Stability of a spherical flame ball in a porous medium

Gaseous flame balls and their stability to symmetric disturbances are studied numerically and asymptotically, for large activation temperature, within a porous medium that serves only to exchange heat with the gas. Heat losses to a distant ambient environment, affecting only the gas, are taken to be radiative in nature and are represented using two alternative models. One of these treats the heat loss as being constant in the burnt gases and linearizes the radiative law in the unburnt gas (as has been studied elsewhere without the presence of a solid). The other does not distinguish between burnt and unburnt gas and is a continuous dimensionless form of Stefan's law, having a linear part that dominates close to ambient temperatures and a fourth power that dominates at higher temperatures.

Numerical results are found to require unusually large activation temperatures in order to approach the asymptotic results. The latter involve two branches of solution, a smaller and a larger flame ball, provided heat losses are not too high. The two radiative heat loss models give completely analogous steady asymptotic solutions, to leading order, that are also unaffected by the presence of the solid which therefore only influences their stability. For moderate values of the dimensionless heat-transfer time between the solid and gas all flame balls are unstable for Lewis numbers greater than unity. At Lewis numbers less than unity, part of the branch of larger flame balls becomes stable, solutions with the continuous radiative law being stable over a narrower range of parameters. In both cases, for moderate heat-transfer times, the stable region is increased by the heat capacity of the solid in a way that amounts, simply, to decreasing an effective Lewis number for determining stability, just as if the heat-transfer time was zero.     

边界条件对二维斜面颗粒流颗粒分布的影响

实验研究了粗糙边界条件、不同通道宽度以及不同斜面倾斜角下二维颗粒流的颗粒分布 ，发现颗粒流在稀疏流状态下尽管在现象上表现出类似流体的性质，但由于颗粒流系统的能 量耗散,所以颗粒流的速度和密度分布和牛顿流体完全不同，颗粒流在通道中的分布既受通 道宽度的影响同时也受重力场的影响，颗粒在通道中以通道中轴线呈对称分布，通道两侧的 颗粒密度明显高于通道中间，由于通道中颗粒受剪切力的作用，导致颗粒在通道中的密度分 布的变化，通道边界粗糙度明显影响着颗粒流横向分布. 关键词： 二维颗粒流 颗粒物质 稀疏流     

Search for free quark production at the CERN SPS collider

No experiment has found convincing evidence for free quarks [1, 2]. A possible explanation for this non-observation in accelerator experiments is that their cross-section for interaction with matter is much larger than those of hadrons [3], and hence they are scattered and/or absorbed by material between their production point and the detector. We have examined this possibility by exposing 1/4 mm diameter steel balls near an intersection point of the CERN SPS \(p\bar p\) collider beams. These balls have been subsequently examined for the presence of fractional charge, using the Rutherford Laboratory room temperature magnetic levitation system [4]. None of the 60 balls tested to date shows evidence of fractional charge.     

倾斜多孔腔体内瞬态自然对流的实验研究

本文对充满水和玻璃球的倾斜多孔介质腔体,在其一侧壁温维持恒定值、而相对侧壁温以此恒定值为时均值并按一定的幅值随时间周期性变化的情况下,实验研究了周期性壁温变化和腔体倾角对多孔介质腔体内的瞬态自然对流换热的影响.结果表明,通过多孔介质腔体的净热流总是从腔体的较低壁面指向较高壁面.     

