The stability of some Kaluza-Klein cosmological models

The stability is examined of some exact Kaluza-Klein cosmological solutions that possess static extra dimensions and standard Friedman behaviour for the remaining three spatial dimensions. It is shown that even when the extra dimensions are asymptotically static they produce a deviation from Friedman behaviour in the observable three spatial dimensions. A similar effect is found for models with anisotropically expanding three-spaces.     

Study of the interglobular structure of metal powders obtained by a borohydride reduction process

The effect of borohydride reduction duration and magnetic field in the specific surface area, metal particle size and form, and form and dimensions of the textural arrangement of pores in metal powders is investigated. It is established that the particle chains formed involve interglobular voids, i.e. the chains may be considered as consisting of successivly ordered globule layers. The interglobular void is found to possess definite dimensions and configuration. Powders obtained in a magnetic field have tetrahedral voids, while octahedral voids correspond to powders prepared without a magnetic field. The magnetic parameters of the powders have displayed a similar difference which needs additional studies.     

Zipf’s law for fractal voids and a new void-finder

Voids are a prominent feature of fractal point distributions but there is no precise definition of what is a void (except in one dimension). Here we propose a definition of voids that uses methods of discrete stochastic geometry, in particular, Delaunay and Voronoi tessellations, and we construct a new algorithm to search for voids in a point set. We find and rank-order the voids of suitable examples of fractal point sets in one and two dimensions to test whether Zipf’s power-law holds. We conclude affirmatively and, furthermore, that the rank-ordering of voids conveys similar information to the number-radius function, as regards the scaling regime and the transition to homogeneity. So it is an alternative tool in the analysis of fractal point distributions with crossover to homogeneity and, in particular, of the distribution of galaxies.     

Analytical Expressions of Matrix Elements of Physical Quantities for Dirac Oscillator

The analytical expressions of the matrix elements for physical quantities are obtained for the Dirac oscillator in two and three spatial dimensions. Their behaviour for the case of operator's square is discussed in details. The two-dimensional Dirac oscillator has similar behavior to that for three-dimensional one.     

The Casimir force on a piston in the spacetime with extra compactified dimensions

A Casimir piston for massless scalar fields obeying Dirichlet boundary conditions in high-dimensional spacetimes within the frame of Kaluza–Klein theory is analyzed. We derive and calculate the exact expression for the Casimir force on the piston. We also compute the Casimir force in the limit that one outer plate is moved to the extremely distant place to show that the reduced force is associated with the properties of additional spatial dimensions. The more dimensionality the spacetime has, the stronger the extra-dimension influence is. The Casimir force for the piston in the model including a third plate under the background with extra compactified dimensions always keeps attractive. Further we find that when the limit is taken the Casimir force between one plate and the piston will change to be the same form as the corresponding force for the standard system consisting of two parallel plates in the four-dimensional spacetimes if the ratio of the plate-piston distance and extra dimensions size is large enough.     

Three-dimensional structure of polystyrene colloidal crystal by synchrotron radiation X-ray phase-contrast computed tomography

Colloidal crystal with long-range ordered structure has attracted great attention for their applications in various fields. Although perfect colloidal crystals have been achieved by some fabrications for utilization, little is known about their exact structures and internal defects. In this study, we use synchrotron radiation (SR) phase-contrast computed tomography (CT) to noninvasively access the internal structure of polystyrene (PS) colloidal crystals in three dimensions (3D). The phase-attenuation duality Paganin algorithm phase retrieval was employed to achieve a satisfactory contrast and outline of the spheres. After CT reconstruction, the positions of individual PS particles and structural defects are identified in three dimensions, and the local crystal structure is revealed. Further quantitative analysis of the void system in colloidal crystal illustrates that single voids can be mostly attributed to tetrahedron void of sphere close packing, but the interconnected voids with large volume induce a sphere volume fraction of 59.39 % that reflects a metastable glass behavior of colloidal crystal arrangement. The void orientation result reveals that the 3D close-packing difficulty mainly lies in the stacking of interlayer.     

