV-1 prevention in Africa. In addition to community prevalence of HIV-1 discordance, national infrastructure for HIV-1 testing and healthcare delivery and effective community outreach strategies impact recruitment of HIV-1 discordant couples into HIV-1 prevention trials
Interferometric imaging with the 32 element Murchison Wide-field Array
The Murchison Wide-field Array (MWA) is a low frequency radio telescope,
currently under construction, intended to search for the spectral signature of
the epoch of re-ionisation (EOR) and to probe the structure of the solar
corona. Sited in Western Australia, the full MWA will comprise 8192 dipoles
grouped into 512 tiles, and be capable of imaging the sky south of 40 degree
declination, from 80 MHz to 300 MHz with an instantaneous field of view that is
tens of degrees wide and a resolution of a few arcminutes. A 32-station
prototype of the MWA has been recently commissioned and a set of observations
taken that exercise the whole acquisition and processing pipeline. We present
Stokes I, Q, and U images from two ~4 hour integrations of a field 20 degrees
wide centered on Pictoris A. These images demonstrate the capacity and
stability of a real-time calibration and imaging technique employing the
weighted addition of warped snapshots to counter extreme wide field imaging
distortions.Comment: Accepted for publication in PASP. This is the draft before journal
typesetting corrections and proofs so does contain formatting and journal
style errors, also has with lower quality figures for space requirement
Timing of immune escape linked to success or failure of vaccination
Successful vaccination against HIV should limit viral replication sufficiently to prevent the emergence of viral immune escape mutations. Broadly directed immunity is likely to be required to limit opportunities for immune escape variants to flourish. We studied the emergence of an SIV Gag cytotoxic T cell immune escape variant in pigtail macaques expressing the Mane-A*10 MHC I allele using a quantitative RT-PCR to measure viral loads of escape and wild type variants. Animals receiving whole Gag expressing vaccines completely controlled an SIVmac251 challenge, had broader CTL responses and exhibited minimal CTL escape. In contrast, animals vaccinated with only a single CTL epitope and challenged with the same SIVmac251 stock had high levels of viral replication and rapid CTL escape. Unvaccinated naïve animals exhibited a slower emergence of immune escape variants. Thus narrowly directed vaccination against a single epitope resulted in rapid immune escape and viral levels equivalent to that of naïve unvaccinated animals. These results emphasize the importance of inducing broadly directed HIV-specific immunity that effectively quashes early viral replication and limits the generation of immune escape variants. This has important implications for the selection of HIV vaccines for expanded human trials.<br /
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