1,963 research outputs found
A 3D radiative transfer framework: X. Arbitrary Velocity Fields in the Co-moving Frame
3-D astrophysical atmospheres will have random velocity fields. We seek to
combine the methods we have developed for solving the 1-D problem with
arbitrary flows to those that we have developed for solving the fully 3-D
relativistic radiative transfer problem in the case of monotonic flows. The
methods developed in the case of 3-D atmospheres with monotonic flows, solving
the fully relativistic problem along curves defined by an affine parameter, are
very flexible and can be extended to the case of arbitrary velocity fields in
3-D. Simultaneously, the techniques we developed for treating the 1-D problem
with arbitrary velocity fields are easily adapted to the 3-D problem. The
algorithm we present allows the solution of 3-D radiative transfer problems
that include arbitrary wavelength couplings. We use a quasi-analytic formal
solution of the radiative transfer equation that significantly improves the
overall computation speed. We show that the approximate lambda operator
developed in previous work gives good convergence, even neglecting wavelength
coupling. Ng acceleration also gives good results. We present tests that are of
similar resolution to what has been presented using Monte-Carlo techniques,
thus our methods will be applicable to problems outside of our test setup.
Additional domain decomposition parallelization strategies will be explored in
future work.Comment: 9 pages, 9 figures, A&A, in press, new version matches copy edited
version, definition restore
General Relativistic Radiative Transfer
We present a general method to calculate radiative transfer including
scattering in the continuum as well as in lines in spherically symmetric
systems that are influenced by the effects of general relativity (GR). We
utilize a comoving wavelength ansatz that allows to resolve spectral lines
throughout the atmosphere. The used numerical solution is an operator splitting
(OS) technique that uses a characteristic formal solution. The bending of
photon paths and the wavelength shifts due to the effects of GR are fully taken
into account, as is the treatment of image generation in a curved spacetime. We
describe the algorithm we use and demonstrate the effects of GR on the
radiative transport of a two level atom line in a neutron star like atmosphere
for various combinations of continuous and line scattering coefficients. In
addition, we present grey continuum models and discuss the effects of different
scattering albedos on the emergent spectra and the determination of effective
temperatures and radii of neutron star atmospheres
Mechatronic design of a fast and long range 4 degrees of freedom humanoid neck
This paper describes the mechatronic design of a humanoid neck. To research human machine interaction, the head and neck combination should be able to approach the human behavior as much as possible. We present a novel humanoid neck concept that is both fast, and has a long range of motion in 4 degrees of freedom (DOFs). This enables the head to track fast objects, and the neck design is suitable for mimicking expressions. The humanoid neck features a differential drive design for the lower 2 DOFs resulting in a low moving mass and the ability to use strong actuators. The performance of the neck has been\ud
optimized by minimizing backlash in the mechanisms, and by using gravity compensation. Two cameras in the head are used for scanning and interaction with the environment
Introgressive replacement of natives by invading Arion pest slugs.
Hybridization with invasive species is one of the major threats to the phenotypic and genetic persistence of native organisms worldwide. Arion vulgaris (syn. lusitanicus) is a major agricultural pest slug that successfully invaded many European countries in recent decades, but its impact on closely related native species remains unclear. Here, we hypothesized that the regional decline of native A. rufus is connected with the spread of invasive A. vulgaris, and tested whether this can be linked to hybridization between the two species by analyzing 625 Arion sp. along altitudinal transects in three regions in Switzerland. In each region, we observed clear evidence of different degrees of genetic admixture, suggesting recurrent hybridization beyond the first generation. We found spatial differences in admixture patterns that might reflect distinct invasion histories among the regions. Our analyses provide a landscape level perspective for the genetic interactions between invasive and native animals during the invasion. We predict that without specific management action, A. vulgaris will further expand its range, which might lead to local extinction of A. rufus and other native slugs in the near future. Similar processes are likely occurring in other regions currently invaded by A. vulgaris
Classification of double flag varieties of complexity 0 and 1
A classification of double flag varieties of complexity 0 and 1 is obtained.
