1,826 research outputs found
Mini High Velocity Clouds
Neutral hydrogen mapping of several small, very low column density High
Velocity Clouds (HVC), using the Arecibo telescope, is reported. Some were
found serendipitously superimposed at distinct velocities on largerHVCs; others
were found to comprise the four most isolated low column density sources in the
observations of Lockman et al. but proved to be individually much smaller than
the Green Bank 140 Foot telescope beam. We call these clouds ``mini-HVC'' to
distinguish them from the larger and denser Compact High Velocity Clouds
(CHVC). Peak column densities are typically a few X 10^18 cm^-2 averaged over
the Arecibo 3.2' beam, and diameters to our detection limit ~ 10^18 cm^-2 range
from 9' to 35'. These column densities and angular diameters overlap with those
for CHVC, but are typically smaller. We consider three possiblities: (1) that
most mini-HVC are related to the Magellanic Stream, (2) that most mini-HVC are
part of M31's retinue of CHVC, or (3) that the mini-HVC are simply the low
column density tail of the distribution of CHVC. None of these possibilities
can be rejected as yet, given the selection biases in our sample. We also
discuss controversies about the amount of ionized hydrogen in CHVC and
mini-HVC, which may be mainly ionized, and the implications of these small
clouds for Lyman Limit Systems.Comment: Astronomical Journal, in pres
Minimum Tillage for Corn
Minimum tillage for corn discusses methods of planting, including strip processing, modified rototilling, hard ground listing, chisel plowing, wheel track planting, plow planting, and once-over tillage. Included are advantages and disadvantages of water infiltration, soil erosion and temperature, water evaporation, soil compaction and aeration, soil structure, and adaption to soil types. Problems, such as weed control, fertilizer placement, insect control, and the economics of minimum tillage, as also discussed
Minimum Tillage for Corn
This publication explains several minimum tillage planting methods such as strip processing, till, chisel, wheel track, and plow planting, modified rototilling, hard ground listing, and once-over tillage. Problems, advantages, and disadvantages of minimum tillage are also discussed
High Velocity Cloud Edges and Mini-HVCs
Arecibo mapping is reported of the neutral hydrogen distribution along
selected directions out from the centers of two small High Velocity Clouds
(HVC), W486 and W491. Both HVCs have a small inner region where the neutral
hydrogen column density N_HI decreases slowly and a larger outer region where
N_HI declines more rapidly, smoothly and exponentially from ~ 2 X 10^19 atoms
cm^-2 down to < 10^18 atoms cm^-2. Line widths, and presumably temperature and
turbulence, do not increase in the outermost regions. Therefore pressure
decreases smoothly, making confinement by dark matter gravity more likely than
confinement by external pressure.
The more extended HVC, W491, has a superimposed small cloud (which we dub a
``mini-HVC''), offset by 66 km s^-1 in velocity along the line of sight with
peak column density about 5 X 10^18 atoms cm^-2. Preliminary data toward future
mapping of two more HVCs reveals two more mini-HVCs of similarly small size and
central column density a bit less than 1 X 10^19 atoms cm^-2. We suggest that
these three mini-HVCs are not physically associated with the HVCs on which they
are superimposed, but are either very small outlyers of the extended Magellanic
Stream HVC complex or more distant and/or smaller isolated CHVCs.
The value of N_HI at the point where the neutral and ionized column densities
are equal is ~ 2 X 10^19 atoms cm^-2 for the two mapped HVCs. Therefore the
angular scalelength of the total hydrogen is appreciably larger than the
observed HI scalelength. Previous distance estimates, related to absolute size
and mass of the total hydrogen cloud, may have to be scaled down because of the
undetected, more extended ionized hydrogen.Comment: 24 pages, 8 figures, ApJ in pres
Small changes at single nodes can shift global network dynamics
Understanding the sensitivity of a system's behavior with respect to
parameter changes is essential for many applications. This sensitivity may be
desired - for instance in the brain, where a large repertoire of different
dynamics, particularly different synchronization patterns, is crucial - or may
be undesired - for instance in power grids, where disruptions to
synchronization may lead to blackouts. In this work, we show that the dynamics
of networks of phase oscillators can acquire a very large and complex
sensitivity to changes made in either their units' parameters or in their
connections - even modifications made to a parameter of a single unit can
radically alter the global dynamics of the network in an unpredictable manner.
As a consequence, each modification leads to a different path to phase
synchronization manifested as large fluctuations along that path. This
dynamical malleability occurs over a wide parameter region, around the
network's two transitions to phase synchronization. One transition is induced
by increasing the coupling strength between the units, and another is induced
by increasing the prevalence of long-range connections. Specifically, we study
Kuramoto phase oscillators connected under either Watts-Strogatz or
distance-dependent topologies to analyze the statistical properties of the
fluctuations along the paths to phase synchrony. We argue that this increase in
the dynamical malleability is a general phenomenon, as suggested by both
previous studies and the theory of phase transitions.Comment: 14 pages, 8 figure
THE CO LUM BIA RIVER GORGE THE STORY OF THE RIVER AND TH E ROCKS
xxxxxxxxxxxxxxxxxxxx Subscription rate- .25 each Second closs postage pai
Simulating the Clamped Tapered Beam Specimen Under Quasi-Static and Fatigue Loading Using Floating Node Method
As part of the NASA Advanced Composites Project (ACP), a sub-element has been designed to provide validation data for progressive damage analysis models. The clamped tapered beam is a cross-ply laminated composite specimen designed to validate the simulation of the onset of matrix cracks and their interaction with delaminations, including delamination migration. A tapered geometry was used to localize the first damage occurrence in the tapered region, without prescribing an initial crack. The boundary and loading conditions were chosen to favor delamination growth and subsequent migration after the first damage occurrence. The typical sequence of events consists of a matrix crack located at the tapered region, leading to delamination onset, followed by delamination growth and subsequent delamination migration to a different interface via a dominant matrix crack. The Clamped Tapered Beam (CTB) was tested in both quasi-static and fatigue regimes. The results obtained are used in this study to assess and validate a methodology based on the Floating Node Method (FNM) implemented as an Extended Interface Element. In this methodology, quasi-static and fatigue damage formation and development are modeled by combining FNM to represent crack networks, with Directional Cohesive Zone Elements (DCZE) and Virtual Crack Closure Technique (VCCT), respectively. Qualitatively, the methodology is capable of predicting the sequence of events and overall failure morphology. Quantitatively, the simulation results generally bound the experimental data, based on the range of the characterization data used. In this paper, the results from quasi static and fatigue simulations are compared and correlated with experimental data
- …