3,009 research outputs found
An Efficient Algorithm for Automatic Structure Optimization in X-ray Standing-Wave Experiments
X-ray standing-wave photoemission experiments involving multilayered samples
are emerging as unique probes of the buried interfaces that are ubiquitous in
current device and materials research. Such data require for their analysis a
structure optimization process comparing experiment to theory that is not
straightforward. In this work, we present a new computer program for optimizing
the analysis of standing-wave data, called SWOPT, that automates this
trial-and-error optimization process. The program includes an algorithm that
has been developed for computationally expensive problems: so-called black-box
simulation optimizations. It also includes a more efficient version of the Yang
X-ray Optics Program (YXRO) [Yang, S.-H., Gray, A.X., Kaiser, A.M., Mun, B.S.,
Sell, B.C., Kortright, J.B., Fadley, C.S., J. Appl. Phys. 113, 1 (2013)] which
is about an order of magnitude faster than the original version. Human
interaction is not required during optimization. We tested our optimization
algorithm on real and hypothetical problems and show that it finds better
solutions significantly faster than a random search approach. The total
optimization time ranges, depending on the sample structure, from minutes to a
few hours on a modern laptop computer, and can be up to 100x faster than a
corresponding manual optimization. These speeds make the SWOPT program a
valuable tool for realtime analyses of data during synchrotron experiments
Magneto-elastic oscillations modulating the emission of magnetars
Magneto-elastic oscillations of neutron stars are believed to explain
observed quasi-periodic oscillations (QPOs) in the decaying tail of the giant
flares of highly magnetized neutron stars (magnetars). Strong efforts of the
theoretical modelling from different groups have increased our understanding of
this phenomenon significantly. Here, we discuss some constraints on the matter
in neutron stars that arise if the interpretation of the observations in terms
of superfluid, magneto-elastic oscillations is correct. To explain the observed
modulation of the light curve of the giant flare, we describe a model that
allows the QPOs to couple to the stellar exterior through the magnetic field.
In this magnetosphere, the shaking magnetic field induces currents that provide
scattering targets for resonant cyclotron scattering of photons, which is
calculated with a Monte-Carlo approach and coupled to a code that calculates
the momentum distribution of the charge carriers as a one-dimensional
accelerator problem. We show first results of a simplified, but self-consistent
momentum distribution, i.e. a waterbag distribution, and of the corresponding
spectra.Comment: 7 pages, 4 figures, proceedings of stars2017 and 2017smfn
The linear matching method applied to the high temperature life integrity of structures. Part 1. Assessments involving constant residual stress fields
Design and life assessment procedures for high temperatures are based on 'expert knowledge' in structural mechanics and materials science, combined with simplified methods of structural analysis. Of these R5 is one of the most widely used life assessment methods internationally with procedures based on reference stress techniques and shakedown calculations using linear elastic solutions. These have been augmented by full finite element analysis and, recently, the development of a new programming method, the Linear Matching Method (LMM), that allows a range of direct solutions that include shakedown methods and simplified analysis in excess of shakedown. In this paper LMM procedures are compared with calculations typical of those employed in R5 for cyclic loading problems when the assumption of a constant residual stress field is appropriate including shakedown and limit analyses, creep rupture analysis and the evaluation of accumulated creep deformation. A typical example of a 3D holed plate subjected to a cyclic thermal load and a constant mechanical load is assessed in detail. These comparisons demonstrate the significant advantages of linear matching methods for a typical case. For a range of cyclic problems when the residual stress field varies during the cycle, which include the evaluation of plastic strain amplitude, ratchet limit and accumulated creep strains during a high temperature dwell periods, the corresponding LMM and R5 procedures are discussed in an accompanying paper
VLSI implementation of a multi-mode turbo/LDPC decoder architecture
Flexible and reconfigurable architectures have gained wide popularity in the communications field. In particular, reconfigurable architectures for the physical layer are an attractive solution not only to switch among different coding modes but also to achieve interoperability. This work concentrates on the design of a reconfigurable architecture for both turbo and LDPC codes decoding. The novel contributions of this paper are: i) tackling the reconfiguration issue introducing a formal and systematic treatment that, to the best of our knowledge, was not previously addressed; ii) proposing a reconfigurable NoCbased turbo/LDPC decoder architecture and showing that wide flexibility can be achieved with a small complexity overhead. Obtained results show that dynamic switching between most of considered communication standards is possible without pausing the decoding activity. Moreover, post-layout results show that tailoring the proposed architecture to the WiMAX standard leads to an area occupation of 2.75 mm2 and a power consumption of 101.5 mW in the worst case
