1,111 research outputs found
The Numerical Simulation of Radiative Shocks I: The elimination of numerical shock instabilities using a localized oscillation filter
We address a numerical instability that arises in the directionally split
computation of hydrodynamic flows when shock fronts are parallel to a grid
plane. Transverse oscillations in pressure, density and temperature are
produced that are exacerbated by thermal instability when cooling is present,
forming post--shock `stripes'. These are orthogonal to the classic post--shock
'ringing' fluctuations. The resulting post--shock `striping' substantially
modifies the flow. We discuss three different methods to resolve this problem.
These include (1) a method based on artificial viscosity; (2) grid--jittering
and (3) a new localized oscillation filter that acts on specific grid cells in
the shock front. These methods are tested using a radiative wall shock problem
with an embedded shear layer. The artificial viscosity method is unsatisfactory
since, while it does reduce post--shock ringing, it does not eliminate the
stripes and the excessive shock broadening renders the calculation of cooling
inaccurate, resulting in an incorrect shock location. Grid--jittering
effectively counteracts striping. However, elsewhere on the grid, the shear
layer is unphysically diffused and this is highlighted in an extreme case. The
oscillation filter method removes stripes and permits other high velocity
gradient regions of the flow to evolve in a physically acceptable manner. It
also has the advantage of only acting on a small fraction of the cells in a two
or three dimensional simulation and does not significantly impair performance.Comment: 20 pages, 6 figures, revised version submitted to ApJ Supplement
Serie
Space station integrated propulsion and fluid systems study
The program study was performed in two tasks: Task 1 addressed propulsion systems and Task 2 addressed all fluid systems associated with the Space Station elements, which also included propulsion and pressurant systems. Program results indicated a substantial reduction in life cycle costs through integrating the oxygen/hydrogen propulsion system with the environmental control and life support system, and through supplying nitrogen in a cryogenic gaseous supercritical or subcritical liquid state. A water sensitivity analysis showed that increasing the food water content would substantially increase the amount of water available for propulsion use and in all cases, the implementation of the BOSCH CO2 reduction process would reduce overall life cycle costs to the station and minimize risk. An investigation of fluid systems and associated requirements revealed a delicate balance between the individual propulsion and fluid systems across work packages and a strong interdependence between all other fluid systems
A remark on the trace-map for the Silver mean sequence
In this work we study the Silver mean sequence based on substitution rules by
means of a transfer-matrix approach. Using transfer-matrix method we find a
recurrence relation for the traces of general transfer-matrices which
characterizes electronic properties of the quasicrystal in question. We also
find an invariant of the trace-map.Comment: 5 pages, minor improvements in style and presentation of calculation
Spatially Resolved Far-Ultraviolet Spectroscopy of the Nuclear Region of NGC 1068
We carry out high-resolution FUSE spectroscopy of the nuclear region of NGC
1068. The first set of spectra was obtained with a 30" square aperture that
collects all emission from the narrow-line region. The data reveal a strong
broad OVI component of FWHM ~3500 kms-1 and two narrow OVI 1031/1037 components
of ~350 kms-1. The CIII 977 and NIII 991 emission lines in this spectrum can be
fitted with a narrow component of FWHM ~1000 kms-1 and a broad one of ~2500
kms-1. Another set of seven spatially resolved spectra were made using a long
slit of 1.25" X 20", at steps of ~1" along the axis of the emission-line cone.
