8,707 research outputs found
Protoplanetary Disk Turbulence Driven by the Streaming Instability: Non-Linear Saturation and Particle Concentration
We present simulations of the non-linear evolution of streaming instabilities
in protoplanetary disks. The two components of the disk, gas treated with grid
hydrodynamics and solids treated as superparticles, are mutually coupled by
drag forces. We find that the initially laminar equilibrium flow spontaneously
develops into turbulence in our unstratified local model. Marginally coupled
solids (that couple to the gas on a Keplerian time-scale) trigger an upward
cascade to large particle clumps with peak overdensities above 100. The clumps
evolve dynamically by losing material downstream to the radial drift flow while
receiving recycled material from upstream. Smaller, more tightly coupled solids
produce weaker turbulence with more transient overdensities on smaller length
scales. The net inward radial drift is decreased for marginally coupled
particles, whereas the tightly coupled particles migrate faster in the
saturated turbulent state. The turbulent diffusion of solid particles, measured
by their random walk, depends strongly on their stopping time and on the
solids-to-gas ratio of the background state, but diffusion is generally modest,
particularly for tightly coupled solids. Angular momentum transport is too weak
and of the wrong sign to influence stellar accretion. Self-gravity and
collisions will be needed to determine the relevance of particle overdensities
for planetesimal formation.Comment: Accepted for publication in ApJ (17 pages). Movies of the simulations
can be downloaded at http://www.mpia.de/~johansen/research_en.ph
Ray optics in flux avalanche propagation in superconducting films
Experimental evidence of wave properties of dendritic flux avalanches in
superconducting films is reported. Using magneto-optical imaging the
propagation of dendrites across boundaries between a bare NbN film and areas
coated by a Cu-layer was visualized, and it was found that the propagation is
refracted in full quantitative agreement with Snell's law. For the studied film
of 170 nm thickness and a 0.9 mkm thick metal layer, the refractive index was
close to n=1.4. The origin of the refraction is believed to be caused by the
dendrites propagating as an electromagnetic shock wave, similar to damped modes
considered previously for normal metals. The analogy is justified by the large
dissipation during the avalanches raising the local temperature significantly.
Additional time-resolved measurements of voltage pulses generated by segments
of the dendrites traversing an electrode confirm the consistency of the adapted
physical picture.Comment: 4 pages, 4 figure
Stock mechanics: predicting recession in S&P500, DJIA, and NASDAQ
An original method, assuming potential and kinetic energy for prices and
conservation of their sum is developed for forecasting exchanges. Connections
with power law are shown. Semiempirical applications on S&P500, DJIA, and
NASDAQ predict a coming recession in them. An emerging market, Istanbul Stock
Exchange index ISE-100 is found involving a potential to continue to rise.Comment: 14 pages, 4 figure
Dust sedimentation and self-sustained Kelvin-Helmholtz turbulence in protoplanetary disk mid-planes. I. Radially symmetric simulations
We perform numerical simulations of the Kelvin-Helmholtz instability in the
mid-plane of a protoplanetary disk. A two-dimensional corotating slice in the
azimuthal--vertical plane of the disk is considered where we include the
Coriolis force and the radial advection of the Keplerian rotation flow. Dust
grains, treated as individual particles, move under the influence of friction
with the gas, while the gas is treated as a compressible fluid. The friction
force from the dust grains on the gas leads to a vertical shear in the gas
rotation velocity. As the particles settle around the mid-plane due to gravity,
the shear increases, and eventually the flow becomes unstable to the
Kelvin-Helmholtz instability. The Kelvin-Helmholtz turbulence saturates when
the vertical settling of the dust is balanced by the turbulent diffusion away
from the mid-plane. The azimuthally averaged state of the self-sustained
Kelvin-Helmholtz turbulence is found to have a constant Richardson number in
the region around the mid-plane where the dust-to-gas ratio is significant.