CFD Assessment of Forward Booster Separation Motor Ignition Overpressure on ET XT 718 Ice/Frost Ramp
Computational fluid dynamics assessment of the forward booster separation motor ignition over-pressure was performed on the space shuttle external tank X(sub T) 718 ice/frost ramp using the flow solver OVERFLOW. The main objective of this study was the investigation of the over-pressure during solid rocket booster separation and its affect on the local pressure and air-load environments. Delta pressure and plume impingement were investigated as a possible contributing factor to the cause of the debris loss on shuttle missions STS-125 and STS-127. A simplified computational model of the Space Shuttle Launch Vehicle was developed consisting of just the external tank and the solid rocket boosters with separation motor nozzles and plumes. The simplified model was validated by comparison to full fidelity computational model of the Space Shuttle without the separation motors. Quasi steady-state plume solutions were used to calibrate the thrust of the separation motors. Time-accurate simulations of the firing of the booster-separation motors were performed. Parametric studies of the time-step size and the number of sub-iterations were used to find the best converged solution. The computed solutions were compared to previous OVERFLOW steady-state runs of the separation motors with reaction control system jets and to ground test data. The results indicated that delta pressure from the overpressure was small and within design limits, and thus was unlikely to have contributed to the foam losses
Parametric high resolution techniques for radio astronomical imaging
The increased sensitivity of future radio telescopes will result in
requirements for higher dynamic range within the image as well as better
resolution and immunity to interference. In this paper we propose a new matrix
formulation of the imaging equation in the cases of non co-planar arrays and
polarimetric measurements. Then we improve our parametric imaging techniques in
terms of resolution and estimation accuracy. This is done by enhancing both the
MVDR parametric imaging, introducing alternative dirty images and by
introducing better power estimates based on least squares, with positive
semi-definite constraints. We also discuss the use of robust Capon beamforming
and semi-definite programming for solving the self-calibration problem.
Additionally we provide statistical analysis of the bias of the MVDR beamformer
for the case of moving array, which serves as a first step in analyzing
iterative approaches such as CLEAN and the techniques proposed in this paper.
Finally we demonstrate a full deconvolution process based on the parametric
imaging techniques and show its improved resolution and sensitivity compared to
the CLEAN method.Comment: To appear in IEEE Journal of Selected Topics in Signal Processing,
Special issue on Signal Processing for Astronomy and space research. 30 page
Non-recursive max* operator with reduced implementation complexity for turbo decoding
In this study, the authors deal with the problem of how to effectively approximate the max?? operator when having n > 2 input values, with the aim of reducing implementation complexity of conventional Log-MAP turbo decoders. They show that, contrary to previous approaches, it is not necessary to apply the max?? operator recursively over pairs of values. Instead, a simple, yet effective, solution for the max?? operator is revealed having the advantage of being in non-recursive form and thus, requiring less computational effort. Hardware synthesis results for practical turbo decoders have shown implementation savings for the proposed method against the most recent published efficient turbo decoding algorithms by providing near optimal bit error rate (BER) performance
A System for Compressive Sensing Signal Reconstruction
An architecture for hardware realization of a system for sparse signal
reconstruction is presented. The threshold based reconstruction method is
considered, which is further modified in this paper to reduce the system
complexity in order to provide easier hardware realization. Instead of using
the partial random Fourier transform matrix, the minimization problem is
reformulated using only the triangular R matrix from the QR decomposition. The
triangular R matrix can be efficiently implemented in hardware without
calculating the orthogonal Q matrix. A flexible and scalable realization of
matrix R is proposed, such that the size of R changes with the number of
available samples and sparsity level.Comment: 6 page
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