An experimental and statistical study of the behavior of the vibration field in two coupled lightweight wooden joist floors

The aim of this study was to evaluate the vibration level attenuation of a common wooden floor structure and to present the results together with the statistical precision of the evaluation. Linear regression was used to determine the attenuation rate in the two main directions of the floor structure. The probability for the attenuation rate to be zero was calculated. The attenuation rate was found to be high in the direction perpendicular to the beams with a probability to be zero lower than 0.001. In the direction parallel to the beams the attenuation rate was found to be close to zero.     

Numbers,scale and symbols: the public understanding of nanotechnology

Nanotechnology will be an increasing part of the everyday lives of most people in the world. There is a general recognition that few people understand the implications of the technology, the technology itself or even the definition of the word. This lack of understanding stems from a lack of knowledge about science in general but more specifically difficulty in grasping the size scale and symbolism of nanotechnology. A potential key to informing the general public is establishing the ability to comprehend the scale of nanotechnology. Transitioning from the macro to the nanoscale seems to require an ability to comprehend scales of one-billion. Scaling is a skill not common in most individuals and tests of their ability to extrapolate size based upon scaling a common object demonstrates that most individuals cannot scale to the extent needed to make the transition to nanoscale. Symbolism is another important vehicle to providing the general public with a basis to understand the concepts of nanotechnology. With increasing age, individuals are able to draw representations of atomic scale objects, but these tend to be iconic and the different representations not easily translated. Ball and stick models are most recognized by the public, which provides an opportunity to present not only useful symbolism but also a reference point for the atomic scale.     

经典洛伦兹磁力和广义洛伦兹磁力解释电子感应加速器的本质

介绍了虚构的法拉第定律与电子感应加速器不合理,用完整洛伦兹磁力解释电子感应加速器合理有效。     

Deflagration to detonation transition in fast flames and tracking with chemical explosive mode analysis

In the context of vapour cloud explosion, the flame acceleration process can lead to conditions promoting deflagration to detonation transition (DDT), potentially leading to increased damages in accidental scenarios. This study focuses on this phenomenon by performing simulations of detonation reinitiation for fast flames in the Chapman–Jouguet deflagration regime. It is obtained experimentally by the attenuation of an incident detonation by an array of obstacles. A primary objective of the paper is to demonstrate the ability of the numerical model to reproduce the major experimental trends, namely the variation of the reinitiation propensity for different initial pressures and blockage ratios (BRs). Chemical explosive mode analysis (CEMA) is also adapted to the context of this study, in order to identify locally the propagation regime and to provide insights on the reinitiation mechanism. An a priori validation of the CEMA methodology is first performed on relevant canonical one-dimensional configurations. Subsequently, ensembles of five realizations are computed at different initial pressures and BRs and compared to experimental data. They are shown to reproduce the major observed trends in terms of detonation reinitiation length with respect to the operating conditions, with significant variability from one realization to another. In addition, the reinitiation mechanism is also found to be consistent with experimental observations and a previous numerical study of the same configuration. The CEMA methodology adapted to this context is able to identify locally the different propagation regimes, and to track the highly reactive zones that coherently couple with transverse pressure perturbations, leading to the formation of a strongly reacting kernel which eventually triggers the detonation reinitiation.     

Adaptive digital twins of combustion systems using sparse sensing strategies

This work proposes to implement a sparse sensing framework to build a hybrid numerical-experimental Digital Twin of a practical combustion system. The goal is to find the optimal sensor placement that minimizes the prediction error, and to predict the distribution of reacting scalars using few measurements. Three-dimensional CFD simulations with detailed chemistry were used to build the design space by varying the fuel composition (from pure methane to pure hydrogen), the equivalence ratio (from 0.7 to 1) and the air velocity. The Proper Orthogonal Decomposition (POD) was applied to the numerical data to find a tailored basis for dimensionality reduction. Then, the QR decomposition with column pivoting was applied to the tailored basis to find the optimal sensor placement. Finally, the model was employed to predict the three-dimensional temperature distribution in the unexplored part of the design space, using the experimental samples as input. The optimal placement of the sensors provides valuable information on the key locations and features, which can then be used in the design of reactor network models, for example. Also, the results show that the hybrid Digital Twin could predict an adjusted temperature distribution which reduces the error with the experimental measurements, when compared to the original CFD temperature distribution.     

Gravitational critical phenomena in the realm of the galaxies and Ising magnets

In the non-relativistic and quasi-static limit, it is possible to map exactly the system of galaxies in the observable universe onto an Ising magnet. Techniques from the theory of critical phenomena as applied to magnets can then be employed to calculate rigorously the galaxy-to-galaxy correlation function, whose critical exponent is predicted to be between 1.530 to 1.862, to be compared to the empirical/observational value of 1.6 to 1.8.This essay received the fifth award from the Gravity Research Foundation, 1996—Ed.     

