85 research outputs found
Dissipation Instability in the Accretion Disk
The model of a geometrically thin gaseous disk in the external gravitational
potential is considered. The dinamics of small nonaxisymmetric perturbations in
the plane of the accretion disk with dissipative effects is investigated. It is
showed, that conditions of development and parameters of unstable oscillation
modes in the opticaly thick accretion disk are strongly depended on the models
of viscosity and opacity.Comment: Plain TeX, 6 pages, 2 figures (GIF), Submitted to Astron. Astrophys.
Transaction
Fluorescence Lidar Multicolor Imaging of Vegetation
Multicolor imaging of vegetation fluorescence following laser excitation is reported for distances of 50 m. A mobile laser-radar system equipped with a Nd:YAG laser transmitter and a 40-cm-diameter telescope was utilized. The laser light was Raman shifted to 397 nm with pulse energies of approximately 30 mJ. An image-intensified CCD camera with a specially designed split-mirror Cassegrainian telescope was utilized for the simultaneous recording of fluorescence images of leaves and branches in four different spectral bands. Additionally, fluorescence spectra at selected points within the detection area were measured with an image-intensified diode array system. Image processing permits extraction of information related to the physiological status of the vegetation and might prove useful in forest decline research
Influence of surface oxide characteristics and speciation on corrosion, electrochemical properties and metal release of atomized 316L stainless steel powders
Surface oxide characteristics of powder particles are important to consider for any toxicological risk assessment based on in-vitro or in-vivo tests. This study focuses on a multi-analytical approach (X-ray photoelectron spectroscopy, Auger electron spectroscopy, scanning- and transmission electron microscopy, and different electrochemical techniques) for in-depth characterization of surface oxides of inert-gas-atomized (GA) AISI 316L stainless steel powder, compared with massive sheet and a water-atomized (WA) 316L powder. Implications of differences in surface oxide phases and their surface distribution on corrosion, electrochemical properties and metal release are systematically discussed. Cr was enriched in an inner surface layer for both GA powders, with Mn and S enriched in the outermost surface oxide. The surface oxide was 2-5 nm thick for both GA powder size fractions, amorphous for the GA powder size
Influence of surface oxide characteristics and speciation on corrosion, electrochemical properties and metal release of atomized 316L stainless steel powders
Surface oxide characteristics of powder particles are important to consider for any toxicological risk assessment based on in-vitro or in-vivo tests. This study focuses on a multi-analytical approach (X-ray photoelectron spectroscopy, Auger electron spectroscopy, scanning- and transmission electron microscopy, and different electrochemical techniques) for in-depth characterization of surface oxides of inert-gas-atomized (GA) AISI 316L stainless steel powder, compared with massive sheet and a water-atomized (WA) 316L powder. Implications of differences in surface oxide phases and their surface distribution on corrosion, electrochemical properties and metal release are systematically discussed. Cr was enriched in an inner surface layer for both GA powders, with Mn and S enriched in the outermost surface oxide. The surface oxide was 2-5 nm thick for both GA powder size fractions, amorphous for the GA powder size
New Analytical Formula for Supercritical Accretion Flows
We examine a new family of global analytic solutions for optically thick
accretion disks, which includes the supercritical accretion regime. We found
that the ratio of the advection cooling rate, , to the viscous
heating rate, , i.e., , can be
represented by an analytical form dependent on the radius and the mass
accretion rate. The new analytic solutions can be characterized by the
photon-trapping radius, \rtrap, inside which the accretion time is less than
the photon diffusion time in the vertical direction; the nature of the
solutions changes significantly as this radius is crossed. Inside the trapping
radius,
approaches , which corresponds to the advection-dominated
limit (), whereas outside the trapping radius, the radial dependence
of changes to , which corresponds to the
radiative-cooling-dominated limit. The analytical formula for derived here
smoothly connects these two regimes. The set of new analytic solutions
reproduces well the global disk structure obtained by numerical integration
over a wide range of mass accretion rates, including the supercritical
accretion regime. In particular, the effective temperature profiles for our new
solutions are in good agreement with those obtained from numerical solutions.
Therefore, the new solutions will provide a useful tool not only for evaluating
the observational properties of accretion flows, but also for investigating the
mass evolution of black holes in the presence of supercritical accretion flows.Comment: 14 pages, 7 figures, accepted for publication in the Astrophysical
Journa
Welding fume nanoparticles from solid and flux-cored wires: Solubility, toxicity, and role of fluorides
Welding fume particles are hazardous. Their toxicity likely depends on their composition and reactivity. This study aimed at exploring the role of sodium or other fluorides (NaF), which are intentionally added to flux-cored wire electrodes for stainless steel welding, on the solubility (in phosphate buffered saline) and toxicity of the generated welding fume particles. A multi-analytical particle characterization approach along with in-vitro cell assays was undertaken. The release of Cr(VI) and Mn from the particles was tested as a function of fluoride solution concentration. The welding fume particles containing NaF released significantly higher amounts of Cr(VI) compared with solid wire reference fumes, which was associated with increased cytotoxicity and genotoxicity in-vitro. No crystalline Na or potassium (K) containing chromates were observed. Cr(VI) was incorporated in an amorphous mixed oxide. Solution-added fluorides did not increase the solubility of Cr(VI), but contributed to a reduced Mn release from both solid and flux-cored wire fume particles and the reduction of Cr(VI) release from solid wire fume particles. Chemical speciation modeling suggested that metal fluoride complexes were not formed. The presence of NaF in the welding electrodes did not have any direct, but possibly an indirect, role in the Cr(VI) solubility of welding fumes
Radiation pressure instability driven variability in the accreting black holes
The time dependent evolution of the accretion disk around black hole is
computed. The classical description of the -viscosity is adopted so the
evolution is driven by the instability operating in the innermost
radiation-pressure dominated part of the accretion disk. We assume that the
optically thick disk always extends down to the marginally stable orbit so it
is never evacuated completely. We include the effect of the advection, coronal
dissipation and vertical outflow. We show that the presence of the corona
and/or the outflow reduce the amplitude of the outburst. If only about half of
the energy is dissipated in the disk (with the other half dissipated in the
corona and carried away by the outflow) the outburst amplitude and duration are
consistent with observations of the microquasar GRS 1915+105. Viscous evolution
explains in a natural way the lack of direct transitions from the state C to
the state B in color-color diagram of this source. Further reduction of the
fraction of energy dissipated in the optically thick disk switches off the
outbursts which may explain why they are not seen in all high accretion rate
sources being in the Very High State.Comment: 31 pages, 14 figures; accepted to Ap
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