The three-body problem with short-range interactions

The quantum mechanical three-body problem is studied for general short-range interactions. We work in coordinate space to facilitate accurate computations of weakly bound and spatially extended systems. Hyperspherical coordinates are used in both the interpretation and as an integral part of the numerical method. Universal properties and model independence are discussed throughout the report. We present an overview of the hyperspherical adiabatic Faddeev equations. The wave function is expanded on hyperspherical angular eigenfunctions which in turn are found numerically using the Faddeev equations. We generalize the formalism to any dimension of space d greater or equal to two. We present two numerical techniques for solving the Faddeev equations on the hypersphere. These techniques are effective for short and intermediate/large distances including use for hard core repulsive potentials. We study the asymptotic limit of large hyperradius and derive the analytic behaviour of the angular eigenvalues and eigenfunctions. We discuss four applications of the general method. We first analyze the Efimov and Thomas effects for arbitrary angular momenta and for arbitrary dimensions d. Second we apply the method to extract the general behaviour of weakly bound three-body systems in two dimensions. Third we illustrate the method in three dimensions by structure computations of Borromean halo nuclei, the hypertriton and helium molecules. Fourth we investigate in three dimensions three-body continuum properties of Borromean halo nuclei and recombination reactions of helium atoms as an example of direct relevance for the stability of Bose–Einstein condensates.     

{100} Faceted anion voids in electron irradiated fluorite

High fluence electron irradiation of fluorite crystals in the temperature range 150 to 320 K results in formation of a simple cubic anion void superlattice. Above 320 K the damage structure changes to a random distribution of large {001} faceted anion voids. This voidage behaviour, similar to that observed in a range of irradiated metals, is discussed in terms points defect rather than conventional colour centre terminology.     

Positrons in metals with voids,vacancies and surfaces

The behaviour of positrons near monovacancies and voids in a metal is discussed in terms of three contributions to the positron work function for the metal, namely the positron zero-point energy, the positron-electron correlation energy and the surface dipole barrier. The image potential when a positron comes near to a metal surface leads to a deep potential ‘trough’ just outside an exterior metal surface or just inside a void surface. Calculations indicate positron bound states localized at metal surfaces for most metals which should be manifested in a long lived positron lifetime component. The behaviour of positronium near metal surfaces is also discussed.     

The Periodic Oscillation of an Adiabatic Piston in Two or Three Dimensions

We study a heavy piston of mass M that separates finitely many ideal, unit mass gas particles moving in two or three dimensions. Neishtadt and Sinai previously determined a method for finding this system’s averaged equation and showed that its solutions oscillate periodically. Using averaging techniques, we prove that the actual motions of the piston converge in probability to the predicted averaged behavior on the time scale M ^1/2 when M tends to infinity while the total energy of the system is bounded and the number of gas particles is fixed.     

基于孔洞体胞模型铸造镁合金ML308的本构方程

铸造镁合金不可避免地包含许多微孔洞,这些微孔洞在材料的后续加工及服役过程中将发生演化,并对材料的力学行为产生重要影响.基于球形孔洞体胞模型,提出微孔洞长大及形核方程,它们构成微孔洞的演化方程.根据孔洞演化将造成材料性质弱化的物理机制,将微孔洞演化以弱化函数的形式引入到非经典弹塑性本构方程,得到考虑孔洞演化的铸造镁合金弹塑性本构方程.发展与本构方程相应的有限元数值分析程序,用其模拟了铸造镁合金ML308的微孔洞演化及力学行为,计算结果与实验结果符合较好. 关键词： 铸造镁合金 孔洞体胞模型 孔洞演化方程 本构方程     

Bowl piston geometry as an alternative to enlarged crevice pistons for rapid compression machines

Thermal inhomogeneity and physical processes like fluid dynamics reduce the utility of rapid compression machine (RCM) facilities to accurately study fuel combustion phenomenon relevant to internal combustion engines. Most current RCMs incorporate a large crevice volume in the piston to capture roll-up vortices that encroach into the combustion zone during compression. In this work, a bowl piston design similar to those used in diesel engines is proposed as an alternative to enlarged creviced pistons for creating a sufficiently thermally homogenous gas mixture prior to ignition without undesirable fluid motion found in flat piston configurations. The bowl piston also eliminates the possibility of cold unreacted gases entering the combustion chamber when the piston is retracted in rapid compression and expansion machines (RCEMs) like in creviced piston designs. In the work, a bowl piston was compared to creviced piston and flat piston configurations numerically and experimentally. Through non-reacting computational fluid dynamics simulations, the bowl piston reduced the roll-up vortex found for the flat piston and led to a higher temperature and more thermally uniform core of gas at peak compression compared to the enlarged crevice piston. Experimentally, three pistons were studied in a RCM facility with ethanol and n-butane as fuels. Results showed that the bowl piston yielded benefits over conventional piston geometries including: reduced heat loss due to lower surface area, higher turbulent Reynolds Number, stronger ignition, and higher heat release rate and combustion efficiency as estimated using heat release analysis. Based on the findings presented here, we conclude that bowl piston geometries are a promising alternative to creviced pistons for conducting fuel ignition studies in RCM and RCEM facilities.     

