6,866 research outputs found
Extent of force indeterminacy in packings of frictional rigid disks
Static packings of frictional rigid particles are investigated by means of
discrete element simulations. We explore the ensemble of allowed force
realizations in the space of contact forces for a given packing structure. We
estimate the extent of force indeterminacy with different methods. The
indeterminacy exhibits a nonmonotonic dependence on the interparticle friction
coefficient. We verify directly that larger force-indeterminacy is accompanied
by a more robust behavior against local perturbations. We also investigate the
local indeterminacy of individual contact forces. The probability distribution
of local indeterminacy changes its shape depending on friction. We find that
local indeterminacy tends to be larger on force chains for intermediate
friction. This correlation disappears in the large friction limit.Comment: 5 pages, 6 figure
TB134: A Laboratory Study to Assess Methods for Predicting pH Change of Ash Amended Forest Soils
The objective of this study was to compare several commonly used lime requirement tests and soil capacity factors for their ability to predict pH change following wood-ash amendment. The ability to predict pH change is important because it is one of the criteria used to prescribe rates of ash amendment to forest soils.https://digitalcommons.library.umaine.edu/aes_techbulletin/1070/thumbnail.jp
A Dense Gas Trigger for OH Megamasers
HCN and CO line diagnostics provide new insight into the OH megamaser (OHM)
phenomenon, suggesting a dense gas trigger for OHMs. We identify three physical
properties that differentiate OHM hosts from other starburst galaxies: (1) OHMs
have the highest mean molecular gas densities among starburst galaxies; nearly
all OHM hosts have = 10^3-10^4 cm^-3 (OH line-emitting clouds likely
have n(H2) > 10^4 cm^-3). (2) OHM hosts are a distinct population in the
nonlinear part of the IR-CO relation. (3) OHM hosts have exceptionally high
dense molecular gas fractions, L(HCN)/L(CO)>0.07, and comprise roughly half of
this unusual population. OH absorbers and kilomasers generally follow the
linear IR-CO relation and are uniformly distributed in dense gas fraction and
L(HCN), demonstrating that OHMs are independent of OH abundance. The fraction
of non-OHMs with high mean densities and high dense gas fractions constrains
beaming to be a minor effect: OHM emission solid angle must exceed 2 pi
steradians. Contrary to conventional wisdom, IR luminosity does not dictate OHM
formation; both star formation and OHM activity are consequences of tidal
density enhancements accompanying galaxy interactions. The OHM fraction in
starbursts is likely due to the fraction of mergers experiencing a temporal
spike in tidally driven density enhancement. OHMs are thus signposts marking
the most intense, compact, and unusual modes of star formation in the local
universe. Future high redshift OHM surveys can now be interpreted in a star
formation and galaxy evolution context, indicating both the merging rate of
galaxies and the burst contribution to star formation.Comment: 5 pages, 3 figures, 1 table, accepted by ApJ Letter
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
Heparan sulfate proteoglycans are concentrated on the sinusoidal plasmalemmal domain and in intracellular organelles of hepatocytes
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