A Dynamic Observer to Capture and Control Perturbation Energy in Noise Amplifier Flows

AbstractIn this article, we present a technique to extract a reduced-order model of a transitional flat-plate boundary layer from simultaneous velocity snapshots and wall-shear stress measurements. The proposed approach combines a reduction of the degrees of freedom of the system by a projection of the velocity snapshots onto a POD basis together with a system-identification technique to obtain a state-space model of the flow. Such a model is then used in an optimal control framework to reduce the kinetic energy of the perturbation field and therefore delay transition

Collective Dynamics of Josephson Vortices in Intrinsic Josephson Junctions :Exploration of In-phase Locked Superradiant Vortex Flow States

In order to clarify the ``superradiant'' conditions for the moving Josephson vortices to excite in-phase AC electromagnetic fields over all junctions, we perform large scale simulations of realistic dimensions for intrinsic Josephson junctions under the layer parallel magnetic field. Three clear step-like structures in the I-V curve are observed above a certain high field ($H > 1T$ in the present simulations), at which we find structural transitions in the moving flux-line lattice. The Josephson vortex flow states are accordingly classified into four regions (region I $\sim$ IV with increasing current), in each of which the power spectrum for the electric field oscillations at the sample edge are measured and typical snapshots for Josephson vortex configurations are displayed. Among the four regions, especially in the region III, an in-phase rectangular vortex lattice flow state emerges and the power spectrum shows remarkably sharp peak structure, i.e., superradiant state. Comparison of the simulation results with an eigenmode analysis for the transverse propagating Josephson plasma oscillations reveals that the resonances between Josephson vortex flow states and some of the eigenmodes are responsible for the clear flux lattice structural transitions. Furthermore, the theoretical analysis clarifies that the width of the superradiant state region in the I-V characteristics enlarges with decreasing both the superconducting and insulating layer thickness.Comment: 8 pages, Revtex, 7 figures; figure arrangements improved. no changes in tex

Entity Type Prediction in Knowledge Graphs using Embeddings

Open Knowledge Graphs (such as DBpedia, Wikidata, YAGO) have been recognized as the backbone of diverse applications in the field of data mining and information retrieval. Hence, the completeness and correctness of the Knowledge Graphs (KGs) are vital. Most of these KGs are mostly created either via an automated information extraction from Wikipedia snapshots or information accumulation provided by the users or using heuristics. However, it has been observed that the type information of these KGs is often noisy, incomplete, and incorrect. To deal with this problem a multi-label classification approach is proposed in this work for entity typing using KG embeddings. We compare our approach with the current state-of-the-art type prediction method and report on experiments with the KGs

The Occurrence of the Hall--Instability in Crusts of Isolated Neutron Stars

In former papers we showed that during the decay of a neutron star's magnetic field under the influence of the Hall--drift, an unstable rise of small--scale field structures at the expense of the large--scale background field may happen. This linear stability analysis was based on the assumption of a uniform density throughout the neutron star crust, whereas in reality the density and all transport coefficients vary by many orders of magnitude. Here, we extend the investigation of the Hall--drift induced instability by considering realistic profiles of density and chemical composition, as well as background fields with more justified radial profiles. Two neutron star models are considered differing primarily in the assumption on the core matter equation of state. For their cooling history and radial profiles of density and composition we use known results to infer the conductivity profiles. These were fed into linear calculations of a dipolar field decay starting from various initial configurations. At different stages of the decay, snapshots of the magnetic fields at the equator were taken to yield background field profiles for the stability analysis. The main result is that the Hall instability may really occur in neutron star crusts. Characteristic growth times are in the order of \lesssim 10^4 ... 10^6 yrs depending on cooling age and background field strength. The influence of the equation of state and of the initial field configuration is discussed.Comment: 16 pages, 16 figures, PS, submitted to A&A. Justification/discussion slightly changed/extended in replying to the referee. Changes on p. 3, 11, 13, framed by XXX mark

The Effects of Magnetic Fields and Inhomogeneities on Accretion Disk Spectra and Polarization

We present the results of one and three-dimensional radiative transfer calculations of polarized spectra emerging from snapshots of radiation magnetohydrodynamical simulations of the local vertical structure of black hole accretion disks. The simulations cover a wide range of physical regimes relevant for the high/soft state of black hole X-ray binaries. We constrain the uncertainties in theoretical spectral color correction factors due to the presence of magnetic support of the disk surface layers and strong density inhomogeneities. For the radiation dominated simulation, magnetic support increases the color correction factor by about ten percent, but this is largely compensated by a ten percent softening due to inhomogeneities. We also compute the effects of inhomogeneities and Faraday rotation on the resulting polarization. Magnetic fields in the simulations are just strong enough to produce significant Faraday depolarization near the spectral peak of the radiation field. X-ray polarimetry may therefore be a valuable diagnostic of accretion disk magnetic fields, being able to directly test simulations of magnetorotational turbulence.Comment: 18 pages, accepted for publication in Ap