An application of this problem to decomposing tensor products of irreducible
representations of semisimple Lie groups is considered
A 3D radiative transfer framework: VII. Arbitrary velocity fields in the Eulerian frame
A solution of the radiative-transfer problem in 3D with arbitrary velocity
fields in the Eulerian frame is presented. The method is implemented in our 3D
radiative transfer framework and used in the PHOENIX/3D code. It is tested by
comparison to our well- tested 1D co-moving frame radiative transfer code,
where the treatment of a monotonic velocity field is implemented in the
Lagrangian frame. The Eulerian formulation does not need much additional memory
and is useable on state-of-the-art computers, even large-scale applications
with 1000's of wavelength points are feasible
A new formal solution of the radiative transfer in arbitrary velocity fields
We present a new formal solution of the Lagrangian equation of radiative
transfer that is useful in solving the equation of radiative transfer in the
presence of arbitrary velocity fields. Normally a term due to the inclusion of
the wavelength derivative in the Lagrangian equation of radiative transfer is
associated with a generalised opacity. In non-monotonic velocity fields, this
generalised opacity may become negative. To ensure that the opacity remains
positive, this term of the derivative is included in the formal solution of the
radiative transfer problem.
The new definition of the generalised opacity allows for a new solution of
the equation of radiative transfer in the presence of velocity fields. It is
especially useful for arbitrary velocity fields, where it effectively prevents
the occurrences of negative generalised opacities and still allows the explicit
construction of the Lambda-operator of the system needed for an accelerated
Lambda-iteration. We performed test calculations, where the results of old,
established solutions were compared with the new solution. The relative
deviations never exceeded 1% and so the new solution is indeed suitable for use
in radiative-transfer modelling. Non-monotonic velocity fields along photon
paths frequently occur in three-dimensional hydrodynamical models of
astrophysical atmospheres. Therefore, the formal solution will be of use for
multidimensional radiative transfer and has immediate applications in the
modelling of pulsating stars and astrophysical shock fronts.Comment: Astronomy & Astrophysics, in press 8 figure
Comparison of different droplet measurement techniques in the Braunschweig Icing Wind Tunnel
The generation, transport and characterization of supercooled droplets in multiphase wind tunnel test facilities is of great importance for conducting icing experiments and to better understand cloud microphysical processes such as coalescence, ice nucleation, accretion and riming. To this end, a spray system has been developed, tested and calibrated in the Braunschweig Icing Wind Tunnel. Liquid droplets in the size range of 1 to 150â”m produced by pneumatic atomizers were accelerated to velocities between 10 and 40âmâsâ1 and supercooled to temperatures between 0 and â20ââC. Thereby, liquid water contents between 0.07 and 2.5âgâmâ3 were obtained in the test section. The wind tunnel conditions were stable and reproducible within 3â% standard variation for median volumetric diameter (MVD) and 7â% standard deviation for liquid water content (LWC). Different instruments were integrated in the icing wind tunnel measuring the particle size distribution (PSD), MVD and LWC. Phase Doppler interferometry (PDI), laser spectroscopy with a fast cloud droplet probe (FCDP) and shadowgraphy were systematically compared for present wind tunnel conditions. MVDs measured with the three instruments agreed within 15â% in the range between 8 and 35â”m and showed high coefficients of determination (R2) of 0.985 for FCDP and 0.799 for shadowgraphy with respect to PDI data. Between 35 and 56â”m MVD, the shadowgraphy data exhibit a low bias with respect to PDI. The instruments' trends and biases for selected droplet conditions are discussed. LWCs determined from mass flow calculations in the range of 0.07â1.5âgâmâ3 are compared to measurements of the bulk phase rotating cylinder technique (RCT) and the above-mentioned single-particle instruments. For RCT, agreement with the mass flow calculations of approximately 20â% in LWC was achieved. For PDI 84â% of measurement points with LWC<0.5âgâmâ3 agree with mass flow calculations within a range of ±0.1âgâmâ3. Using the different techniques, a comprehensive wind tunnel calibration for supercooled droplets was achieved, which is a prerequisite for providing well-characterized liquid cloud conditions for icing tests for aerospace, wind turbines and power networks
Study of Magnetic Excitation in Singlet-Ground-State Magnets CsFeCl and RbFeCl by Nuclear Magnetic Relaxation
The temperature dependences of spin-lattice relaxation time of
Cs in CsFeCl and Rb in RbFeCl were measured in the
temperature range between 1.5 K and 22 K, at various fields up to 7 T applied
parallel (or perpendicular) to the c-axis, and the analysis was made on the
basis of the DCEFA. The mechanism of the nuclear magnetic relaxation is
interpreted in terms of the magnetic fluctuations which are characterized by
the singlet ground state system. In the field region where the phase transition
occurs, exhibited the tendency of divergence near , and
this feature was ascribed to the transverse spin fluctuation associated with
the mode softening at the -point. It was found that the damping constant of
the soft mode is remarkably affected by the occurrence of the magnetic ordering
at lower temperature, and increases largely in the field region where the phase
transition occurs.Comment: 12 pages, 18 figures, submitted to J. Phys. Soc. Jp
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