18,038 research outputs found
Hybridized polymer matrix composite
Under certain conditions of combined fire and impact, graphite fibers are released to the atmosphere by graphite fiber composites. The retention of graphite fibers in these situations is investigated. Hybrid combinations of graphite tape and cloth, glass cloth, and resin additives are studied with resin systems. Polyimide resins form the most resistant composites and resins based on simple novolac epoxies the least resistant of those tested. Great improvement in the containment of the fibers is obtained in using graphite/glass hybrids, and nearly complete prevention of individual fiber release is made possible by the use of resin additives
Dynamics of fingering convection II: The formation of thermohaline staircases
Regions of the ocean's thermocline unstable to salt fingering are often
observed to host thermohaline staircases, stacks of deep well-mixed convective
layers separated by thin stably-stratified interfaces. Decades after their
discovery, however, their origin remains controversial. In this paper we use 3D
direct numerical simulations to shed light on the problem. We study the
evolution of an analogous double-diffusive system, starting from an initial
statistically homogeneous fingering state and find that it spontaneously
transforms into a layered state. By analysing our results in the light of the
mean-field theory developed in Paper I, a clear picture of the sequence of
events resulting in the staircase formation emerges. A collective instability
of homogeneous fingering convection first excites a field of gravity waves,
with a well-defined vertical wavelength. However, the waves saturate early
through regular but localized breaking events, and are not directly responsible
for the formation of the staircase. Meanwhile, slower-growing, horizontally
invariant but vertically quasi-periodic gamma-modes are also excited and grow
according to the gamma-instability mechanism. Our results suggest that the
nonlinear interaction between these various mean-field modes of instability
leads to the selection of one particular gamma-mode as the staircase
progenitor. Upon reaching a critical amplitude, this progenitor overturns into
a fully-formed staircase. We conclude by extending the results of our
simulations to real oceanic parameter values, and find that the progenitor
gamma-mode is expected to grow on a timescale of a few hours, and leads to the
formation of a thermohaline staircase in about one day with an initial spacing
of the order of one to two metres.Comment: 18 pages, 9 figures, associated mpeg file at
http://earth.uni-muenster.de/~stellma/movie_small.mp4, submitted to JF
Scale Transformations on the Noncommutative Plane and the Seiberg-Witten Map
We write down three kinds of scale transformations {\tt i-iii)} on the
noncommutative plane. {\tt i)} is the analogue of standard dilations on the
plane, {\tt ii)} is a re-scaling of the noncommutative parameter , and
{\tt iii)} is a combination of the previous two, whereby the defining relations
for the noncommutative plane are preserved. The action of the three
transformations is defined on gauge fields evaluated at fixed coordinates and
.
The transformations are obtained only up to terms which transform covariantly
under gauge transformations. We give possible constraints on these terms. We
show how the transformations {\tt i)} and {\tt ii)} depend on the choice of
star product, and show the relation of {\tt ii)} to Seiberg-Witten
transformations. Because {\tt iii)} preserves the fundamental commutation
relations it is a symmetry of the algebra. One has the possibility of
implementing it as a symmetry of the dynamics, as well, in noncommutative field
theories where is not fixed.Comment: 20 page
Empirical modeling of the quiet time nightside magnetosphere
Empirical modeling of plasma pressure and magnetic field for the quiet time nightside magnetosphere is investigated. Two models are constructed for this study. One model, referred to here as T89R, is basically the magnetic field model of Tsyganenko (1989) but is modified by the addition of an inner eastward ring current at a radial distance of ∼3 RE as suggested by observation. The other is a combination of the T89R model and the long version of the magnetic field model of Tsyganenko (1987) such that the former dominates the magnetic field in the inner magnetosphere, whereas the latter prevails in the distant tail. The distribution of plasma pressure, which is required to balance the magnetic force for each of these two field models, is computed along the tail axis in the midnight meridian. The occurrence of pressure anisotropy in the inner magnetospheric region is also taken into account by determining an empirical fit to the observed plasma pressure anisotropy. This effort is the first attempt to obtain the plasma pressure distribution in force equilibrium with magnetic stresses from an empirical field model with the inclusion of pressure anisotropy. The inclusion of pressure anisotropy alters the plasma pressure by as much as a factor of ∼3 in the inner magnetosphere. The deduced plasma pressure profile along the tail axis is found to be in good agreement with the observed quiet time plasma pressure for geocentric distances between ∼2 and ∼35 RE
Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds
