42,375 research outputs found
Tracing the ISM magnetic field morphology: The potential of multi-wavelength polarization measurements
We present a case study to demonstrate the potential of
multi-wavelength polarization measurements. The aim is to investigate the
effects that dichroic polarization and thermal re-emission have on tracing the
magnetic field in the interstellar medium (ISM). Furthermore, we analyze the
crucial influence of imperfectly aligned compact dust grains on the resulting
synthetic continuum polarization maps. We developed an
extended version of the well-known 3D Monte-Carlo radiation transport code MC3D
for multi-wavelength polarization simulations running on an adaptive grid.We
investigated the interplay between radiation, magnetic fields and dust grains.
Our results were produced by post-processing both ideal density distributions
and sophisticated magnetohydrodynamic (MHD) collapse simulations with radiative
transfer simulations. We derived spatially resolved maps of intensity, optical
depth, and linear and circular polarization at various inclination angles and
scales in a wavelength range from 7 to 1 . We
predict unique patterns in linear and circular polarization maps for different
types of density distributions and magnetic field morphologies for test setups
and sophisticated MHD collapse simulations. We show that alignment processes of
interstellar dust grains can significantly influence the resulting synthetic
polarization maps. Multi-wavelength polarization measurements allow one to
predict the morphology of the magnetic field inside the ISM. The interpretation
of polarization measurements of complex structures still remains ambiguous
because of the large variety of the predominant parameters in the ISM.Comment: 14 pages, 12 figures, 1 table, Paper accepted 2014 by A&
Duality in matrix lattice Boltzmann models
The notion of duality between the hydrodynamic and kinetic (ghost) variables
of lattice kinetic formulations of the Boltzmann equation is introduced. It is
suggested that this notion can serve as a guideline in the design of matrix
versions of the lattice Boltzmann equation in a physically transparent and
computationally efficient way.Comment: 12 pages, 3 figure
A deterministic sandpile automaton revisited
The Bak-Tang-Wiesenfeld (BTW) sandpile model is a cellular automaton which
has been intensively studied during the last years as a paradigm for
self-organized criticality. In this paper, we reconsider a deterministic
version of the BTW model introduced by Wiesenfeld, Theiler and McNamara, where
sand grains are added always to one fixed site on the square lattice. Using the
Abelian sandpile formalism we discuss the static properties of the system. We
present numerical evidence that the deterministic model is only in the BTW
universality class if the initial conditions and the geometric form of the
boundaries do not respect the full symmetry of the square lattice.Comment: 7 pages, 8 figures, EPJ style, accepted for publication in European
Physical Journal
Fully-coupled analysis of jet mixing problems. Part 1. Shock-capturing model, SCIPVIS
A computational model, SCIPVIS, is described which predicts the multiple cell shock structure in imperfectly expanded, turbulent, axisymmetric jets. The model spatially integrates the parabolized Navier-Stokes jet mixing equations using a shock-capturing approach in supersonic flow regions and a pressure-split approximation in subsonic flow regions. The regions are coupled using a viscous-characteristic procedure. Turbulence processes are represented via the solution of compressibility-corrected two-equation turbulence models. The formation of Mach discs in the jet and the interactive analysis of the wake-like mixing process occurring behind Mach discs is handled in a rigorous manner. Calculations are presented exhibiting the fundamental interactive processes occurring in supersonic jets and the model is assessed via comparisons with detailed laboratory data for a variety of under- and overexpanded jets
Self streamlining wind tunnel: Low speed testing and transonic test section design
Comprehensive aerodynamic data on an airfoil section were obtained through a wide range of angles of attack, both stalled and unstalled. Data were gathered using a self streamlining wind tunnel and were compared to results obtained on the same section in a conventional wind tunnel. The reduction of wall interference through streamline was demonstrated
Vacuum pumping system for spaceborne passive hydrogen masers
The ultimate utility of hydrogen masers as highly accurate clocks aboard navigation satellites depends on the feasibility of making the maser lightweight, compact, and capable of a 5 to 7 year unattended operation. A vacuum pumping system for the maser that is believed to meet these criteria was designed and fabricated. The pumping system was fabricated almost completely from 6Al-4V titanium alloy and incorporates two or four sintered zirconium carbon getter pumps with integral activation heaters. The manner in which the getter pumps were mounted to insure that they will stand both the activation and the shock of launch is illustrated. Data on the total hydrogen capacity and pumping of this system is also presented
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