1,194 research outputs found
Manufacturing with the Sun
Concentrated solar radiation is now a viable alternative source for many advanced manufacturing processes. Researchers at the National Renewable Energy Laboratory (NREL) have demonstrated the feasibility of processes such as solar induced surface transformation of materials (SISTM), solar based manufacturing, and solar pumped lasers. Researchers are also using sunlight to decontaminate water and soils polluted with organic compounds; these techniques could provide manufacturers with innovative alternatives to traditional methods of waste management. The solar technology that is now being integrated into today's manufacturing processes offer greater potential for tomorrow, especially as applied to the radiation abundant environment available in space and on the lunar surface
VLT observations of the asymmetric Etched Hourglass Nebula, MyCn 18
Context. The mechanisms that form extreme bipolar planetary nebulae remain
unclear. Aims. The physical properties, structure, and dynamics of the bipolar
planetary nebula, MyCn 18, are investigated in detail with the aim of
understanding the shaping mechanism and evolutionary history of this object.
Methods. VLT infrared images, VLT ISAAC infrared spectra, and long-slit optical
Echelle spectra are used to investigate MyCn 18. Morpho-kinematic modelling was
used to firmly constrain the structure and kinematics of the source. A
timescale analysis was used to determine the kinematical age of the nebula and
its main components. Results. A spectroscopic study of MyCn 18's central and
offset region reveals the detailed make-up of its nebular composition.
Molecular hydrogen, atomic helium, and Bracket gamma emission are detected from
the central regions of MyCn 18. ISAAC spectra from a slit position along the
narrow waist of the nebula demonstrate that the ionised gas resides closer to
the centre of the nebula than the molecular emission. A kinematical age of the
nebula and its components were obtained by the P-V arrays and timescale
analysis. Conclusions. The structure and kinematics of MyCn 18 are better
understood using an interactive 3-D modelling tool called shape. A dimensional
and timescale analysis of MyCn 18's major components provides a possible
mechanism for the nebula's asymmetry. The putative central star is somewhat
offset from the geometric centre of the nebula, which is thought to be the
result of a binary system. We speculate that the engulfing and destruction of
an exoplanet during the AGB phase may have been a key event in shaping MyCn 18
and generating of its hypersonic knotty outflow.Comment: 15 pages, 3 tables, 13 figures. Accepted for publication by A&
The Population of Tiny Near-Earth Objects Observed by NEOWISE
Only a very small fraction of the asteroid population at size scales
comparable to the object that exploded over Chelyabinsk, Russia has been
discovered to date, and physical properties are poorly characterized. We
present previously unreported detections of 106 close approaching near-Earth
objects (NEOs) by the Wide-field Infrared Survey Explorer mission's NEOWISE
project. These infrared observations constrain physical properties such as
diameter and albedo for these objects, many of which are found to be smaller
than 100 m. Because these objects are intrinsically faint, they were detected
by WISE during very close approaches to the Earth, often at large apparent
on-sky velocities. We observe a trend of increasing albedo with decreasing
size, but as this sample of NEOs was discovered by visible light surveys, it is
likely that selection biases against finding small, dark NEOs influence this
finding.Comment: Accepted to Ap
Coordinating government and community support for community language teaching in Australia: Overview with special attention to New South Wales
An overview of formal government language-in-education planning for community languages (CLs) that has been undertaken in Australia and New South Wales is provided, moving from the more informal programmes provided in the 1980s to school-oriented programmes and training at the turn of the century. These programmes depend on community support; for many of the teachers from the communities, methodological training is needed to complement their language and cultural skills. At the same time, Commonwealth (Federal) and State support for CL programmes has improved their quality and provides students with opportunities to study CLs at the senior secondary matriculation level. The paper concludes with specific recommendations for greater recognition of CL schools and for greater attention to CL teacher preparation
Magnetohydrodynamic activity inside a sphere
We present a computational method to solve the magnetohydrodynamic equations
in spherical geometry. The technique is fully nonlinear and wholly spectral,
and uses an expansion basis that is adapted to the geometry:
Chandrasekhar-Kendall vector eigenfunctions of the curl. The resulting lower
spatial resolution is somewhat offset by being able to build all the boundary
conditions into each of the orthogonal expansion functions and by the
disappearance of any difficulties caused by singularities at the center of the
sphere. The results reported here are for mechanically and magnetically
isolated spheres, although different boundary conditions could be studied by
adapting the same method. The intent is to be able to study the nonlinear
dynamical evolution of those aspects that are peculiar to the spherical
geometry at only moderate Reynolds numbers. The code is parallelized, and will
preserve to high accuracy the ideal magnetohydrodynamic (MHD) invariants of the
system (global energy, magnetic helicity, cross helicity). Examples of results
for selective decay and mechanically-driven dynamo simulations are discussed.
