Optical filters consisting of metallic waveguide arrays

We demonstrate high-pass optical filters with cutoffs in the 0.3-10-micron spectral region. These filters consist of uniform arrays of hollow metallic waveguides, obtained by coating wafers of the previously developed channel-glass (CG) materials with a thin metal film. In these filters the channel diameter controls the cutoff frequency, the channel length controls the sharpness of the cutoff, and the channel density determines the transmission efficiency at cutoff. All of these parameters can be controlled in the CG starting material. The properties of the metal coatings that influence the filter properties are also discussed. Cutoff wavelengths near 300 nm have been achieved to date by using CG materials with submicrometer channel diameters. At all channel diameters, the transmission spectra include a peak just above the cutoff wavelength, where the transmission value can exceed that expected on the basis of the geometrical open area of the CG structure.     

Prediction of Distortion of Cylinders without Phase Transformations

The rule of Ameen whereby steel made cylinders with no phase transformations during quenching approach the spherical shape is contrary to the finding of Berger, who observes an elongation of thin and long work pieces. If the prediction of Ameen is correct, cylinders should decrease in length during heat treatment. This paper describes a first step of broader investigation of strain hardening and distortion of cylinders during gas quenching in a gas nozzle field. To make more general predictions about the distortion of cylinders which show no phase transformations during heat treatment, different dimensions of cylinders (lengths 50 mm, 100 mm and 200 mm, diameters 10 mm up to 100 mm) were investigated by means of numerical and experimental methods. The prediction of dimension and shape changes during gas quenching of steel cylinders has been performed by numerical simulation using the commercial Finite Element Program SYSWELD. The austenitic steel SAE30300 (German grade X8CrNiS18.9) was selected as investigated material; it shows no phase transformations during performed heating and cooling. The investigations show a good agreement between the kinematic strain hardening model and the experimental data, whereas the isotropic model is not in line with the experimental data. If the ratio of length and diameter is greater than 3, the relative changes in length displayed against the Biot number give uniform curves for all investigated geometries and heat transfer coefficients.     

Transport properties of fluid mixtures in the critical region

Transport properties of fluid mixtures exhibit anomalous behavior near the vapor-liquid critical line. These anomalies are a result of long-range fluctuations in the system in the vicinity of a critical point. We use mode-coupling theory to describe the critical enhancements of the thermal conductivity, the viscosity, the mutual diffusivity, and the thermal-diffusion coefficients of binary mixtures. The resulting expressions not only are valid in the asymptotic critical region but also describe the crossover to regular behavior far away from a critical point. The crossover functions depend on the thermodynamic properties of the mixtures, background values of all transport coefficients, and two concentration-dependent cutoff wave numbers. We compare the proposed crossover model with experimental thermal-conductivity data for mixtures of carbon dioxide and ethane in the critical region and find good agreement between theory and experiment.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

Thermal Conductivity and Critical Boundary Resistance of Helium Near the Lambda Point

We report measurements of the temperature dependence of the thermal conductivity of helium in the region just above the lambda point and the critical boundary resistance on both sides of the transition. The data were obtained using a cell with a gap that was varied from 0.12 to 0.73 mm. The range of the data was from about 2 mK below to 8 mK above the transition, with a maximum resolution of about 5×10⁻⁹ K, well within the gravity rounded region. We find good agreement with the expected behavior of the thermal conductivity based on the dynamic renormalization group theory. The boundary resistance results above the transition are in fair agreement with the theoretical prediction of Frank and Dohm. Below the transition the wide range results are in approximate agreement with additional predictions and other measurements at higher powers. Closer to the transition on the low temperature side phenomena are observed which appear to be apparatus-dependent.      

The specific heat of confined helium near the lambda point

Heat capacity measurements of the liquid helium lambda transition in confined geometries have long been in disagreement with scaling theories of second order phase transitions. We present measurements of a 19 micron film which extend the results of Chen and Gasparini (CG) to near macroscopic size. We find that the new data as well as most of the CG data are in reasonable agreement with recent renormalization group theory calculations.     

