870 research outputs found
Nonlinear dynamics and band transport in a superlattice driven by a plane wave
A quantum particle transport induced in a spatially-periodic potential by a
propagating plane wave has a number important implications in a range of
topical physical systems. Examples include acoustically driven semiconductor
superlattices and cold atoms in optical crystal. Here we apply kinetic
description of the directed transport in a superlattice beyond standard linear
approximation, and utilize exact path-integral solutions of the semiclassical
transport equation. We show that the particle drift and average velocities have
non-monotonic dependence on the wave amplitude with several prominent extrema.
Such nontrivial kinetic behaviour is related to global bifurcations developing
with an increase of the wave amplitude. They cause dramatic transformations of
the system phase space and lead to changes of the transport regime. We describe
different types of phase trajectories contributing to the directed transport
and analyse their spectral content
Demonstration of the Dynamic Flowgraph Methodology using the Titan 2 Space Launch Vehicle Digital Flight Control System
Dynamic Flowgraph Methodology (DFM) is a new approach developed to integrate the modeling and analysis of the hardware and software components of an embedded system. The objective is to complement the traditional approaches which generally follow the philosophy of separating out the hardware and software portions of the assurance analysis. In this paper, the DFM approach is demonstrated using the Titan 2 Space Launch Vehicle Digital Flight Control System. The hardware and software portions of this embedded system are modeled in an integrated framework. In addition, the time dependent behavior and the switching logic can be captured by this DFM model. In the modeling process, it is found that constructing decision tables for software subroutines is very time consuming. A possible solution is suggested. This approach makes use of a well-known numerical method, the Newton-Raphson method, to solve the equations implemented in the subroutines in reverse. Convergence can be achieved in a few steps
GEANT4 low energy electromagnetic models for electrons and photons
A set of physics processes has been developed in the Geant4 Simulation Toolkit to describe the electromagnetic interactions of photons and electrons with matter down to 250 eV. Preliminary comparisons of the models with experimental data show a satisfactory agreement
Postoperative peri-axillary seroma following axillary artery cannulation for surgical treatment of acute type A aortic dissection
The arterial cannulation site for optimal tissue perfusion and cerebral protection during cardiopulmonary bypass (CPB) for surgical treatment of acute type A aortic dissection remains controversial. Right axillary artery cannulation confers significant advantages, because it provides antegrade arterial perfusion during cardiopulmonary bypass, and allows continuous antegrade cerebral perfusion during hypothermic circulatory arrest, thereby minimizing global cerebral ischemia. However, right axillary artery cannulation has been associated with serious complications, including problems with systemic perfusion during cardiopulmonary bypass, problems with postoperative patency of the artery due to stenosis, thrombosis or dissection, and brachial plexus injury. We herein present the case of a 36-year-old Caucasian man with known Marfan syndrome and acute type A aortic dissection, who had direct right axillary artery cannulation for surgery of the ascending aorta. Postoperatively, the patient developed an axillary perigraft seroma. As this complication has, not, to our knowledge, been reported before in cardiothoracic surgery, we describe this unusual complication and discuss conservative and surgical treatment options
Findings of a review of spacecraft fire safety needs
Discussions from a workshop to guide UCLA and NASA investigators on the state of knowledge and perceived needs in spacecraft fire safety and its risk management are reviewed, for an introduction to an analytical and experimental project in this field. The report summarizes the workshop discussions and includes the visual aids used in the presentations. Probabilistic Safety Assessment (PSA) methods, which are currently not used, would be of great value to the designs and operation of future human-crew spacecraft. Key points in the discussions were the importance of understanding and testing smoldering as a likely fire scenario in space and the need for smoke damage modeling, since many fire-risk models ignore this mechanism and consider only heat damage
Recommended from our members
Optimized, Competitive Supercritical-CO2 Cycle GFR for Gen IV Service
An overall plant design was developed for a gas-cooled fast reactor employing a direct supercritical Brayton power conversion system. The most important findings were that (1) the concept could be capital-cost competitive, but startup fuel cycle costs are penalized by the low core power density, specified in large part to satisfy the goal of significatn post-accident passive natural convection cooling; (2) active decay heat removal is preferable as the first line of defense, with passive performance in a backup role; (3) an innovative tube-in-duct fuel assembly, vented to the primpary coolant, appears to be practicable; and (4) use of the S-Co2 GFR to support hydrogen production is a synergistic application, since sufficient energy can be recuperated from the product H2 and 02 to allow the electrolysis cell to run 250 C hotter than the reactor coolant, and the water boilers can be used for reactor decay heat removal. Increasing core poer density is identified as the top priority for future work on GFRs of this type
A test of Local Realism with entangled kaon pairs and without inequalities
We propose the use of entangled pairs of neutral kaons, considered as a
promising tool to close the well known loopholes affecting generic Bell's
inequality tests, in a specific Hardy-type experiment. Hardy's contradiction
without inequalities between Local Realism and Quantum Mechanics can be
translated into a feasible experiment by requiring ideal detection efficiencies
for only one of the observables to be alternatively measured. Neutral kaons are
near to fulfil this requirement and therefore to close the efficiency loophole.Comment: 4 RevTeX page
Kinetic models for dilute solutions of dumbbells in non-homogeneous flows revisited
We propose a two fluid theory to model a dilute polymer solution assuming
that it consists of two phases, polymer and solvent, with two distinct
macroscopic velocities. The solvent phase velocity is governed by the
macroscopic Navier-Stokes equations with the addition of a force term
describing the interaction between the two phases. The polymer phase is
described on the mesoscopic level using a dumbbell model and its macroscopic
velocity is obtained through averaging. We start by writing down the full
phase-space distribution function for the dumbbells and then obtain the
inertialess limits for the Fokker-Planck equation and for the averaged friction
force acting between the phases from a rigorous asymptotic analysis. The
resulting equations are relevant to the modelling of strongly non-homogeneous
flows, while the standard kinetic model is recovered in the locally homogeneous
case
Risk-based Spacecraft Fire Safety Experiments
Viewgraphs on risk-based spacecraft fire safety experiments are presented. Spacecraft fire risk can never be reduced to a zero probability. Probabilistic risk assessment is a tool to reduce risk to an acceptable level
- …