5,294 research outputs found
Feature Lines for Illustrating Medical Surface Models: Mathematical Background and Survey
This paper provides a tutorial and survey for a specific kind of illustrative
visualization technique: feature lines. We examine different feature line
methods. For this, we provide the differential geometry behind these concepts
and adapt this mathematical field to the discrete differential geometry. All
discrete differential geometry terms are explained for triangulated surface
meshes. These utilities serve as basis for the feature line methods. We provide
the reader with all knowledge to re-implement every feature line method.
Furthermore, we summarize the methods and suggest a guideline for which kind of
surface which feature line algorithm is best suited. Our work is motivated by,
but not restricted to, medical and biological surface models.Comment: 33 page
Jeans Instability in a Tidally Disrupted Halo Satellite Galaxy
We use a hybrid test particle/N-body simulation to integrate 4 million
massless test particle trajectories within a fully self-consistent 10^5
particle N-body simulation. The number of massless particles allows us to
resolve fine structure in the spatial distribution and phase space of a dwarf
galaxy as it is disrupted in the tidal field of a Milky Way type galaxy. The
tidal tails exhibit nearly periodic clumping or a smoke-like appearance. By
running simulations with different satellite particle mass, halo particle mass,
number of massive and massless particles and with and without a galaxy disk, we
have determined that the instabilities are not due to numerical noise,
amplification of structure in the halo, or shocking as the satellite passes
through the disk of the Galaxy. We measure Jeans wavelengths and growth
timescales in the tidal tail and show that the Jeans instability is a viable
explanation for the clumps. We find that the instability causes velocity
perturbations of order 10 km/s. Clumps in tidal tails present in the Milky Way
could be seen in stellar radial velocity surveys as well as number counts. We
find that the unstable wavelength growth is sensitive to the simulated mass of
dark matter halo particles. A simulation with a smoother halo exhibits colder
and thinner tidal tails with more closely spaced clumps than a simulation with
more massive dark matter halo particles. Heating by the halo particles
increases the Jeans wavelength in the tidal tail affecting substructure
development, suggesting an intricate connection between tidal tails and dark
matter halo substructure.Comment: 15 pages, 7 figures, submitted to MNRAS, May 25 201
Molecular ferroelectric contributions to anomalous hysteresis in hybrid perovskite solar cells
We report a model describing the molecular orientation disorder in
CH3NH3PbI3, solving a classical Hamiltonian parametrised with electronic
structure calculations, with the nature of the motions informed by ab-initio
molecular dynamics. We investigate the temperature and static electric field
dependence of the equilibrium ferroelectric (molecular) domain structure and
resulting polarisability. A rich domain structure of twinned molecular dipoles
is observed, strongly varying as a function of temperature and applied electric
field. We propose that the internal electrical fields associated with
microscopic polarisation domains contribute to hysteretic anomalies in the
current--voltage response of hybrid organic-inorganic perovskite solar cells
due to variations in electron-hole recombination in the bulk.Comment: 10 pages; 4 figures, 2 SI figure
Oceanographic Weather Maps: Using Oceanographic Models to Improve Seabed Mapping Planning and Acquisition
In a world of high precision sensors, one of the few remaining challenges in multibeam echosounding is that of refraction based uncertainty. A poor understanding of oceanographic variability can lead to inadequate sampling of the water mass and the uncertainties that result from this can dominate the uncertainty budget of even state-of-the-art echosounding systems. Though dramatic improvements have been made in sensor accuracies over the past few decades, survey accuracy and efficiency is still potentially limited by a poor understanding of the “underwater weather”. Advances in the sophistication of numerical oceanographic forecast modeling, combined with ever increasing computing power, allow for the timely operation and dissemination of oceanographic nowcast and forecast model systems on regional and global scales. These sources of information, when examined using sound speed uncertainty analysis techniques, have the potential to change the way hydrographers work by increasing our understanding of what to expect from the ocean and when to expect it. Sound speed analyses derived from ocean modeling system’s three-dimensional predictions could provide guidance for hydrographers during survey planning, acquisition and post-processing of hydrographic data. In this work, we examine techniques for processing and visualizing of predictions from global and regional operational oceanographic forecast models and climatological analyses from an ocean atlas to better understand how these data could best be put to use to in the field of hydrograph
Super-orbital re-entry in Australia - laboratory measurement, simulation and flight observation
There are large uncertainties in the aerothermodynamic modelling of super-orbital re-entry which impact the design of spacecraft thermal protection systems (TPS). Aspects of the thermal environment of super-orbital re-entry flows can be simulated in the laboratory using arc- and plasma jet facilities and these devices are regularly used for TPS certification work [5]. Another laboratory device which is capable of simulating certain critical features of both the aero and thermal environment of super-orbital re-entry is the expansion tube, and three such facilities have been operating at the University of Queensland in recent years[10]. Despite some success, wind tunnel tests do not achieve full simulation, however, a virtually complete physical simulation of particular re-entry conditions can be obtained from dedicated flight testing, and the Apollo era FIRE II flight experiment [2] is the premier example which still forms an important benchmark for modern simulations. Dedicated super-orbital flight testing is generally considered too expensive today, and there is a reluctance to incorporate substantial instrumentation for aerothermal diagnostics into existing missions since it may compromise primary mission objectives. An alternative approach to on-board flight measurements, with demonstrated success particularly in the ‘Stardust’ sample return mission, is remote observation of spectral emissions from the capsule and shock layer [8]. JAXA’s ‘Hayabusa’ sample return capsule provides a recent super-orbital reentry example through which we illustrate contributions in three areas: (1) physical simulation of super-orbital re-entry conditions in the laboratory; (2) computational simulation of such flows; and (3) remote acquisition of optical emissions from a super-orbital re entry event
Convergence of invariant densities in the small-noise limit
This paper presents a systematic numerical study of the effects of noise on
the invariant probability densities of dynamical systems with varying degrees
of hyperbolicity. It is found that the rate of convergence of invariant
densities in the small-noise limit is frequently governed by power laws. In
addition, a simple heuristic is proposed and found to correctly predict the
power law exponent in exponentially mixing systems. In systems which are not
exponentially mixing, the heuristic provides only an upper bound on the power
law exponent. As this numerical study requires the computation of invariant
densities across more than 2 decades of noise amplitudes, it also provides an
opportunity to discuss and compare standard numerical methods for computing
invariant probability densities.Comment: 27 pages, 19 figures, revised with minor correction
Atomically thin dilute magnetism in Co-doped phosphorene
Two-dimensional dilute magnetic semiconductors can provide fundamental
insights in the very nature of magnetic orders and their manipulation through
electron and hole doping. Despite the fundamental physics, due to the large
charge density control capability in these materials, they can be extremely
important in spintronics applications such as spin valve and spin-based
transistors. In this article, we studied a two-dimensional dilute magnetic
semiconductors consisting of phosphorene monolayer doped with cobalt atoms in
substitutional and interstitial defects. We show that these defects can be
stabilized and are electrically active. Furthermore, by including holes or
electrons by a potential gate, the exchange interaction and magnetic order can
be engineered, and may even induce a ferromagnetic-to-antiferromagnetic phase
transition in p-doped phosphorene.Comment: 7 pages, 4 colorful figure
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