轴承滚珠等离子体浸没离子注入过程的数值模拟

 利用轴对称PIC模型对轴承滚珠等离子体浸没离子注入（PIII）过程进行了数值模拟，对归一化电势的扩展情况进行了研究。在滚珠批量处理过程中，为了避免相邻滚珠周围鞘层的相互重叠对注入均匀性造成不良影响，对滚珠在靶台上摆放的最小距离进行了数值计算，计算结果表明：在电压为－40kV, 氮等离子体密度为4.8×10⁹ cm^-3，脉冲宽度为10μs时，滚珠摆放的最小距离应大于34.18cm。分析了滚珠圆周方向注入剂量的分布情况，针对静止滚珠改性处理后剂量分布很不均匀的问题，通过旋转靶台使滚珠注入均匀性明显得到改善。利用朗谬尔探针测量了滚珠周围鞘层扩展的情况测量，模拟结果和实验测量结果相吻合，最大相对误差小于8.4 %。     

Effect of surface defects on vortex penetration in small superconducting squares

We report the dependence of the vortex penetration on the number and positions of surface defects in mesoscopic superconducting squares. The vortex penetration is detected by the change in the transport properties of small tunnel junctions attached to the sample (small-tunnel-junction method). We find that for vorticities L = 0 and 1, a defect at the center of a side facilitate the vortex penetration, while a defect at a corner does not affect the vortex penetration field. For higher vorticities, the influence of defects becomes complicated, indicating that not only the existence of surface defects but also the vortex arrangement influences the vortex penetration. Simple explanation for this mesoscopic vortex penetration effect is given.     

Fourier transform of the collagen triple-helical structure and its significance

The Fourier transforms of the collagen molecular structure have been calculated taking into consideration various side chain atoms, as well as the presence of bound water molecules. There is no significant change in the calculated intensity distribution on including the side chain atoms of non-imino-acid residues. Taking into account the presence of about two bound water molecules per tripeptide unit, the agreement with the observed x-ray pattern is slightly improved. Fourier transforms have also been calculated for the detailed molecular geometries proposed from other laboratories. It is found that there are no major differences between them, as compared to our structure, either in the positions of peak intensity or in the intensity distribution. Hence it is not possible to judge the relative merits of the various molecular geometries for the collagen triple helix from a comparison of the calculated transforms with the meagre data available from its x-ray fibre pattern. It is also concluded that the collagen molecular structure should be regarded as a somewhat flexible chain structure, capable of adapting itself to the requirements of the different side groups which occur in each local region.     

Optimization and scale-freeness for complex networks

Complex networks are mapped to a model of boxes and balls where the balls are distinguishable. It is shown that the scale-free size distribution of boxes maximizes the information associated with the boxes provided configurations including boxes containing a finite fraction of the total amount of balls are excluded. It is conjectured that for a connected network with only links between different nodes, the nodes with a finite fraction of links are effectively suppressed. It is hence suggested that for such networks the scale-free node-size distribution maximizes the information encoded on the nodes. The noise associated with the size distributions is also obtained from a maximum entropy principle. Finally, explicit predictions from our least bias approach are found to be borne out by metabolic networks.     

Characteristics of deep penetration laser welding of dissimilar metal Ni-based cast superalloy K418 and alloy steel 42CrMo

Experimental trials of autogenous deep penetration welding between dissimilar cast Ni-based superalloy K418 and alloy steel 42CrMo flat plates with 5.0 mm thickness were conducted using a 3 kW continuous wave (CW) Nd:YAG laser. The influences of laser output power, welding velocity and defocusing distance on the morphology, welding depth and width as well as quality of the welded seam were investigated. Results show that full keyhole welding is not formed on both K418 and 42CrMo side, simultaneously, due to the relatively low output power. Partial fusion is observed on the welded seam near 42CrMo side because of the large disparity of thermal–physical and high-temperature mechanical properties of these two materials. The microhardness of the laser-welded joint was also examined and analyzed. It is suggested that applying negative defocusing in the range of Raylei length can increase the welding depth and improve the coupling efficiency of the laser materials interaction.     