Strong and Weak Damping in the Adiabatic Motion of the Simple Piston

We investigate the microscopic mechanisms associated with strong and weak damping in the adiabatic motion of the simple piston with finite mass and finite amount of gas. The velocity of the piston relative to the thermal velocity of the gas particles is the principal factor in determining the behaviour. When the piston velocity is always smaller than the thermal velocity we observe weak damping. When it is greater than the thermal velocity for part of its motion there is strong damping.     

Intermittency in the Fisher's droplet model

Intermittent behaviour of fragment multiplicity distributions in the nuclear liquid-gas phase transition is studied in terms of the droplet model of Fisher. The anomalous fractal dimensions are compared with data on heavy ion reactions and classical molecular dynamics simulations. A signature of the transition in the anomalous fractal dimensions is shown.We thank Profs. S. Ayik, M. Di Toro and V. Kondratyev for discussions. One of us (T.K.) acknowledges the support of INFN-LNS.     

空心活塞运动产生的激波

普通活塞在管道中运动, 不论活塞速度高低都能驱动产生激波.空心活塞在管道中运动与普通活塞不同, 当其运动Mach数低于壅塞Mach数, 活塞前不会形成运动激波; 只有活塞运动Mach数超出壅塞Mach数, 才能驱动产生激波.壅塞Mach数由空心活塞中心孔直径、长度和内壁面摩擦系数以及外直径横截面积决定.火车通过隧道类似空心活塞在管道中运动.将火车车箱与隧道间的空隙换算成有效水力直径后, 可利用空心活塞在管道中运动计算壅塞Mach数的方法, 求出火车通过隧道出现激波的条件.文章提出一种新颖的间接测量列车平均摩擦系数新方案.      

Dilatancy in liquid foams

It is shown that dilatancy (a change in liquid fraction due to shear) can be a large effect in foams. Theory and simulations for the standard idealized model of a two-dimensional liquid foam are found to be in good agreement. The estimate of the maximum effect of static shear at constant osmotic pressure gives an absolute increase in liquid fraction of roughly 1.5%, halfway between the wet and the dry limits. The same theory, which uses the established dependence of shear rnodulus on liquid fraction, predicts a qualitatively and quantitatively similar behaviour in three dimensions.     

粘塑性介质中球形孔洞的动态增长

 提出了一个新的粘塑性介质中球形孔洞在高加载率作用下的演化方程。方程中考虑了应变率、局部惯性和介质的硬化效应，并对这些影响进行了数值分析和讨论。数值分析结果表明：孔洞的增长对外加载率和应变率十分敏感，在高加载率条件下局部惯性效应对孔洞增长有着重要影响，随着加载率的增加，这种影响增大。另外，得到了使孔洞增长的临界应力值的表达式。     

Numerical study of the equilibrium thermodynamics of the Coulomb glass

We discuss the results of large‐scale Monte Carlo simulations of the lattice Coulomb glass model. Using the Exchange Monte Carlo algorithm we are able to equilibrate the system to very low temperature and obtain evidence against the existence of an equilibrium glass transition in three dimensions (3D). We also discuss results for the 3D Random Field Ising model that highlight the role of the interaction range. Finally, we report results for the 2D Coulomb glass, which show a behaviour very similar to the 3D case.     

Corrosion properties of aluminium coatings deposited on sintered NdFeB by ion-beam-assisted deposition

Pure Al coatings were deposited by direct current (DC) magnetron sputtering to protect sintered NdFeB magnets. The effects of Ar⁺ ion-beam-assisted deposition (IBAD) on the structure and the corrosion behaviour of Al coatings were investigated. The Al coating prepared by DC magnetron sputtering with IBAD (IBAD-Al-coating) had fewer voids than the coating without IBAD (Al-coating). The corrosion behaviour of the Al-coated NdFeB specimens was investigated by potentiodynamic polarisation, a neutral salt spray (NSS) test, and electrochemical impedance spectroscopy (EIS). The pitting corrosion of the Al coatings always began at the voids of the grain boundaries. Bombardment by the Ar⁺ ion-beams effectively improved the corrosion resistance of the IBAD-Al-coating.