Nevertheless the dust density has a strong non-axisymmetric component. We
identify a powerful clumping mechanism, caused by the dependence of the
rotation velocity of the dust grains on the dust-to-gas ratio, as the source of
the non-axisymmetry. Our simulations confirm recent findings that the critical
Richardson number for Kelvin-Helmholtz instability is around unity or larger,
rather than the classical value of 1/4Comment: Accepted for publication in ApJ. Some minor changes due to referee
report, most notably that the clumping mechanism has been identified as the
streaming instability of Youdin & Goodman (2005). Movies of the simulations
are still available at http://www.mpia.de/homes/johansen/research_en.ph
Synchronization and Coarsening (without SOC) in a Forest-Fire Model
We study the long-time dynamics of a forest-fire model with deterministic
tree growth and instantaneous burning of entire forests by stochastic lightning
strikes. Asymptotically the system organizes into a coarsening self-similar
mosaic of synchronized patches within which trees regrow and burn
simultaneously. We show that the average patch length grows linearly with
time as t-->oo. The number density of patches of length L, N(L,t), scales as
^{-2}M(L/), and within a mean-field rate equation description we find
that this scaling function decays as e^{-1/x} for x-->0, and as e^{-x} for
x-->oo. In one dimension, we develop an event-driven cluster algorithm to study
the asymptotic behavior of large systems. Our numerical results are consistent
with mean-field predictions for patch coarsening.Comment: 5 pages, 4 figures, 2-column revtex format. To be submitted to PR
Decay dynamics of quantum dots influenced by the local density of optical states of two-dimensional photonic crystal membranes
We have performed time-resolved spectroscopy on InAs quantum dot ensembles in
photonic crystal membranes. The influence of the photonic crystal is
investigated by varying the lattice constant systematically. We observe a
strong slow down of the quantum dots' spontaneous emission rates as the
two-dimensional bandgap is tuned through their emission frequencies. The
measured band edges are in full agreement with theoretical predictions. We
characterize the multi-exponential decay curves by their mean decay time and
find enhancement of the spontaneous emission at the bandgap edges and strong
inhibition inside the bandgap in good agreement with local density of states
calculations.Comment: 9 pages (preprint), 3 figure
Addressing \mu-b_\mu and proton lifetime problems and active neutrino masses in a U(1)^\prime-extended supergravity model
We present a locally supersymmetric extension of the minimal supersymmetric
Standard Model (MSSM) based on the gauge group where, except for the supersymmetry breaking scale
which is fixed to be GeV, we require that all non-Standard-Model
parameters allowed by the {\it local} spacetime and gauge symmetries assume
their natural values. The symmetry, which is spontaneously broken
at the intermediate scale, serves to ({\it i}) explain the weak scale
magnitudes of and terms, ({\it ii}) ensure that dimension-3 and
dimension-4 baryon-number-violating superpotential operators are forbidden,
solving the proton-lifetime problem, ({\it iii}) predict {\it bilinear lepton
number violation} in the superpotential at just the right level to accommodate
the observed mass and mixing pattern of active neutrinos (leading to a novel
connection between the SUSY breaking scale and neutrino masses), while
corresponding trilinear operators are strongly supppressed. The phenomenology
is like that of the MSSM with bilinear R-parity violation, were the would-be
lightest supersymmetric particle decays leptonically with a lifetime of s. Theoretical consistency of our model requires the
existence of multi-TeV, stable, colour-triplet, weak-isosinglet scalars or
fermions, with either conventional or exotic electric charge which should be
readily detectable if they are within the kinematic reach of a hadron collider.
Null results of searches for heavy exotic isotopes implies that the re-heating
temperature of our Universe must have been below their mass scale which, in
turn, suggests that sphalerons play a key role for baryogensis. Finally, the
dark matter cannot be the weakly interacting neutralino.Comment: 33 pages, 2 figures, Discussion on proton decay and radiative
neutrino masses augmented, and references adde
Dendritic flux patterns in MgB2 films
Magneto-opitcal studies of a c-oriented epitaxial MgB2 film with critical
current density 10^7 A/cm^2 demonstrate a breakdown of the critical state at
temperatures below 10 K [cond-mat/0104113]. Instead of conventional uniform and
gradual flux penetration in an applied magnetic field, we observe an abrupt
invasion of complex dendritic structures. When the applied field subsequently
decreases, similar dendritic structures of the return flux penetrate the film.
The static and dynamic properties of the dendrites are discussed.Comment: Accepted to Supercond. Sci. Techno
Construction and validation of a low cost paediatric pelvis phantom
PURPOSE: Imaging phantoms can be cost prohibitive, therefore a need exists to produce low cost alternatives which are fit for purpose. This paper describes the development and validation of a low cost paediatric pelvis phantom based on the anatomy of a 5-year-old child.
METHODS: Tissue equivalent materials representing paediatric bone (Plaster of Paris; PoP) and soft tissue (Poly methyl methacrylate; PMMA) were used. PMMA was machined to match the bony anatomy identified from a CT scan of a 5-year-old child and cavities were created for infusing the PoP. Phantom validation comprised physical and visual measures. Physical included CT density comparison between a CT scan of a 5-year old child and the phantom and Signal to Noise Ratio (SNR) comparative analysis of anteroposterior phantom X-ray images against a commercial anthropomorphic phantom. Visual analysis using a psychometric image quality scale (face validity).
RESULTS: CT density, the percentage difference between cortical bone, soft tissue and their equivalent tissue substitutes were -4.7 to -4.1% and -23.4%, respectively. For SNR, (mAs response) there was a strong positive correlation between the two phantoms (r>0.95 for all kVps). For kVp response, there was a strong positive correlation between 1 and 8 mAs (r=0.85), this then decreased as mAs increased (r=-0.21 at 20 mAs). Psychometric scale results produced a Cronbach’s Alpha of almost 0.8.
CONCLUSIONS: Physical and visual measures suggest our low-cost phantom has suitable anatomical characteristics for X-ray imaging. Our phantom could have utility in dose and image quality optimisation studies.
Keywords: Pelvis phantom, low-cost, dose optimisation, validation, development
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