Inclusion (particle) removal by interception and gravity in ceramic foam filters

An analytical model regarding the ceramic foam filter (CFF) as a network of branches (cylinders) has been developed to describe inclusion removal in CFFs used for aluminium filtration. The model based on measurements of filters with 30 pores per inch, should also predict behaviour of commercial filters with finer pores. Filtration efficiency is a function of particle size, metal velocity, particle settling velocity, and filter properties—the branch diameter, filter thickness, porosity, and specific surface area. The model takes into account interception with cylinders and settling on branch surfaces. The velocities are calculated from the Forchheimer and Ergun’s equation. There is good agreement between the model and plant experiments. Removal by interception increases strongly with decreasing cylinder diameter. There is indication that Al₂O₃ and SiC filters differ in their capture of inclusions.     

Specific Heat Near the Superfluid Transition of a 0.9869 μm 4He Film

We report measurements of the specific heat of ⁴He near the superfluid transition while confined between silicon wafers at 0.9869 m separation. These data are analyzed to check on the behavior expected from correlation-length scaling. Comparison is also made with other data for planar confinement, as well as data for cylindrical confinement. These represent different lower-dimensional crossovers. We find that the present data scale very well above the bulk transition temperature, and in the region immediately below it. Near the specific heat maximum however, the data for planar confinement do not collapse on a universal curve. We compare these results with specific theoretical scaling functions. In particular we find that on the normal side, and for large enough values of the scaling variable, one can describe the data well using the concept of the surface specific heat. The locus of the data in this region agrees well with the most recent theoretical calculations.     

串列多圆柱气动力干扰效应的试验研究

通过刚性模型测压风洞试验,在均匀流场中对比研究了不同数量和不同间距串列多圆柱气动力的干扰效应。串列多圆柱两相邻圆柱的中心距L与圆柱的直径D之比L/D的变化范围为1.2~12.0。圆柱数量的变化范围为1~4。试验的雷诺数为3.4×10⁴。试验结果发现:串列多圆柱发生流态切换的临界间距比(L/D)cr为3.5~4.0,在临界间距附近,前两个圆柱的时均阻力系数和脉动升力系数突升,其余圆柱则突降,所有圆柱的斯托罗哈数均突升;气动干扰对串列多圆柱时均阻力系数和斯托罗哈数的影响主要表现为减小效应;后方干扰圆柱数量的增加对上游第一个圆柱气动力的影响基本可以忽略;前方干扰圆柱数量的增加对下游最后一个圆柱的气动力影响显著。     

Continuous vortex cutting in type II superconductors with longitudinal current

The consequences of stipulating translational symmetry for a type II superconductor to which longitudinal electric current and longitudinal magnetic field are applied are investigated. The magnetic flux lines must cut each other continuously in order to generate an electric field in this symmetry. We describe the steady state by two interpenetrating vortex lattices moving into and out of the sample. We find for the slab and cylinder geometries that cutting, crossjoining, and subsequent straightening of the flux lines reduce the electric field, as compared with the normal conducting state, by a factor which is of the order of one over the total number of flux lines in the cylinder. We conclude that the much larger voltages observed in cylinders of several millimeters diameter can be explained only by a breakdown of translational symmetry. With translational ¥mmetry, the voltage initially increases as the third power of the current. The resulting vortex configuration is force-free. The transverse flux component increases and the longitudinal component decreases from the axis to the surface, leading to a paramagnetic moment. The drift or oscillation velocity of the flux lines is reduced by the same factor as the electric field. We predict low-frequency oscillations of the vortices near the surface of thin superconducting wires.     

The effect of diameter/thickness ratio on the ratchetting behaviour of pressurised plain pipework subjected to simulated seismic loading

A series of five tests have been conducted using cylindrical specimens having mean diameter/thickness (D_m lt) ratios in the range 9≤D_m lt ≤25. The cylinders were subjected to an internal pressure (calculated as the design pressure for each cylinder) and alternating bending moments at frequencies typical of seismic events. Hoop ratchetting was recorded in all cylinders at bending moments greater than the yield moment. The moment level for the onset of ratchetting was found to be in agreement with theoretical predictions. The variations in behaviour caused by changing D/t ratios is discussed.     

