207,981 research outputs found
Examining EXPRESS with Simulation
The Execution and Prioritization of Repair Support System (EXPRESS) is a database tool used by the Air Force to prioritize depot maintenance of reparable spare parts to maximize responsiveness to warfighter need. Many studies have examined individual portions of EXPRESS, though few have examined it as an entire system. This effort proposes a modeling approach for examining the overall system behavior of EXPRESS using discrete event simulation. The emphasis of the model is to be flexible enough to provide useful insight into system performance, while also remaining open-ended enough to provide a foundation for future expansion and analysis. A case study involving three reparable parts managed by EXPRESS, based on 6 months of real world data, focuses on total Mission Capability (MICAP) hours as a measure of responsiveness to customer need. The model is validated using data on actual MICAP hours for the modeled period. The case study simulation is then used to study the impact on responsiveness and repair behavior resulting from running EXPRESS less frequently. Output data points to increases in total MICAP hours and variance in repair workload as run frequency decreases. The conclusion is that running EXPRESS less frequently negatively impacts system performance for both the maintenance and warfighter communitie
DESQ: Frequent Sequence Mining with Subsequence Constraints
Frequent sequence mining methods often make use of constraints to control
which subsequences should be mined. A variety of such subsequence constraints
has been studied in the literature, including length, gap, span,
regular-expression, and hierarchy constraints. In this paper, we show that many
subsequence constraints---including and beyond those considered in the
literature---can be unified in a single framework. A unified treatment allows
researchers to study jointly many types of subsequence constraints (instead of
each one individually) and helps to improve usability of pattern mining systems
for practitioners. In more detail, we propose a set of simple and intuitive
"pattern expressions" to describe subsequence constraints and explore
algorithms for efficiently mining frequent subsequences under such general
constraints. Our algorithms translate pattern expressions to compressed finite
state transducers, which we use as computational model, and simulate these
transducers in a way suitable for frequent sequence mining. Our experimental
study on real-world datasets indicates that our algorithms---although more
general---are competitive to existing state-of-the-art algorithms.Comment: Long version of the paper accepted at the IEEE ICDM 2016 conferenc
Plasma Edge Kinetic-MHD Modeling in Tokamaks Using Kepler Workflow for Code Coupling, Data Management and Visualization
A new predictive computer simulation tool targeting the development of the H-mode pedestal at the plasma edge in tokamaks and the triggering and dynamics of edge localized modes (ELMs) is presented in this report. This tool brings together, in a coordinated and effective manner, several first-principles physics simulation codes, stability analysis packages, and data processing and visualization tools. A Kepler workflow is used in order to carry out an edge plasma simulation that loosely couples the kinetic code, XGC0, with an ideal MHD linear stability analysis code, ELITE, and an extended MHD initial value code such as M3D or NIMROD. XGC0 includes the neoclassical ion-electron-neutral dynamics needed to simulate pedestal growth near the separatrix. The Kepler workflow processes the XGC0 simulation results into simple images that can be selected and displayed via the Dashboard, a monitoring tool implemented in AJAX allowing the scientist to track computational resources, examine running and archived jobs, and view key physics data, all within a standard Web browser. The XGC0 simulation is monitored for the conditions needed to trigger an ELM crash by periodically assessing the edge plasma pressure and current density profiles using the ELITE code. If an ELM crash is triggered, the Kepler workflow launches the M3D code on a moderate-size Opteron cluster to simulate the nonlinear ELM crash and to compute the relaxation of plasma profiles after the crash. This process is monitored through periodic outputs of plasma fluid quantities that are automatically visualized with AVS/Express and may be displayed on the Dashboard. Finally, the Kepler workflow archives all data outputs and processed images using HPSS, as well as provenance information about the software and hardware used to create the simulation. The complete process of preparing, executing and monitoring a coupled-code simulation of the edge pressure pedestal buildup and the ELM cycle using the Kepler scientific workflow system is described in this paper
Non-equilibrium dynamics in the dual-wavelength operation of Vertical external-cavity surface-emitting lasers
Microscopic many-body theory coupled to Maxwell's equation is used to
investigate dual-wavelength operation in vertical external-cavity
surface-emitting lasers. The intrinsically dynamic nature of coexisting
emission wavelengths in semiconductor lasers is associated with characteristic
non-equilibrium carrier dynamics which causes significant deformations of the
quasi-equilibrium gain and carrier inversion. Extended numerical simulations
are employed to efficiently investigate the parameter space to identify the
regime for two-wavelength operation. Using a frequency selective intracavity
etalon, two families of modes are stabilized with dynamical interchange of the
strongest emission peaks. For this operation mode, anti-correlated intensity
noise is observed in agreement with the experiment. A method using effective
frequency selective filtering is suggested for stabilization genuine
dual-wavelength output.Comment: 15 pages, 7 figure
Retrieval of phase relation and emission profile of quantum cascade laser frequency combs
The major development recently undergone by quantum cascade lasers has
effectively extended frequency comb emission to longer-wavelength spectral
regions, i.e. the mid and far infrared. Unlike classical pulsed frequency
combs, their mode-locking mechanism relies on four-wave mixing nonlinear
processes, with a temporal intensity profile different from conventional
short-pulses trains. Measuring the absolute phase pattern of the modes in these
combs enables a thorough characterization of the onset of mode-locking in
absence of short-pulses emission, as well as of the coherence properties. Here,
by combining dual-comb multi-heterodyne detection with Fourier-transform
analysis, we show how to simultaneously acquire and monitor over a wide range
of timescales the phase pattern of a generic frequency comb. The technique is
applied to characterize a mid-infrared and a terahertz quantum cascade laser
frequency comb, conclusively proving the high degree of coherence and the
remarkable long-term stability of these sources. Moreover, the technique allows
also the reconstruction of electric field, intensity profile and instantaneous
frequency of the emission.Comment: 20 pages. Submitted to Nature Photonic
Full-wave parallel dispersive finite-difference time-domain modeling of three-dimensional electromagnetic cloaking structures
A parallel dispersive finite-difference time-domain (FDTD) method for the
modeling of three-dimensional (3-D) electromagnetic cloaking structures is
presented in this paper. The permittivity and permeability of the cloak are
mapped to the Drude dispersion model and taken into account in FDTD simulations
using an auxiliary differential equation (ADE) method. It is shown that the
correction of numerical material parameters and the slow switching-on of source
are necessary to ensure stable and convergent single-frequency simulations.
Numerical results from wideband simulations demonstrate that waves passing
through a three-dimensional cloak experience considerable delay comparing with
the free space propagations, as well as pulse broadening and blue-shift
effects
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