808 research outputs found
Applications of the finite element method to modeling the atmospheric boundary layer
AbstractThe finite element method is used to describe the detailed properties of the atmospheric boundary layer by use of a high-resolution model and its bulk properties by use of a simple vertically integrated model. Features of the finite element method that can be exploited for applications to the atmospheric boundary layer include the capability to use different basis functions in different parts of the domain, ability to grid over irregular terrain, ease of using time-dependent basis functions and the natural way that surface boundary conditions and vertically integrated properties enter the model
Exploring Minimal Scenarios to Produce Transversely Bright Electron Beams Using the Eigen-Emittance Concept
Next generation hard X-ray free electron lasers require electron beams with
low transverse emittance. One proposal to achieve these low emittances is to
exploit the eigen-emittance values of the beam. The eigen-emittances are
invariant under linear beam transport and equivalent to the emittances in an
uncorrelated beam. If a correlated beam with two small eigen-emittances can be
produced, removal of the correlations via appropriate optics will lead to two
small emittance values, provided non-linear effects are not too large. We study
how such a beam may be produced using minimal linear correlations. We find it
is theoretically possible to produce such a beam, however it may be more
difficult to realize in practice. We identify linear correlations that may lead
to physically realizable emittance schemes and discuss promising future
avenues.Comment: 7 pages, 2 figures, to appear in NIM
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Containment barrier metals for high-level waste packages in a Tuff repository
The Nevada Nuclear Waste Storage Investigations (NNWSI) Waste Package project is part of the US Department of Energy`s Civilian Radioactive Waste Management (CRWM) Program. The NNWSI project is working towards the development of multibarriered packages for the disposal of spent fuel and high-level waste in tuff in the unsaturated zone at Yucca Mountain at the Nevada Test Site (NTS). The final engineered barrier system design may be composed of a waste form, canister, overpack, borehole liner, packing, and the near field host rock, or some combination thereof. Lawrence Livermore National Laboratory`s (LLNL) role is to design, model, and test the waste package subsystem for the tuff repository. At the present stage of development of the nuclear waste management program at LLNL, the detailed requirements for the waste package design are not yet firmly established. In spite of these uncertainties as to the detailed package requirements, we have begun the conceptual design stage. By conceptual design, we mean design based on our best assessment of present and future regulatory requirements. We anticipate that changes will occur as the detailed requirements for waste package design are finalized. 17 references, 4 figures, 10 tables
Bound states between dark matter particles and emission of gravitational radiation
Bound states of two weakly interactive massive particles are studied. It is
assumed that the WIMPonium is formed due to the gravitational interaction,
since the weak interaction can sometimes be repulsive. The lifetimes of the
spontaneous emission of gravitational radiation and of the WIMPs annihilation
into a pair of gravitons are computed, and are shown to be many orders of
magnitude larger than the age of the universe.Comment: Accepted for publication in GER
Narrow-Band Waveguide Taps Using Photonic Surface Modes Supported by Multilayer Dielectric Stacks
The photonic surface modes supported by multilayer dielectric stacks are highly dispersive. They can therefore be used to design mode-selective taps and narrow-band notch transmission filters for waveguide applications
An overview of jets and outflows in stellar mass black holes
In this book chapter, we will briefly review the current empirical
understanding of the relation between accretion state and and outflows in
accreting stellar mass black holes. The focus will be on the empirical
connections between X-ray states and relativistic (`radio') jets, although we
are now also able to draw accretion disc winds into the picture in a systematic
way. We will furthermore consider the latest attempts to measure/order jet
power, and to compare it to other (potentially) measurable quantities, most
importantly black hole spin.Comment: Accepted for publication in Space Science Reviews. Also to appear in
the Space Sciences Series of ISSI - The Physics of Accretion on to Black
Holes (Springer Publisher
Novae Ejecta as Colliding Shells
Following on our initial absorption-line analysis of fifteen novae spectra we
present additional evidence for the existence of two distinct components of
novae ejecta having different origins. As argued in Paper I one component is
the rapidly expanding gas ejected from the outer layers of the white dwarf by
the outburst. The second component is pre-existing outer, more slowly expanding
circumbinary gas that represents ejecta from the secondary star or accretion
disk. We present measurements of the emission-line widths that show them to be
significantly narrower than the broad P Cygni profiles that immediately precede
them. The emission profiles of novae in the nebular phase are distinctly
rectangular, i.e., strongly suggestive of emission from a relatively thin,
roughly spherical shell. We thus interpret novae spectral evolution in terms of
the collision between the two components of ejecta, which converts the early
absorption spectrum to an emission-line spectrum within weeks of the outburst.
The narrow emission widths require the outer circumbinary gas to be much more
massive than the white dwarf ejecta, thereby slowing the latter's expansion
upon collision. The presence of a large reservoir of circumbinary gas at the
time of outburst is suggestive that novae outbursts may sometime be triggered
by collapse of gas onto the white dwarf, as occurs for dwarf novae, rather than
steady mass transfer through the inner Lagrangian point.Comment: 12 pages, 3 figures; Revised manuscript; Accepted for publication in
Astrophysics & Space Scienc
Galactic X-ray binary jets
With their relatively fast variability time-scales, Galactic X-ray binaries
provide an excellent laboratory to explore the physics of accretion and related
phenomena, most notably outflows, over different regimes. After comparing the
phenomenology of jets in black hole X-ray binary systems to that of neutron
stars, here I discuss the role of the jet at very low Eddington ratios, and
present preliminary results obtained by fitting the broadband spectral energy
distribution of a quiescent black hole binary with a `maximally jet-dominated'
model.Comment: Refereed version, accepted for publication in Astrophysics & Space
Scienc
Processing and mechanical properties of magnesium-lithium composites containing steel fibers
Deformation-processed metal-metal composites (DMMC) of Mg-Li alloys containing steel reinforcing fibers were prepared by infiltrating a preform of steel wool with the molten matrix. The Li content was varied to control the crystal structure of the matrix; Mg-4 wt pct Li is hexagonal close packed (hcp), while Mg-12 wt pct Li is body-centered cubic (bcc). The low carbon steel used as the reinforcing fiber is essentially bcc. The hcp/bcc and bcc/bcc composites were subsequently deformed by rolling and by extrusion/swaging and mechanically tested to relate the tensile strength of the composites to true deformation strain. The hcp/bcc composites had limited formability at temperatures up to 400 °C, while the bcc/bcc composites had excellent formability during sheet rolling at room temperature but limited formability during swaging at room temperature. The tensile strengths of the hcp/bcc composite rod and the bcc/bcc composite sheet and rod increased moderately with deformation, though less than predicted from rule-of-mixtures (ROM) calculations. This article presents the experimental data for these DMMC materials and comments on the possible effect of texture development in the matrix and fiber phases on the deformation characteristics of the composite material
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