We find that (1) Major emission lines in the FUSE wavelength range consist of a
broad and a narrow component; (2) There is a gradient in the velocity field for
the narrow OVI component of ~200 kms-1 from ~2" southwest of the nucleus to ~4"
northeast. A similar pattern is also observed with the broad OVI component,
with a gradient of ~3000 kms-1. These are consistent with the HST/STIS findings
and suggest a biconical structure in which the velocity field is mainly radial
outflow; (3) A major portion of the CIII and NIII line flux is produced in the
compact core. They are therefore not effective temperature diagnostics for the
conical region; and (4) The best-fitted UV continuum suggests virtually no
reddening, and the HeII 1085/1640 ratio suggests a consistently low extinction
factor across the cone.Comment: To appear in the Astrophysical Journal. 37 pages with 12 figure
Human bladder cancer invasion model using rat bladder in vitro and its use to test mechanisms and therapeutic inhibitors of invasion
As well as being a passive support, the extracellular matrix also regulates key biological processes such as invasion, differentiation and angiogenesis. We have therefore developed an in vitro model of bladder cancer invasion using de-epithelialized rat bladder to allow for tumour cell–extracellular matrix interactions. Onto this we have seeded a panel of human bladder cancer cell lines (RT4, RT112, 253J and EJ28 (T24)) representing progression from well to poorly differentiated phenotypes and used as models of superficial to invasive bladder cancer. The better differentiated cell lines RT4 and RT112 reproducibly grew as stratified epithelium, whereas poorly differentiated EJ28 cells invaded across a broad front. Invasion was not simply related to proliferation rate, measured either as doubling time on plastic (non-invasive 253J and invasive EJ28 having the same doubling time) or by Ki-67 proliferation index within the model. We used the model to test the ability of 4 compounds that interfere with tumour cell–extracellular matrix interactions (suramin, N-acetylcysteine and the urokinase plasminogen activator pathway antagonists Å5 compound and monoclonal antibody Mab 3936) to inhibit invasion. At non-toxic concentrations, all significantly inhibited invasion (P< 0.05), although to varying degrees, suramin and Å5 almost completely and N-acetylcysteine the least. In conclusion, this model shows the urokinase system is important for bladder invasion and can be used to investigate other mechanisms of bladder cancer invasion and also for the testing of intravesical drugs. © 2001 Cancer Research Campaign http://www.bjcancer.co
Elevated transcription of transposable elements is accompanied by het-siRNA-driven de novo DNA methylation in grapevine embryogenic callus
Background: Somatic variation is a valuable source of trait diversity in clonally propagated crops. In grapevine, which has been clonally propagated worldwide for centuries, important phenotypes such as white berry colour are the result of genetic changes caused by transposable elements. Additionally, epiallele formation may play a role in determining geo-specific (‘terroir’) differences in grapes and thus ultimately in wine. This genomic plasticity might be co-opted for crop improvement via somatic embryogenesis, but that depends on a species-specific understanding of the epigenetic regulation of transposable element (TE) expression and silencing in these cultures. For this reason, we used whole-genome bisulphite sequencing, mRNA sequencing and small RNA sequencing to study the epigenetic status and expression of TEs in embryogenic callus, in comparison with leaf tissue.
Results: We found that compared with leaf tissue, grapevine embryogenic callus cultures accumulate relatively high genome-wide CHH methylation, particularly across heterochromatic regions. This de novo methylation is associated with an abundance of transcripts from highly replicated TE families, as well as corresponding 24 nt heterochromatic siRNAs. Methylation in the TE-specific CHG context was relatively low over TEs located within genes, and the expression of TE loci within genes was highly correlated with the expression of those genes.
Conclusions: This multi-‘omics analysis of grapevine embryogenic callus in comparison with leaf tissues reveals a high level of genome-wide transcription of TEs accompanied by RNA-dependent DNA methylation of these sequences in trans. This provides insight into the genomic conditions underlying somaclonal variation and epiallele formation in plants regenerated from embryogenic cultures, which is an important consideration when using these tissues for plant propagation and genetic improvement
Interactions of a Light Hypersonic Jet with a Non-Uniform Interstellar Medium
We present three dimensional simulations of the interaction of a light
hypersonic jet with an inhomogeneous thermal and turbulently supported disk in
an elliptical galaxy. We model the jet as a light, supersonic non-relativistic
flow with parameters selected to be consistent with a relativistic jet with
kinetic power just above the FR1/FR2 break.
We identify four generic phases in the evolution of such a jet with the
inhomogeneous interstellar medium: 1) an initial ``flood and channel'' phase,
where progress is characterized by high pressure gas finding changing weak
points in the ISM, flowing through channels that form and re-form over time, 2)
a spherical, energy-driven bubble phase, were the bubble is larger than the
disk scale, but the jet remains fully disrupted close to the nucleus, 3) a
rapid, jet break--out phase the where jet breaks free of the last dense clouds,
becomes collimated and pierces the spherical bubble, and 4) a classical phase,
the jet propagates in a momentum-dominated fashion leading to the classical jet
+ cocoon + bow-shock structure.
Mass transport in the simulations is investigated, and we propose a model for
the morphology and component proper motions in the well-studied Compact
Symmetric Object 4C31.04.Comment: 66 pages, 22 figures, PDFLaTeX, aastex macros, graphicx and amssymb
packages, Accepted, to be published 2007 ApJ
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