32,734 research outputs found
SPSA-Based Tracking Method for Single-Channel-Receiver Array
A novel tracking method in the phased antenna array with a single-channel receiver for the moving signal source is presented in this paper. And the problems of the direction-of-arrival track and beamforming in the array system are converted to the power maximization of received signal in the free-interference conditions, which is different from the existing algorithms that maximize the signal to interference and noise ratio. The proposed tracking method reaches the global optimum rather than local by injecting the extra noise terms into the gradient estimation. The antenna beam can be steered to coincide with the direction of the moving source fast and accurately by perturbing the output of the phase shifters during motion, due to the high efficiency and easy implementation of the proposed beamforming algorithm based on the simultaneous perturbation stochastic approximation (SPSA). Computer simulations verify that the proposed tracking scheme is robust and effective
The effect of Mach number on unstable disturbances in shock/boundary-layer interactions
The effect of Mach number on the growth of unstable disturbances in a boundary layer undergoing a strong interaction with an impinging oblique shock wave is studied by direct numerical simulation and linear stability theory (LST). To reduce the number of independent parameters, test cases are arranged so that both the interaction location Reynolds number (based on the distance from the plate leading edge to the shock impingement location for a corresponding inviscid flow) and the separation bubble length Reynolds number are held fixed. Small-amplitude disturbances are introduced via both white-noise and harmonic forcing and, after verification that the disturbances are convective in nature, linear growth rates are extracted from the simulations for comparison with parallel flow LST and solutions of the parabolized stability equations (PSE). At Mach 2.0, the oblique modes are dominant and consistent results are obtained from simulation and theory. At Mach 4.5 and Mach 6.85, the linear Navier-Stokes results show large reductions in disturbance energy at the point where the shock impinges on the top of the separated shear layer. The most unstable second mode has only weak growth over the bubble region, which instead shows significant growth of streamwise structures. The two higher Mach number cases are not well predicted by parallel flow LST, which gives frequencies and spanwise wave numbers that are significantly different from the simulations. The PSE approach leads to good qualitative predictions of the dominant frequency and wavenumber at Mach 2.0 and 4.5, but suffers from reduced accuracy in the region immediately after the shock impingement. Three-dimensional Navier-Stokes simulations are used to demonstrate that at finite amplitudes the flow structures undergo a nonlinear breakdown to turbulence. This breakdown is enhanced when the oblique-mode disturbances are supplemented with unstable Mack modes
Exploring the Dynamics in the Environmental Discourse: The Longitudinal Interaction Among Public Opinion, Presidential Opinion, Media Coverage, Policymaking in 3 Decades and an Integrated Model of Media Effects
Using data on environmental issues drawn from 41 series of poll questions and federal outlay in 43 years (1965-2007) and a content analysis of newspaper articles, television news summaries, and presidential documents in 28 years (1980-2007), with the multivariate Granger Causality test based on Vector Autoregression (VAR) models and bivariate Granger Causality (F and Chi-squire) tests, the study finds that public opinion has little influences on federal environmental expenditure in the past several decades. It also finds that for the presidents, the media, and the public, their agendas (volume of information) cause a change in the agenda and frame building and setting processes are essentially confirmed by the multivariate test, but feedback influences are also identified through the bivariate tests. The multivariate test also shows an influence from the presidential agenda to the public agenda on environmental issues, with no influence identified the other way around. The implications of the proposed five-level integrated model of media effects are also discussed
Unforgeable Noise-Tolerant Quantum Tokens
The realization of devices which harness the laws of quantum mechanics
represents an exciting challenge at the interface of modern technology and
fundamental science. An exemplary paragon of the power of such quantum
primitives is the concept of "quantum money". A dishonest holder of a quantum
bank-note will invariably fail in any forging attempts; indeed, under
assumptions of ideal measurements and decoherence-free memories such security
is guaranteed by the no-cloning theorem. In any practical situation, however,
noise, decoherence and operational imperfections abound. Thus, the development
of secure "quantum money"-type primitives capable of tolerating realistic
infidelities is of both practical and fundamental importance. Here, we propose
a novel class of such protocols and demonstrate their tolerance to noise;
moreover, we prove their rigorous security by determining tight fidelity
thresholds. Our proposed protocols require only the ability to prepare, store
and measure single qubit quantum memories, making their experimental
realization accessible with current technologies.Comment: 18 pages, 5 figure
Prewetting transition on a weakly disordered substrate : evidence for a creeping film dynamics
We present the first microscopic images of the prewetting transition of a
liquid film on a solid surface. Pictures of the local coverage map of a helium
film on a cesium metal surface are taken while the temperature is raised
through the transition. The film edge is found to advance at constant
temperature by successive avalanches in a creep motion with a macroscopic
correlation length. The creep velocity varies strongly in a narrow temperature
range. The retreat motion is obtained only at much lower temperature,
conforming to the strong hysteresis observed for prewetting transition on a
disordered surface. Prewetting transition on such disordered surfaces appears
to give rise to dynamical phenomena similar to what is observed for domain wall
motions in 2D magnets.Comment: 7 pages, 3 figures, to be published in Euro.Phys.Let
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