The fate of subducted oceanic lithosphere and its role in the planet-scale geochemical cycle is a key problem in solid Earth studies. Asthenospheric and transition zone minerals included in diamond have been interpreted as representing subducted oceanic crust based on inclusion REE patterns and strong 13C depletion of their host diamond (δ13C as low as -23 ‰). This view/explanation, however, has been challenged by alternative interpretations that variable carbon isotopic compositions either result from high temperature fractionation involving carbides, or reflect primordial, unhomogenised mantle reservoirs. Here, we present the first oxygen isotope analyses of inclusions in such ultradeep diamonds – majoritic garnets in diamond from Jagersfontein (South Africa). The oxygen isotope compositions provide unambiguous evidence for derivation of the inclusions from subducted crustal materials. The δ18OVSMOW values of the majorites range from +8.6 ‰ to +10.0 ‰, well outside that of ambient mantle (+5.5 ±0.4 ‰) and indicate that the protoliths were very heavily weathered at relatively low temperatures. When this information is combined with the broadly eclogitic composition of the majoritic garnets, a derivation from subducted sea-floor basalts is implied. Based on the association between the heavy oxygen and light carbon, the light carbon isotope composition cannot relate to deep mantle processes and is also ultimately derived from the crust
Location and Direction Dependent Effects in Collider Physics from Noncommutativity
We examine the leading order noncommutative corrections to the differential
and total cross sections for e+ e- --> q q-bar. After averaging over the
earth's rotation, the results depend on the latitude for the collider, as well
as the direction of the incoming beam. They also depend on scale and direction
of the noncommutativity. Using data from LEP, we exclude regions in the
parameter space spanned by the noncommutative scale and angle relative to the
earth's axis. We also investigate possible implications for phenomenology at
the future International Linear Collider.Comment: version to appear in PR
The transition zone as a host for recycled volatiles: Evidence from nitrogen and carbon isotopes in ultra-deep diamonds from Monastery and Jagersfontein (South Africa)
Sublithospheric (ultra-deep) diamonds provide a unique window into the deepest parts of Earth's mantle, which otherwise remain inaccessible. Here, we report the first combined C- and N-isotopic data for diamonds from the Monastery and Jagersfontein kimberlites that sample the deep asthenosphere and transition zone beneath the Kaapvaal Craton, in the mid Cretaceous, to investigate the nature of mantle fluids at these depths and the constraints they provide on the deep volatile cycle.
Both diamond suites exhibit very light δ13C values (down to − 26‰) and heavy δ15N (up to + 10.3‰), with nitrogen abundances generally below 70 at. ppm but varying up to very high concentrations (2520 at. ppm) in rare cases. Combined, these signatures are consistent with derivation from subducted crustal materials. Both suites exhibit variable nitrogen aggregation states from 25 to 100% B defects. Internal growth structures, revealed in cathodoluminescence (CL) images, vary from faintly layered, through distinct cores to concentric growth patterns with intermittent evidence for dissolution and regular octahedral growth layers in places.
Modelling the internal co-variations in δ13C-δ15N-N revealed that diamonds grew from diverse C-H-O-N fluids involving both oxidised and reduced carbon species. The diversity of the modelled diamond-forming fluids highlights the complexity of the volatile sources and the likely heterogeneity of the deep asthenosphere and transition zone. We propose that the Monastery and Jagersfontein diamonds form in subducted slabs, where carbon is converted into either oxidised or reduced species during fluid-aided dissolution of subducted carbon before being re-precipitated as diamond. The common occurrence of recycled C and N isotopic signatures in super-deep diamonds world-wide indicates that a significant amount of carbon and nitrogen is recycled back to the deep asthenosphere and transition zone via subducting slabs, and that the transition zone may be dominated by recycled C and N
Wide-field mid-infrared and millimetre imaging of the high-redshift radio galaxy, 4C41.17
We present deep 350- and 1200-micron imaging of the region around 4C41.17 --
one of the most distant (z = 3.792) and luminous known radio galaxies --
obtained with the Submillimeter High Angular Resolution Camera (SHARC-II) and
the Max Planck Millimeter Bolometer Array (MAMBO). The radio galaxy is robustly
detected at 350- and 1200-micron, as are two nearby 850-micron-selected
galaxies; a third 850-micron source is detected at 350-micron and coincides
with a ~ 2-sigma feature in the 1200-micron map. Further away from the radio
galaxy an additional nine sources are detected at 1200-micron, bringing the
total number of detected (sub)millimeter selected galaxies (SMGs) in this field
to 14. Using radio images from the Very Large Array (VLA) and Spitzer
mid-infrared (mid-IR) data, we find statistically robust radio and/or 24-micron
counterparts to eight of the 14 SMGs in the field around 4C41.17. Follow-up
spectroscopy with Keck/LRIS has yielded redshifts for three of the eight
robustly identified SMGs, placing them in the redshift range 0.5 < z < 2.7,
i.e. well below that of 4C41.17. We infer photometric redshifts for a further
four sources using their 1.6-micron (rest-frame) stellar feature as probed by
the IRAC bands; only one of them is likely to be at the same redshift as
4C41.17. Thus at least four, and as many as seven, of the SMGs within the
4C41.17 field are physically unrelated to the radio galaxy. With the redshift
information at hand we are able to constrain the observed over-densities of
SMGs within radial bins stretching to R=50 and 100" (~ 0.4 and ~ 0.8Mpc at z ~
3.8) from the radio galaxy to ~ 5x and ~ 2x that of the field, dropping off to
the background value at R=150". [Abridged]Comment: 20 pages, 9 figures, accepted for publication in MNRA
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