21,906 research outputs found
Shock wave diffraction phenomena around slotted splitters
In the field of aerospace engineering, the study of the characteristics of vortical flows and their unsteady phenomena finds numerous engineering applications related to improvements in the design of tip devices, enhancement of combustor performance, and control of noise generation. A large amount of work has been carried out in the analysis of the shock wave diffraction around conventional geometries such as sharp and rounded corners, but the employment of splitters with lateral variation has hardly attracted the attention of researchers. The investigation of this phenomenon around two-dimensional wedges has allowed the understanding of the basic physical principles of the flow features. On the other hand, important aspects that appear in the third dimension due to the turbulent nature of the vortices are omitted. The lack of studies that use three-dimensional geometries has motivated the current work to experimentally investigate the evolution of the shock wave diffraction around two splitters with spike-shaped structures for Mach numbers of 1.31 and 1.59. Schlieren photography was used to obtain an insight into the sequential diffraction processes that take place in different planes. Interacting among them, these phenomena generate a complicated turbulent cloud with a vortical arrangement
Two-photon transitions in primordial hydrogen recombination
The subject of cosmological hydrogen recombination has received much
attention recently because of its importance to predictions for and
cosmological constraints from CMB observations. While the central role of the
two-photon decay 2s->1s has been recognized for many decades, high-precision
calculations require us to consider two-photon decays from the higher states
ns,nd->1s (n>=3). Simple attempts to include these processes in recombination
calculations have suffered from physical problems associated with sequences of
one-photon decays, e.g. 3d->2p->1s, that technically also produce two photons.
These correspond to resonances in the two-photon spectrum that are optically
thick, necessitating a radiative transfer calculation. We derive the
appropriate equations, develop a numerical code to solve them, and verify the
results by finding agreement with analytic approximations to the radiative
transfer equation. The related processes of Raman scattering and two-photon
recombination are included using similar machinery. Our results show that early
in recombination the two-photon decays act to speed up recombination, reducing
the free electron abundance by 1.3% relative to the standard calculation at
z=1300. However we find that some photons between Ly-alpha and Ly-beta are
produced, mainly by 3d->1s two-photon decay and 2s->1s Raman scattering. At
later times these photons redshift down to Ly-alpha, excite hydrogen atoms, and
act to slow recombination. Thus the free electron abundance is increased by
1.3% relative to the standard calculation at z=900. The implied correction to
the CMB power spectrum is neligible for the recently released WMAP and ACBAR
data, but at Fisher matrix level will be 7 sigma for Planck. [ABRIDGED]Comment: Matches PRD accepted version. 28 pages, 12 figure
A Space Communications Study Final Report, Sep. 15, 1965 - Sep. 15, 1966
Reception of frequency modulated signals passed through deterministic and random time-varying channel
Characterization and Compensation of Network-Level Anomalies in Mixed-Signal Neuromorphic Modeling Platforms
Advancing the size and complexity of neural network models leads to an ever
increasing demand for computational resources for their simulation.
Neuromorphic devices offer a number of advantages over conventional computing
architectures, such as high emulation speed or low power consumption, but this
usually comes at the price of reduced configurability and precision. In this
article, we investigate the consequences of several such factors that are
common to neuromorphic devices, more specifically limited hardware resources,
limited parameter configurability and parameter variations. Our final aim is to
provide an array of methods for coping with such inevitable distortion
mechanisms. As a platform for testing our proposed strategies, we use an
executable system specification (ESS) of the BrainScaleS neuromorphic system,
which has been designed as a universal emulation back-end for neuroscientific
modeling. We address the most essential limitations of this device in detail
and study their effects on three prototypical benchmark network models within a
well-defined, systematic workflow. For each network model, we start by defining
quantifiable functionality measures by which we then assess the effects of
typical hardware-specific distortion mechanisms, both in idealized software
simulations and on the ESS. For those effects that cause unacceptable
deviations from the original network dynamics, we suggest generic compensation
mechanisms and demonstrate their effectiveness. Both the suggested workflow and
the investigated compensation mechanisms are largely back-end independent and
do not require additional hardware configurability beyond the one required to
emulate the benchmark networks in the first place. We hereby provide a generic
methodological environment for configurable neuromorphic devices that are
targeted at emulating large-scale, functional neural networks
Stimulus-invariant processing and spectrotemporal reverse correlation in primary auditory cortex
The spectrotemporal receptive field (STRF) provides a versatile and
integrated, spectral and temporal, functional characterization of single cells
in primary auditory cortex (AI). In this paper, we explore the origin of, and
relationship between, different ways of measuring and analyzing an STRF. We
demonstrate that STRFs measured using a spectrotemporally diverse array of
broadband stimuli -- such as dynamic ripples, spectrotemporally white noise,
and temporally orthogonal ripple combinations (TORCs) -- are very similar,
confirming earlier findings that the STRF is a robust linear descriptor of the
cell. We also present a new deterministic analysis framework that employs the
Fourier series to describe the spectrotemporal modulations contained in the
stimuli and responses. Additional insights into the STRF measurements,
including the nature and interpretation of measurement errors, is presented
using the Fourier transform, coupled to singular-value decomposition (SVD), and
variability analyses including bootstrap. The results promote the utility of
the STRF as a core functional descriptor of neurons in AI.Comment: 42 pages, 8 Figures; to appear in Journal of Computational
Neuroscienc
A space communication study Final report, 15 Sep. 1967 - 15 Sep. 1968
Transmitting and receiving analog and digital signals through noisy media - space communications stud
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