A Numerical Simulation of Swirling Flow Pneumatic Conveying in a Vertical Pipeline

A numerical prediction for the axial and swirling pneumatic conveying in a vertical pipe was performed based on an Eulerian approach for the gas and a stochastic Lagrangian approach for the particles, where κ – ? turbulence model, the model of particle-particle and particle-wall collisions, was adopted. The numerical results are presented for polyethylene pellets of 3.2mm diameter conveyed through a pipeline of 12m in height with an inner diameter of 80mm. The initial swirl number was 0.0 and 0.68, the mean gas velocity varied from 11 to 17m/s, and the solid mass flow rate was 0.03 and 0.084 kg/s. From the numerical analysis, the swirl decay of the swirling gas-solid flow was found to be rapid in the acceleration region and approached the clean swirling flow in a fully developed region. The turbulent kinetic energy and energy dissipation rates of the swirling gas-solid flow increased near the wall and reduced in other regions. The comparison of predicted values with measured data showed a good agreement.     

Theoretical and experimental investigation of the extinction in a dense distribution of particles: nonlocal effects.

We report experimental measurements of the extinction in a suspension of dielectric spheres. We find that the model originally introduced by Keller is in good agreement with the data provided that nonlocal effects are properly taken into account. We also find that the simple criterion establishing the regime of independent scattering previously introduced is not consistent with our data.     

Numerical simulation of microparticles transport in a concentric annulus by Lattice Boltzmann Method

Dispersion and removal of microaerosol particles are investigated numerically in a horizontal concentric annulus by Lattice Boltzmann Method and Lagrangian Runge–Kutta procedure with the assumption of one-way coupling. Drag, buoyancy, gravity, shear lift, Brownian motion and thermophoretic are forces that are included in particle equation of motion. All simulations were performed at Rayleigh number of 10⁴ and particles specific density of 1000. The effect of aspect ratio and particles diameter were determined on particles behavior such as removal and dispersion. Results show that recirculation power increases by decreasing of cylinders gap. Particles move in a thinner quasi-equilibrium region by increasing of their diameter and decreasing of cylinders gap. Brownian motion is dominant removal mechanism in particle with diameter of 1 μm.     

Modeling the conductance and DNA blockade of solid-state nanopores

We present measurements and theoretical modeling of the ionic conductance G of solid-state nanopores with 5-100 nm diameters, with and without DNA inserted into the pore. First, we show that it is essential to include access resistance to describe the conductance, in particular for larger pore diameters. We then present an exact solution for G of an hourglass-shaped pore, which agrees very well with our measurements without any adjustable parameters, and which is an improvement over the cylindrical approximation. Subsequently we discuss the conductance blockade ΔG due to the insertion of a DNA molecule into the pore, which we study experimentally as a function of pore diameter. We find that ΔG decreases with pore diameter, contrary to the predictions of earlier models that forecasted a constant ΔG. We compare three models for ΔG, all of which provide good agreement with our experimental data.     

激光扫描测量直径

提出一种应用光电共轭原理实现边界检出并进行激光扫描测量直径的新方法。采用该方法 的测量系统中,参考信号是测量信号的反向共轭函数,两函数的一对交点与被测工件边界相对应,工件直径可通过测量两函数交点之间的距离获得。实践验证了该方法的可行性,其边界检出精度±1μm,连续工作漂移不大于±5μm。     

Penetration scaling in atomistic simulations of hypervelocity impact

We present atomistic molecular dynamics simulations of the impact of copper nano particles at 5 km s⁻¹ on copper films ranging in thickness from from 0.5 to 4 times the projectile diameter. We access both penetration and cratering regimes with final cratering morphologies showing considerable similarity to experimental impacts on both micron and millimetre scales. Both craters and holes are formed from a molten region, with relatively low defect densities remaining after cooling and recrystallisation. Crater diameter and penetration limits are compared to analytical scaling models: in agreement with some models we find the onset of penetration occurs for 1.0 < f/d_p < 1.5, where f is the film thickness and d_p is the projectile diameter. However, our results for the hole size agree well with scaling laws based on macroscopic experiments providing enhanced strength of a nano-film that melts completely at the impact region is taken into account.     

