Rotating solutions in critical Lovelock gravitiesMirjam Cvetič
, Xing-Hui Feng
, Hong Lü
, Christopher N. Pope
, 2017, izvirni znanstveni članek
Opis: For appropriate choices of the coupling constants, the equations of motion of Lovelock gravities up to order n in the Riemann tensor can be factorized such that the theories admit a single (A)dS vacuum. In this paper we construct two classes of exact rotating metrics in such critical Lovelock gravities of order n in d = 2n +1 dimensions. In one class, the n angular momenta in the n orthogonal spatial 2-planes are equal, and hence the metric is of cohomogeneity one. We construct these metrics in a Kerr-Schild form, but they can then be recast in terms of Boyer-Lindquist coordinates. The other class involves metrics with only a single non-vanishing angular momentum. Again we construct them in a Kerr-Schild form, but in this case it does not seem to be possible to recast them in Boyer-Lindquist form. Both classes of solutions have naked curvature singularities, arising because of the over rotation of the configurations.
Ključne besede: astrophysics, nuclear physics
Objavljeno: 08.03.2018; Ogledov: 925; Prenosov: 305
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Emergence of target waves in paced populations of cyclically competing speciesLuo-Luo Jiang
, Tao Zhou
, Matjaž Perc
, Xin Huang
, Bing-Hong Wang
, 2009, izvirni znanstveni članek
Opis: We investigate the emergence of target waves in a cyclic predator-prey model incorporating a periodic current of the three competing species in a small area situated at the center of a square lattice. The periodic current acts as a pacemaker, trying to impose its rhythm on the overall spatiotemporal evolution of the three species. We show that the pacemaker is able to nucleate target waves that eventually spread across the whole population, whereby three routes leading to this phenomenon can be distinguished depending on the mobility of the three species and the oscillation period of the localized current. First, target waves can emerge due to the synchronization between the periodic current and oscillations of the density of the three species on the spatial grid. The second route is similar to the first, the difference being that the synchronization sets in only intermittently. Finally, the third route toward target waves is realized when the frequency of the pacemaker is much higher than that characterizing the oscillations of the overall density of the three species. By considering the mobility and frequency of the current as variable parameters, we thus provide insights into the mechanisms of pattern formation resulting from the interplay between local and global dynamics in systems governed by cyclically competing species.
Ključne besede: cyclical interactions, target waves, spatial games, diversity
Objavljeno: 30.06.2017; Ogledov: 774; Prenosov: 285
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Erosion of materials by alumina slurryHong Qing Lou
, Alojz Križman
, 1989, izvirni znanstveni članek
Opis: An examination of the erosion resistance of materials utilized in the construction of alumina pumps was carried out. This included laboratory research and, for the purpose of verification, investigation under actual service conditions. In this paper the effect of material properties on erosion resistance is considered. In the second paper the influence of phase properties and conditions of multiphase materials on material erosion resistance will be described. The results from studies of the seven most frequently utilized and recommended materials show that the erosion resistanceof candidate materials increases to a maximum and, then, decreases with decreasing material hardness. Surface examination reveals that ductile and brittle erosion mechanisms operate on the eroded surfaces of ductile and brittle materials respectively. Fracture toughness is a dominant factor for brittle materials, while hardness is a main factor for ductile materials. Under certain external conditions pump materials should be hard enough to resist material loss by ductile erosion mechanisms and should have sufficient fracture toughness to resist material loss by brittle erosion mechanisms. On the basis of the present work a schematic drawing is constructed to explain irregular patterns in the relation between resistance to material erosion and material hardness in previous works through simultaneous examination of hardness and fracture toughness.The first part of this work Erosion of materials by alumina slurry - part 1 has stated that the erosion resistance of materials in alumina pumps is connected with material properties such as hardness and fracture toughness. Part 2 investigates the erosion mechanisms of multiphase materials. Research into the erosion mechanisms of eroded surfaces as well as microstructural change below the eroded surface led to more thorough analysis of two characteristic wear-resistant materials (Norihard, Fonte30%Cr) and a relatively less resistant material, mottled iron. Material removal processes in different phases of multiphase materials are different, asynchronous and interactive. The erosion mechanisms of material removal from the matrices of different materials differ owing to their dependence on the matrix-phase properties and microstructure of the material. The modes of material removal of mottled iron and Norihard are established. A model of the erosion rate of material with a multiphase microstructure was proposed. In the case of pump elements for alumina slurry in an alkaline medium, a ductile matrix with sufficient hardness is important for higher erosion resistance. The erosion resistance is also affected by the size, shape and proportion of other microstructural components.
Objavljeno: 06.06.2012; Ogledov: 992; Prenosov: 15
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