410 research outputs found

    Reversing cooling flows with AGN jets: shock waves, rarefaction waves, and trailing outflows

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    The cooling flow problem is one of the central problems in galaxy clusters, and active galactic nucleus (AGN) feedback is considered to play a key role in offsetting cooling. However, how AGN jets heat and suppress cooling flows remains highly debated. Using an idealized simulation of a cool-core cluster, we study the development of central cooling catastrophe and how a subsequent powerful AGN jet event averts cooling flows, with a focus on complex gasdynamical processes involved. We find that the jet drives a bow shock, which reverses cooling inflows and overheats inner cool core regions. The shocked gas moves outward in a rarefaction wave, which rarefies the dense core and adiabatically transports a significant fraction of heated energy to outer regions. As the rarefaction wave propagates away, inflows resume in the cluster core, but a trailing outflow is uplifted by the AGN bubble, preventing gas accumulation and catastrophic cooling in central regions. Inflows and trailing outflows constitute meridional circulations in the cluster core. At later times, trailing outflows fall back to the cluster centre, triggering central cooling catastrophe and potentially a new generation of AGN feedback. We thus envisage a picture of cool cluster cores going through cycles of cooling-induced contraction and AGN-induced expansion. This picture naturally predicts an anti-correlation between the gas fraction (or X-ray luminosity) of cool cores and the central gas entropy, which may be tested by X-ray observations.Comment: Slightly revised version, accepted for publication in MNRAS. 14 pages, 10 figure

    Privacy Protection Performance of De-identified Face Images with and without Background

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    Li Meng, 'Privacy Protection Performance of De-identified Face Images with and without Background', paper presented at the 39th International Information and Communication Technology (ICT) Convention. Grand Hotel Adriatic Congress Centre and Admiral Hotel, Opatija, Croatia, May 30 - June 3, 2016.This paper presents an approach to blending a de-identified face region with its original background, for the purpose of completing the process of face de-identification. The re-identification risk of the de-identified FERET face images has been evaluated for the k-Diff-furthest face de-identification method, using several face recognition benchmark methods including PCA, LBP, HOG and LPQ. The experimental results show that the k-Diff-furthest face de-identification delivers high privacy protection within the face region while blending the de-identified face region with its original background may significantly increases the re-identification risk, indicating that de-identification must also be applied to image areas beyond the face region

    Crossover behavior in dynamics of frozen ferrofluids

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    The imaginary part of susceptibility, chi , in frozen ferrofluids was measured as functions of temperature, frequency, and concentration of magnetic particles. In a very dilute region, where magnetic interaction is negligible, the relaxation time is mainly determined by Neel\u27s relaxation in isolated particles and the frequency-dependent peak temperature in chi (T), T-p, obeys the Arrhenius law. In the moderate concentration range, T-p has a Vogel-Fulcher (VF) relaxation with the measuring frequency. However, for concentration larger than a certain value, deviation from VF law occurs. Simulations show that the VF relation could be attributed to Ising spin-glass-like random and frustrated interaction between magnetic moments of particles. We suggest that particle configuration before freezing is critical to determine the distribution of pair exchange parameters. For high concentration, there are more ferromagnetic than antiferromagnetic bonds. The deviation from VF law at high concentration could be due to short range correlation among spins

    Task Scheduling Based on Grey Wolf Optimizer Algorithm for Smart Meter Embedded Operating System

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    In recent years, with the rapid development of electric power informatization, smart meters are gradually developing towards intelligent IOT. Smart meters can not only measure user status, but also interconnect and communicate with cell phones, smart homes and other cloud devices, and these core functions are completed by the smart meter embedded operating system. Due to the dynamic heterogeneity of the user program side and the system processing side of the embedded system, resource allocation and task scheduling is a challenging problem for embedded operating systems of smart meters. Smart meters need to achieve fast response and shortest completion time for user program side requests, and also need to take into account the load balancing of each processing node to ensure the reliability of smart meter embedded systems. In this paper, based on the advanced Grey Wolf Optimizer, we study the scheduling principle of the service program nodes in the smart meter operating system, and analyze the problems of the traditional scheduling algorithm to find the optimal solution. Compared with traditional algorithms and classical swarm intelligence algorithms, the algorithm proposed in this paper avoids the dilemma of local optimization, can quickly allocate operating system tasks, effectively shorten the time consumption of task scheduling, ensure the real-time performance of multi task scheduling, and achieve the system tuning balance. Finally, the effectiveness of the algorithm is verified by simulation experiments

    Synergistic effect between ceria and tungsten oxide on WO3–CeO2–TiO2 catalysts for NH3-SCR reaction

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    AbstractWO3–CeO2–TiO2 catalysts for NO (nitrogen monoxide) reduction by ammonia were prepared by a sol–gel method. The catalysts were characterized by BET, XRD, Raman, NH3/NO adsorption and H2-TPR to investigate the relationships among the catalyst composition, structure, redox property, acidity and deNOx activity. WO3–CeO2–TiO2 catalysts show a high activity in a broad temperature range of 200–480°C. The low-temperature activity of catalysts is sensitive to the catalyst composition especially under low-O2-content atmospheres. It may be related to the synergistic effect between CeOx and WOx in the catalysts. On one hand, the interaction between ceria and tungsten oxide promotes the activation of gaseous oxygen to compensate the lattice oxygen consumed in NH3-SCR (selective catalytic reduction) reaction at low temperatures. Meanwhile, the Brønsted acid sites mainly arise from tungsten oxides, Lewis acid sites mainly arise from ceria. Both of the Brønsted and Lewis acid sites facilitate the adsorption of NH3 on catalysts and improve the stability of the adsorbed ammonia species, which are beneficial to the NH3-SCR reaction