Failure of granite cylinders under impact loading

The temporal variation of axial and radial particle velocity components on the surface of granite cylinders was measured when the latter were struck on end by like cylinders of equal diameter. The experimental results were compared with two-dimensional axisymmetric finite difference calculations in which several different constitutive models were evaluated. Fairly good agreement with experiment was obtained with a simple intuitive representation of the brittle fracture process in which locally linear elastic behavior prevailed until such time as any principal stress component exceeded in tension some small prescribed value. The subsequent failure mechanism admits the opening of cracks, multiple cracking to the point of pulverization and possible reclosure under polyaxial compression. The fracture zone development is strongly influenced by a feedback effect from the interaction of the elastic field in the virgin material with signals propagating forward from the existing fracture zone.     

Applicability of regular particle shapes in light scattering calculations for atmospheric ice particles

We ascertain the usefulness of simple ice particle geometries for modeling the intensity distribution of light scattering by atmospheric ice particles. To this end, similarities and differences in light scattering by axis-equivalent, regular and distorted hexagonal cylindric, ellipsoidal, and circular cylindric ice particles are reported. All the results pertain to particles with sizes much larger than a wavelength and are based on a geometrical optics approximation. At a nonabsorbing wavelength of 0.55 μm, ellipsoids (circular cylinders) have a much (slightly) larger asymmetry parameter g than regular hexagonal cylinders. However, our computations show that only random distortion of the crystal shape leads to a closer agreement with g values as small as 0.7 as derived from some remote-sensing data analysis. This may suggest that scattering by regular particle shapes is not necessarily representative of real atmospheric ice crystals at nonabsorbing wavelengths. On the other hand, if real ice particles happen to be hexagonal, they may be approximated by circular cylinders at absorbing wavelengths.     

Electro-magnetic collapse of thick-walled cylinders to investigate spontaneous shear localization

We present an electro-magnetic (EM) setup in order to collapse thick-walled cylinders, for the investigation of spontaneous formation of multiple adiabatic shear bands. The EM setup is based on a pulsed current generator using a capacitor bank system. The cylindrical specimen is part of an assembly of coaxial cylinders, where the inner and outer cylinders, each attached to an opposite pole, are short-circuited. Upon capacitor discharge, a high current flows through the cylinders, in opposite directions, creating repulsive magnetic forces between them. The outer cylinder is driven outwards and the inner cylinder is driven inwards - in a collapsing manner. This work presents the design procedure of the specimens’ geometry using numerical simulations, and some preliminary experimental results for SS304L steel specimens. The spatial distribution of the multiple adiabatic shear bands in these specimens is in good agreement with that reported in the literature for explosively driven experiments with the same material. Our numerical simulations of the collapsing cylinder show good agreement with the experimental results for both global behavior and shear band distribution.     

高雷诺数下错列双圆柱气动干扰的机理研究

为了进一步澄清小间距错列双圆柱的气动干扰机理,该文采用大涡模拟方法,在高雷诺数下（Re=1.4×105）,研究了间距为2倍圆柱直径的错列双圆柱的气动性能和流场特性随风攻角的变化规律,分析了两个圆柱气动力系数相关性,探讨了下游圆柱气动力与流场结构的内在联系,对下游圆柱平均升力的流场机理提出了新的解释。研究表明,大涡模拟得到的结果与风洞试验值吻合良好;下游圆柱的气动性能、流场结构和两个圆柱气动力相关性均会随风攻角发生剧烈变化;风攻角在0°~10°时,下游圆柱受平均负阻力作用,其原因分别为两圆柱间的回流区和间隙流;风攻角在10°附近时,下游圆柱受很大平均升力作用,风压停滞点偏移、两圆柱间高速间隙流、下游圆柱间隙侧剪切层的提前分离和再附是平均升力出现的三个因素。