3,698 research outputs found
Energy spectra of vortex distributions in two-dimensional quantum turbulence
We theoretically explore key concepts of two-dimensional turbulence in a
homogeneous compressible superfluid described by a dissipative two-dimensional
Gross-Pitaeveskii equation. Such a fluid supports quantized vortices that have
a size characterized by the healing length . We show that for the
divergence-free portion of the superfluid velocity field, the kinetic energy
spectrum over wavenumber may be decomposed into an ultraviolet regime
() having a universal scaling arising from the vortex
core structure, and an infrared regime () with a spectrum that
arises purely from the configuration of the vortices. The Novikov power-law
distribution of intervortex distances with exponent -1/3 for vortices of the
same sign of circulation leads to an infrared kinetic energy spectrum with a
Kolmogorov power law, consistent with the existence of an inertial
range. The presence of these and power laws, together with
the constraint of continuity at the smallest configurational scale
, allows us to derive a new analytical expression for the
Kolmogorov constant that we test against a numerical simulation of a forced
homogeneous compressible two-dimensional superfluid. The numerical simulation
corroborates our analysis of the spectral features of the kinetic energy
distribution, once we introduce the concept of a {\em clustered fraction}
consisting of the fraction of vortices that have the same sign of circulation
as their nearest neighboring vortices. Our analysis presents a new approach to
understanding two-dimensional quantum turbulence and interpreting similarities
and differences with classical two-dimensional turbulence, and suggests new
methods to characterize vortex turbulence in two-dimensional quantum fluids via
vortex position and circulation measurements.Comment: 19 pages, 8 figure
Vortices and turbulence in trapped atomic condensates
After over a decade of experiments generating and studying the physics of
quantized vortices in atomic gas Bose-Einstein condensates, research is
beginning to focus on the roles of vortices in quantum turbulence, as well as
other measures of quantum turbulence in atomic condensates. Such research
directions have the potential to uncover new insights into quantum turbulence,
vortices and superfluidity, and also explore the similarities and differences
between quantum and classical turbulence in entirely new settings. Here we
present a critical assessment of theoretical and experimental studies in this
emerging field of quantum turbulence in atomic condensates
Massively parallel support for a case-based planning system
Case-based planning (CBP), a kind of case-based reasoning, is a technique in which previously generated plans (cases) are stored in memory and can be reused to solve similar planning problems in the future. CBP can save considerable time over generative planning, in which a new plan is produced from scratch. CBP thus offers a potential (heuristic) mechanism for handling intractable problems. One drawback of CBP systems has been the need for a highly structured memory to reduce retrieval times. This approach requires significant domain engineering and complex memory indexing schemes to make these planners efficient. In contrast, our CBP system, CaPER, uses a massively parallel frame-based AI language (PARKA) and can do extremely fast retrieval of complex cases from a large, unindexed memory. The ability to do fast, frequent retrievals has many advantages: indexing is unnecessary; very large case bases can be used; memory can be probed in numerous alternate ways; and queries can be made at several levels, allowing more specific retrieval of stored plans that better fit the target problem with less adaptation. In this paper we describe CaPER's case retrieval techniques and some experimental results showing its good performance, even on large case bases
Snell's Law for a vortex dipole in a Bose-Einstein condensate
A quantum vortex dipole, comprised of a closely bound pair of vortices of
equal strength with opposite circulation, is a spatially localized travelling
excitation of a planar superfluid that carries linear momentum, suggesting a
possible analogy with ray optics. We investigate numerically and analytically
the motion of a quantum vortex dipole incident upon a step-change in the
background superfluid density of an otherwise uniform two-dimensional
Bose-Einstein condensate. Due to the conservation of fluid momentum and energy,
the incident and refracted angles of the dipole satisfy a relation analogous to
Snell's law, when crossing the interface between regions of different density.
The predictions of the analogue Snell's law relation are confirmed for a wide
range of incident angles by systematic numerical simulations of the
Gross-Piteavskii equation. Near the critical angle for total internal
reflection, we identify a regime of anomalous Snell's law behaviour where the
finite size of the dipole causes transient capture by the interface.
Remarkably, despite the extra complexity of the surface interaction, the
incoming and outgoing dipole paths obey Snell's law.Comment: 16 pages, 7 figures, Scipost forma
Economic Impact of Deer Breeding Operations in Texas
The deer breeding industry is a growing industry in the Texas economy, particularly the rural economy. Industry participants were surveyed to provide estimates of economic activity, which was then input into the IMPLAN model. The industry generates an estimated $652 million in economic activity, while supporting 7,335 jobs.Industrial Organization,
In situ imaging of vortices in Bose-Einstein condensates
Laboratory observations of vortex dynamics in Bose-Einstein condensates
(BECs) are essential for determination of many aspects of superfluid dynamics
in these systems. We present a novel application of dark-field imaging that
enables \texttt{\it in situ} detection of two-dimensional vortex distributions
in single-component BECs, a step towards real-time measurements of complex
two-dimensional vortex dynamics within a single BEC. By rotating a Rb
BEC in a magnetic trap, we generate a triangular lattice of vortex cores in the
BEC, with core diameters on the order of 400 nm and cores separated by
approximately 9 m. We have experimentally confirmed that the positions of
the vortex cores can be determined without the need for ballistic expansion of
the BEC.Comment: 5 pages, 4 figure
Financial Impacts of Regional Differences in Dairies
The sensitivity of net cash farm income to changes in selected production variables, output prices, and input costs varies significantly across representative U.S. dairies. Different regions of the country were impacted differently by changes to production and prices.Agricultural Finance, Q12, Q14,
Regional and Structural Impacts of Alternative Dairy Policy Options
Milk and dairy product prices have fallen to their lowest levels in 3 years following the record highs of 2004 and 2005. The large government stockpiles of non-fat dry milk are gone, but threaten to build again as non-fat dry milk and cheese prices decline nearer the support price level. A new farm bill is scheduled to be written in 2007. The Milk Income Loss Contract (MILC) program included in the last farm bill was only authorized through September 2005. Subsequent legislation reinstated the MILC program through August 2007. WTO negotiations are on-going and could influence U.S. farm programs 1/. Dairy’s role in the U.S. amber box limit of 4.2 billion toward the annual amber box limit, but actual spending only averages about 400 million in amber box payments per year. The analysis uses representative dairy farms in major milk producing regions of the country developed by the AFPC for policy analysis.Agribusiness, Agricultural and Food Policy, Livestock Production/Industries,
- …