YOYO as a Dye to Track Penetration of LK15 DNA Complexes in Spheroids: Use and Limits

To elucidate the reasons underlying the poor penetration of non-viral vectors in tissues, relating transport properties to physico-chemical parameters of vectors may be crucial. These properties can be influenced by the presence of multiples labels that are used. Therefore utilizing a vector with minimum of labels preferably not more than one is important to studying penetration in tissues. The cell impermeant bisintercalating dye YOYO-1 was found suitable to both monitor the formation of complexes between DNA and an amphipathic peptide LK15 and, to track their penetration in HCT116 spheroids by confocal microscopy. The results revealed a limited decrease of fluorescence ascribed to the high affinity of YOYO-1 to bind DNA. The residual fluorescence of complexes can be exploited to monitor penetration into spheroids, after correction for YOYO-1 attenuation, and to revealing hyaluronidase-induced reduced binding. Hence high affinity dyes such as YOYO-1 with inefficiently quenched fluorescence may be important to establish a relation between novel medicines characteristics and penetration in tissues.     

双面电弧焊接的传热模型

将等离子焊接(PAW)电弧和钨极氩弧焊(TIG)电弧串接,相对作用于工件的正反面形成双面电弧焊接(DSAW)系统,可以引导焊接电流沿工件厚度方向流过小孔,补偿等离子电弧穿透工件时消耗的能量,以有效地提高等离子弧的穿透能力.综合考虑影响双面电弧焊接正反面熔池几何形状的力学因素,建立了熔池表面变形的控制方程,以此为基础并采用帖体曲线坐标系建立了DSAW焊接传热的数学模型,分析了DSAW,PAW焊接传热的差异,从传热的角度解释了DSAW焊接熔深增加的原因.焊接工艺实验表明,计算结果与实测结果吻合良好. 关键词： 双面电弧焊接(DSAW) 传热模型 熔池表面变形模型     

多功能球-球碰撞实验仪的研制

研制了多功能球-球碰撞实验仪,利用该仪器对三组碰撞实验的研究发现,未退火小球和未退火小球碰撞可近似为弹性碰撞,其恢复系数为0.96;未退火小球和退火小球、退火小球和退火小球之间的碰撞为非弹性碰撞。本实验源自生活中的打靶实验,将其进行改造升级为物理实验中的研究性设计性实验,在启发学生创新思维和培养学生发散思考方面具有显著作用。     

Sound field separation with a double layer velocity transducer array (L)

In near-field acoustic holography sound field separation techniques make it possible to distinguish between sound coming from the two sides of the array. This is useful in cases where the sources are not confined to only one side of the array, e.g., in the presence of additional sources or reflections from the other side. This paper examines a separation technique based on measurement of the particle velocity in two closely spaced parallel planes. The purpose of the technique is to recover the particle velocity radiated by a source in the presence of disturbing sound from the opposite side of the array. The technique has been examined and compared with direct velocity based reconstruction, as well as with a technique based on the measurement of the sound pressure and particle velocity. The double layer velocity method circumvents some of the drawbacks of the pressure-velocity based reconstruction, and it can successfully recover the normal velocity radiated by the source, even in the presence of strong disturbing sound.     

Upward penetration of grains through a granular medium

We study experimentally the creeping penetration of guest (percolating) grains through densely packed granular media in two dimensions. The evolution of the system of the guest grains during the penetration is studied by image analysis. To quantify the changes in the internal structure of the packing, we use Voronoï tessellation and a certain shape factor which is a clear indicator of the presence of different underlying substructures (domains). We first consider the impact of the effective gravitational acceleration on upward penetration of grains. It is found that the higher effective gravity increases the resistance to upward penetration and enhances structural organization in the system of the percolating grains. We also focus our attention on the dependence of the structural rearrangements of percolating grains on some parameters like polydispersity and the initial packing fraction of the host granular system. It is found that the anisotropy of penetration is larger in the monodisperse case than in the bidisperse one, for the same value of the packing fraction of the host medium. Compaction of initial host granular packing also increases anisotropy of penetration of guest grains. When a binary mixture of large and small guest grains is penetrated into the host granular medium, we observe size segregation patterns.     