In the dynamo cases, spontaneous flips of the dipole orientation are observed.Comment: 15 pages, 19 figures. Improved figures, in press in Physics of Fluid
Small scale structures in three-dimensional magnetohydrodynamic turbulence
We investigate using direct numerical simulations with grids up to 1536^3
points, the rate at which small scales develop in a decaying three-dimensional
MHD flow both for deterministic and random initial conditions. Parallel current
and vorticity sheets form at the same spatial locations, and further
destabilize and fold or roll-up after an initial exponential phase. At high
Reynolds numbers, a self-similar evolution of the current and vorticity maxima
is found, in which they grow as a cubic power of time; the flow then reaches a
finite dissipation rate independent of Reynolds number.Comment: 4 pages, 3 figure
Large scale flow effects, energy transfer, and self-similarity on turbulence
The effect of large scales on the statistics and dynamics of turbulent
fluctuations is studied using data from high resolution direct numerical
simulations. Three different kinds of forcing, and spatial resolutions ranging
from 256^3 to 1024^3, are being used. The study is carried out by investigating
the nonlinear triadic interactions in Fourier space, transfer functions,
structure functions, and probability density functions. Our results show that
the large scale flow plays an important role in the development and the
statistical properties of the small scale turbulence. The role of helicity is
also investigated. We discuss the link between these findings and
intermittency, deviations from universality, and possible origins of the
bottleneck effect. Finally, we briefly describe the consequences of our results
for the subgrid modeling of turbulent flows
Convection-driven kinematic dynamos at low Rossby and magnetic Prandtl numbers
Most large-scale planetary magnetic fields are thought to be driven by low Rossby number convection of a low magnetic Prandtl number fluid. Here kinematic dynamo action is investigated with an asymptotic, rapidly rotating dynamo model for the plane layer geometry that is intrinsically low magnetic Prandtl number. The thermal Prandtl number and Rayleigh number are varied to illustrate fundamental changes in flow regime, ranging from laminar cellular convection to geostrophic turbulence in which an inverse energy cascade is present. A decrease in the efficiency of the convection to generate a dynamo, as determined by an increase in the critical magnetic Reynolds number, is observed as the buoyancy forcing is increased. This decreased efficiency may result from both the loss of correlations associated with the increasingly disordered states of flow that are generated, and boundary layer behavior that enhances magnetic diffusion locally. We find that the spatial characteristics of the large-scale magnetic field is dependent only weakly on changes in flow behavior. In contrast, the behavior of the small-scale magnetic field is directly dependent on, and therefore shows significant variations with, the small-scale convective flow field. However, our results are limited to the linear, kinematic dynamo regime; future simulations that include the Lorentz force are therefore necessary to assess the robustness of these results
Assumptions behind grammatical approaches to code-switching: when the blueprint is a red herring
Many of the so-called ‘grammars’ of code-switching are based on various underlying assumptions, e.g. that informal speech can be adequately or appropriately described in terms of ‘‘grammar’’; that deep, rather than surface, structures are involved in code-switching; that one ‘language’ is the ‘base’ or ‘matrix’; and that constraints derived from existing data are universal and predictive. We question these assumptions on several grounds. First, ‘grammar’ is arguably distinct from the processes driving speech production. Second, the role of grammar is mediated by the variable, poly-idiolectal repertoires of bilingual speakers. Third, in many instances of CS the notion of a ‘base’ system is either irrelevant, or fails to explain the facts. Fourth, sociolinguistic factors frequently override ‘grammatical’ factors, as evidence from the same language pairs in different settings has shown. No principles proposed to date account for all the facts, and it seems unlikely that ‘grammar’, as conventionally conceived, can provide definitive answers. We conclude that rather than seeking universal, predictive grammatical rules, research on CS should focus on the variability of bilingual grammars
Sintering Kinetics of Plasma-Sprayed Zirconia TBCs
A model of the sintering exhibited by EB-PVD TBCs, based on principles of free energy minimization, was recently published by Hutchinson et al. In the current paper, this approach is applied to sintering of plasma-sprayed TBCs and comparisons are made with experimental results. Predictions of through-thickness shrinkage and changing pore surface area are compared with experimental data obtained by dilatometry and BET analysis respectively. The sensitivity of the predictions to initial pore architecture and material properties are assessed. The model can be used to predict the evolution of contact area between overlying splats. This is in turn related to the through-thickness thermal conductivity, using a previously-developed analytical model
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