11,695 research outputs found
Influence of Water Depth on the Rate of Expansion of Giant Cutgrass Populations and Management Implications
Giant cutgrass (
Zizaniopsis miliacea
(Michx.) Doell. &
Asch.), a tall emergent grass native to the southeastern United
States, was studied in Lake Seminole where it formed
large expanding stands, and Lake Alice where it was confined
to a stable narrow fringe
Precision Measurements of Stretching and Compression in Fluid Mixing
The mixing of an impurity into a flowing fluid is an important process in
many areas of science, including geophysical processes, chemical reactors, and
microfluidic devices. In some cases, for example periodic flows, the concepts
of nonlinear dynamics provide a deep theoretical basis for understanding
mixing. Unfortunately, the building blocks of this theory, i.e. the fixed
points and invariant manifolds of the associated Poincare map, have remained
inaccessible to direct experimental study, thus limiting the insight that could
be obtained. Using precision measurements of tracer particle trajectories in a
two-dimensional fluid flow producing chaotic mixing, we directly measure the
time-dependent stretching and compression fields. These quantities, previously
available only numerically, attain local maxima along lines coinciding with the
stable and unstable manifolds, thus revealing the dynamical structures that
control mixing. Contours or level sets of a passive impurity field are found to
be aligned parallel to the lines of large compression (unstable manifolds) at
each instant. This connection appears to persist as the onset of turbulence is
approached.Comment: 5 pages, 5 figure
Spin Susceptibility of Noncentrosymmetric Heavy-fermion Superconductor CeIrSi3 under Pressure: 29Si-Knight Shift Study on Single Crystal
We report 29Si-NMR study on a single crystal of the heavy-fermion
superconductor CeIrSi3 without an inversion symmetry along the c-axis. The
29Si-Knight shift measurements under pressure have revealed that the spin
susceptibility for the ab-plane decreases slightly below Tc, whereas along the
c-axis it does not change at all. The result can be accounted for by the spin
susceptibility in the superconducting state being dominated by the strong
antisymmetric (Rashba-type) spin-orbit interaction that originates from the
absence of an inversion center along the c-axis and it being much larger than
superconducting condensation energy. This is the first observation which
exhibits an anisotropy of the spin susceptibility below Tc in the
noncentrosymmetric superconductor dominated by strong Rashba-type spin-orbit
interaction.Comment: 4 pages, 4 figures, Accepted for publication in Phys. Rev. Let
Specifications and programs for computer software validation
Three software products developed during the study are reported and include: (1) FORTRAN Automatic Code Evaluation System, (2) the Specification Language System, and (3) the Array Index Validation System
Parabolic resonances and instabilities in near-integrable two degrees of freedom Hamiltonian flows
When an integrable two-degrees-of-freedom Hamiltonian system possessing a
circle of parabolic fixed points is perturbed, a parabolic resonance occurs. It
is proved that its occurrence is generic for one parameter families
(co-dimension one phenomenon) of near-integrable, t.d.o. systems. Numerical
experiments indicate that the motion near a parabolic resonance exhibits new
type of chaotic behavior which includes instabilities in some directions and
long trapping times in others. Moreover, in a degenerate case, near a {\it flat
parabolic resonance}, large scale instabilities appear. A model arising from an
atmospherical study is shown to exhibit flat parabolic resonance. This supplies
a simple mechanism for the transport of particles with {\it small} (i.e.
atmospherically relevant) initial velocities from the vicinity of the equator
to high latitudes. A modification of the model which allows the development of
atmospherical jets unfolds the degeneracy, yet traces of the flat instabilities
are clearly observed
A configuration system for the ATLAS trigger
The ATLAS detector at CERN's Large Hadron Collider will be exposed to
proton-proton collisions from beams crossing at 40 MHz that have to be reduced
to the few 100 Hz allowed by the storage systems. A three-level trigger system
has been designed to achieve this goal. We describe the configuration system
under construction for the ATLAS trigger chain. It provides the trigger system
with all the parameters required for decision taking and to record its history.
The same system configures the event reconstruction, Monte Carlo simulation and
data analysis, and provides tools for accessing and manipulating the
configuration data in all contexts.Comment: 4 pages, 2 figures, contribution to the Conference on Computing in
High Energy and Nuclear Physics (CHEP06), 13.-17. Feb 2006, Mumbai, Indi
An Assessment of NASA Aeropropulsion Technologies: A System Study
Aviation industry s robust growth rate has given rise to growing concerns about the contribution that aviation emissions will make to local air quality and global climate change. Over the last several years, NASA has been engaged in the development of aeropropulsion technologies with specific objectives to reduce aircraft emissions. A system analysis was performed to evaluate the potential impact of these propulsion technologies on aircraft CO2 (directly proportional to fuel burn) and NOx reductions. A large subsonic aircraft, with two 396-kN thrust (85,000-pound) engines was chosen for the study. Performance benefit estimates are presented for each technology, with a summary of potential emissions reduction possible from the development of these technologies. The results show that NASA s aeropropulsion technologies have the potential to significantly reduce the CO2 and NO(x) emissions. The results are used to support informed decision-making on the development of aeropropulsion technology portfolio for CO2 and NO(x) reductions
Hammerhead, an ultrahigh resolution ePix camera for wavelength-dispersive spectrometers
Wavelength-dispersive spectrometers (WDS) are often used in synchrotron and
FEL applications where high energy resolution (in the order of eV) is
important. Increasing WDS energy resolution requires increasing spatial
resolution of the detectors in the dispersion direction. The common approaches
with strip detectors or small pixel detectors are not ideal. We present a novel
approach, with a sensor using rectangular pixels with a high aspect ratio
(between strips and pixels, further called "strixels"), and strixel
redistribution to match the square pixel arrays of typical ASICs while avoiding
the considerable effort of redesigning ASICs. This results in a sensor area of
17.4 mm x 77 mm, with a fine pitch of 25 m in the horizontal direction
resulting in 3072 columns and 176 rows. The sensors use ePix100 readout ASICs,
leveraging their low noise (43 e, or 180 eV rms). We present results
obtained with a Hammerhead ePix100 camera, showing that the small pitch (25
m) in the dispersion direction maximizes performance for both high and low
photon occupancies, resulting in optimal WDS energy resolution. The low noise
level at high photon occupancy allows precise photon counting, while at low
occupancy, both the energy and the subpixel position can be reconstructed for
every photon, allowing an ultrahigh resolution (in the order of 1 m) in
the dispersion direction and rejection of scattered beam and harmonics. Using
strixel sensors with redistribution and flip-chip bonding to standard ePix
readout ASICs results in ultrahigh position resolution (1 m) and low
noise in WDS applications, leveraging the advantages of hybrid pixel detectors
(high production yield, good availability, relatively inexpensive) while
minimizing development complexity through sharing the ASIC, hardware, software
and DAQ development with existing versions of ePix cameras.Comment: 8 pages, 6 figure
Electrical properties of isotopically enriched neutron-transmutation-doped ^{70} Ge:Ga near the metal-insulator transition
We report the low temperature carrier transport properties of a series of
nominally uncompensated neutron-transmutation doped (NTD) ^{70} Ge:Ga samples
very close to the critical concentration N_c for the metal-insulator
transition. The concentration of the sample closest to N_c is 1.0004N_c and it
is unambiguously shown that the critical conductivity exponent is 0.5.
Properties of insulating samples are discussed in the context of Efros and
Shklovskii's variable range hopping conduction.Comment: 8 pages using REVTeX, 8 figures, published versio
- …