Evaluation of Nanostructured coating layers formed on Ni balls during mechanical alloying of Cu powder

A novel coating method utilizing mechanical alloying was used to deposit a nickel-copper solid solution on the surface of nickel balls in a planetary ball mill. Mechanically milled copper powders were deposited by cold welding on the activated surface of the nickel balls. The impact of the nickel balls causes surface wear, leading to formation of mixed layered particles of copper-nickel and rewelding. In addition, diffusion takes place through the coating layer to form a Ni-Cu solid solution. The hardness of the coating reached a threefold increase (HV_0.01594) in comparison with the substrate hardness. Microstructural characterization of the coating surface performed using an optical microscope, SEM, and EPMA indicates that, with appropriate processing conditions, a thick, fully dense coating can be metallurgically bonded to the nickel balls. XRD results revealed the presence of a solid solution and nanocrystalline structure.     

Interaction between Two Charged Dielectric Balls with Strongly Different Radii

JETP Letters - The interaction between two charged dielectric balls that have significantly different radii and whose surfaces are spaced by a small distance is considered. Analytical solutions are...     

Limit Theorems for Monolayer Ballistic Deposition in the Continuum

We consider a deposition model in which balls rain down at random towards a 2-dimensional surface, roll downwards over existing adsorbed balls, are adsorbed if they reach the surface, and discarded if not. We prove a spatial law of large numbers and central limit theorem for the ultimate number of balls adsorbed onto a large toroidal surface, and also for the number of balls adsorbed on the restriction to a large region of an infinite surface.     

Dielectric permittivity of suspensions

A strict macroscopic analysis of the limiting long-wavelength permittivity of a model suspension is presented in which the suspension is considered as a finely dispersed system consisting of isotropic dielectric balls with piecewise-continuous radial permittivity profile. The analysis is performed within the framework of the notion of compact groups of inhomogeneities and the procedure of field averaging over volumes significantly exceeding the scale of these groups. The indicated value is described by the Lorentz-Lorenz formula. The effective polarizability of balls in the suspension is reconstructed from their parameters and the parameters of the medium by means of integration. The result is valid for any concentration of the balls at which the suspension remains macroscopically homogeneous and isotropic with respect to the field and for an arbitrary difference between the ball and medium permittivities.     

On multiple surface-plasmon-polariton waves guided by the interface of a metal film and a rugate filter in the Kretschmann configuration

The excitation of multiple surface-plasmon-polariton (SPP) waves—of different linear polarization states, phase speeds and spatial profiles—guided by the interface of a metal film and a rugate filter in the Kretschmann configuration was investigated. A plane wave of either of the two linear polarization states was made incident on the metal-capped rugate filter of finite thickness and the absorptances were calculated using a stable algorithm. The excitation of SPP waves was inferred by the presence of those peaks in the absorptance curves that were independent of the thickness of the rugate filter. The absorptance peaks representing the excitation of s-polarized SPP waves are narrower than those representing p-polarized SPP waves. For efficient excitation of multiple SPP waves, the metal film's thickness should be close to the penetration depth of the metal. The solution of a canonical boundary-value problem of SPP-wave propagation guided by a metal film, with a semi-infinite homogeneous dielectric material on one side and a semi-infinite rugate filter on the other side, reinforced the results obtained for the Kretschmann configuration. The thin metal film in the Kretschmann configuration may lead to coupling of its two interfaces.     

Energy,force and field strength in a system of two charged conducting balls

A calculation has been performed for the interaction force and the field strength in a system consisting of two charged conducting balls in three cases: 1) the balls are maintained at the preset potentials; 2) the balls are insulated and have fixed charges; 3) one of the balls has the preset potential, and the second one has the preset charge. Experimental measurements of forces in the first and third cases were performed and fair agreement with calculations was confirmed. A calculation method has been suggested, and the electric field strength on the nearest poles of the balls has been calculated for the three mentioned cases. A good agreement with the other authors' calculation results has been demonstrated.