1,394 research outputs found
Enhanced Cloud Disruption by Magnetic Field Interaction
We present results from the first three-dimensional numerical simulations of
moderately supersonic cloud motion through a tenuous, magnetized medium. We
show that the interaction of the cloud with a magnetic field perpendicular to
its motion has a great dynamical impact on the development of instabilities at
the cloud surface. Even for initially spherical clouds, magnetic field lines
become trapped in surface deformations and undergo stretching. The consequent
field amplification that occurs there and particularly its variation across the
cloud face then dramatically enhance the growth rate of Rayleigh-Taylor
unstable modes, hastening the cloud disruption.Comment: 4 pages, 2 figures, ApJ (Letter) in press. High resolution postscript
figures available at http://www.msi.umn.edu/Projects/twj/mhd3d
Effect of the mycelium, diffused substances and extracts of the fungus Cladosporium herbarum (Pers.) link on the in vitro and in vivo growth of grapevine (Vitis vinifera L.) cv. Cabernet Sauvignon
The fungus Cladosporium herbarum (Pers.) Link was cocultured with plantlets of grapevine (Vitis vinifera L.) cv. Cabernet Sauvignon in vitro and ex vitro. The mycelium of the fungus stimulates growth of the plantlets. This is probably exerted by the diffusion of some growth and/or nutritional substance(s) of unknown chemical nature. In another trial filtrates of the fungus and fungus extracts were added to the culture medium of grapevine plantlets in different concentrations. The fungus extracts stimulated growth of plantlets at low concentrations and inhibited growth at high concentrations
Fast nonadiabatic dynamics of many-body quantum systems
Modeling many-body quantum systems with strong interactions is one of the core challenges of modern physics. A range of methods has been developed to approach this task, each with its own idiosyncrasies, approximations, and realm of applicability. However, there remain many problems that are intractable for existing methods. In particular, many approaches face a huge computational barrier when modeling large numbers of coupled electrons and ions at finite temperature. Here, we address this shortfall with a new approach to modeling many-body quantum systems. On the basis of the Bohmian trajectory formalism, our new method treats the full particle dynamics with a considerable increase in computational speed. As a result, we are able to perform large-scale simulations of coupled electron-ion systems without using the adiabatic Born-Oppenheimer approximation
Proton imaging of stochastic magnetic fields
Recent laser-plasma experiments report the existence of dynamically
significant magnetic fields, whose statistical characterisation is essential
for understanding the physical processes these experiments are attempting to
investigate. In this paper, we show how a proton imaging diagnostic can be used
to determine a range of relevant magnetic field statistics, including the
magnetic-energy spectrum. To achieve this goal, we explore the properties of an
analytic relation between a stochastic magnetic field and the image-flux
distribution created upon imaging that field. We conclude that features of the
beam's final image-flux distribution often display a universal character
determined by a single, field-scale dependent parameter - the contrast
parameter - which quantifies the relative size of the correlation length of the
stochastic field, proton displacements due to magnetic deflections, and the
image magnification. For stochastic magnetic fields, we establish the existence
of four contrast regimes - linear, nonlinear injective, caustic and diffusive -
under which proton-flux images relate to their parent fields in a qualitatively
distinct manner. As a consequence, it is demonstrated that in the linear or
nonlinear injective regimes, the path-integrated magnetic field experienced by
the beam can be extracted uniquely, as can the magnetic-energy spectrum under a
further statistical assumption of isotropy. This is no longer the case in the
caustic or diffusive regimes. We also discuss complications to the
contrast-regime characterisation arising for inhomogeneous, multi-scale
stochastic fields, as well as limitations currently placed by experimental
capabilities on extracting magnetic field statistics. The results presented in
this paper provide a comprehensive description of proton images of stochastic
magnetic fields, with applications for improved analysis of given proton-flux
images.Comment: Main paper pp. 1-29; appendices pp. 30-84. 24 figures, 2 table
XUV Opacity of Aluminum between the Cold-Solid to Warm-Plasma Transition
We present calculations of the free-free XUV opacity of warm, solid-density
aluminum at photon energies between the plasma frequency at 15 eV and the
L-edge at 73 eV, using both density functional theory combined with molecular
dynamics and a semi-analytical model in the RPA framework with the inclusion of
local field corrections. As the temperature is increased from room temperature
to 10 eV, with the ion and electron temperatures equal, we calculate an
increase in the opacity in the range over which the degree of ionization is
constant. The effect is less pronounced if only the electron temperature is
allowed to increase. The physical significance of these increases is discussed
in terms of intense XUV-laser matter interactions on both femtosecond and
picosecond time-scales.Comment: 4 pages, 3 figure
Modelling passengers in air-rail multimodality
Air-rail mobility has the potential to play a significant role in addressing European mobility challenges such as emissions reduction and capacity shortages. Rail can complement the air network in different ways: enlarging airport catchment areas, supporting operations in case of disruption or replacing air links to obtain environmental benefits. There is, however, still a need to better understand the potential role of rail when substituting current air links both from a strategic and a tactical mobility perspective, particularly when passenger connections are considered. This was initially assessed, considering passengers' door-to-door itineraries, as part of the Modus project (H2020 - SESAR 2020) with an innovative approach towards data driven, integrated air-rail modelling. Further considerations, such as the evaluation of strategic and tactical multimodal solutions, will be explored in the MultiModX project (Horizon Europe - SESAR 3). This discussion paper presents the modelling challenges addressed in Modus and the approach defined for MultiModX to evaluate and model multimodal door-to-door solutions
Plastic Deformation in Laser-Induced Shock Compression of Monocrystalline Copper
Copper monocrystals were subjected to shock compression at pressures of 10–60 GPa by a short (3 ns initial) duration laser pulse. Transmission electron microscopy revealed features consistent with previous observations of shock-compressed copper, albeit at pulse durations in the µs regime. The results suggest that the defect structure is generated at the shock front. A mechanism for dislocation generation is presented, providing a realistic prediction of dislocation density as a function of pressure. The threshold stress for deformation twinning in shock compression is calculated from the constitutive equations for slip, twinning, and the Swegle-Grady relationship
On Upward Drawings of Trees on a Given Grid
Computing a minimum-area planar straight-line drawing of a graph is known to
be NP-hard for planar graphs, even when restricted to outerplanar graphs.
However, the complexity question is open for trees. Only a few hardness results
are known for straight-line drawings of trees under various restrictions such
as edge length or slope constraints. On the other hand, there exist
polynomial-time algorithms for computing minimum-width (resp., minimum-height)
upward drawings of trees, where the height (resp., width) is unbounded.
In this paper we take a major step in understanding the complexity of the
area minimization problem for strictly-upward drawings of trees, which is one
of the most common styles for drawing rooted trees. We prove that given a
rooted tree and a grid, it is NP-hard to decide whether
admits a strictly-upward (unordered) drawing in the given grid.Comment: Appears in the Proceedings of the 25th International Symposium on
Graph Drawing and Network Visualization (GD 2017
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