光纤传感实验设计

提出了一种适用于教学的光纤传感器实验研究系统。在实验过程中首先测定了LED光源的I-P特性曲线,接着对光纤的纤端光场分布进行测试,并通过实验得到的数据绘出光纤的纤端光场光强曲线,然后利用光纤传感实验仪对微位移用两种方式进行测量。根据现有的实验条件设计出适合于学生进行的基本实验。     

Simulating coupled dynamics of a rigid-flexible multibody system and compressible fluid

As a subsequent work of previous studies of authors, a new parallel computation approach is proposed to simulate the coupled dynamics of a rigid-flexible multibody system and compressible fluid. In this approach, the smoothed particle hydrodynamics (SPH) method is used to model the compressible fluid, the natural coordinate formulation (NCF) and absolute nodal coordinate formulation (ANCF) are used to model the rigid and flexible bodies, respectively. In order to model the compressible fluid properly and efficiently via SPH method, three measures are taken as follows. The first is to use the Riemann solver to cope with the fluid compressibility, the second is to define virtual particles of SPH to model the dynamic interaction between the fluid and the multibody system, and the third is to impose the boundary conditions of periodical inflow and outflow to reduce the number of SPH particles involved in the computation process. Afterwards, a parallel computation strategy is proposed based on the graphics processing unit (GPU) to detect the neighboring SPH particles and to solve the dynamic equations of SPH particles in order to improve the computation efficiency. Meanwhile, the generalized-alpha algorithm is used to solve the dynamic equations of the multibody system. Finally, four case studies are given to validate the proposed parallel computation approach.     

Flavor symmetry,EMC results and the spin content of the proton

The analysis of the EMC result on the quark contribution to the spin of the proton has caused considerable confusion and is unnecessarily complicated because of the completely unjustified and incorrect use of SU(3) flavor symmetry to provide input on the proton wave function from hyperon decays. There is no reliable method for obtaining information on the proton spin structure from data on the couplings of currents to hyperons. Interesting peculiar results obtained without use of SU(3) and hyperon data show the crucial point to be the apparent contradiction between the contribution of valence u quarks to the proton spin observed in the axial vector contribution to nucleon beta decay and the failure of this contribution to appear in the conventional quark-parton interpretation of the EMC results. When input from hyperon data is not used this paradox can be resolved without requiring the quark contribution to the proton spin to be near zero, but it can be large only if it is due to the strange quarks, with the nonstrange quark contribution opposite to the spin of the proton.     

Lattice dynamics of solid xenon under pressure

We use density-functional perturbation theory to obtain the phonon spectrum of fcc xenon under pressure. Thermodynamic properties obtained within the quasiharmonic approximation are in fair to good agreement with experiment at zero pressure. The transition pressure from the fcc to hcp phase is predicted to occur at 5 GPa. The fcc structure is found to be dynamically stable up to a pressure of 100 GPa, beyond which the phonon modes at the X and L symmetry points soften. We attribute the observed sluggish kinetics of the fcc-hcp transition to the small energy difference between the phases as well as to the high dynamical stability of the fcc phase.     

圆柱螺旋弹簧迟滞的检测和补偿

介绍一种利用微电容测量技术检测圆柱螺旋压缩弹簧迟滞和利用单片机技术对迟滞进行补偿的实验方法,该方法涉及螺旋机构、电子秤和拟合曲线的求法等.通过这一实验,能帮助学生理解真实材料在一定程度上存在迟滞现象,掌握变极距电容传感器的概念,提高知识的综合应用能力和设计动手能力.     

On the problem of finding a suitable distribution of students to universities in Germany

For many years, the problem of how to distribute students to the various universities in Germany according to the preferences of the students has remained unsolved. Various approaches, like the centralized method to let a central agency organize the distribution to the various universities or the decentralized method to let the students apply directly at their preferred universities, turned out to lead to a significant fraction of frustrated students ending up at universities not being on their preference list or even not having a place to study at all. With our centralized approach, we are able to decrease the fraction of frustrated students as well as the bureaucratic expenses for applicants and universities drastically.     

Limit of hanger linearity in suspension footbridge dynamics: A new section model

The mechanical behaviour of suspension bridges is characterised by nonlinearities due to the main cables geometric effects and to the inability of the hangers to sustain compressive loads. The nonlinear effects due to hanger slackening are expected to increase in suspension footbridges due to lightweight decks, that is, low dead to live load ratio, and to shallow plate-girder decks with very low flexural and torsional stiffness. In this paper a new section model is proposed to study the limit of hanger linearity in lightweight suspension footbridges. The model is inspired to a four degrees-of-freedom model already proposed in the literature, but is expressed with a new formalism that allows some interesting properties to be outlined. Specifically, the expression of a particular frequency, herein called relative antiresonance frequency, as a function of the model generalised properties is derived: if the system is loaded with a harmonic force having that frequency, the linear behaviour of the hangers is assured for every value of the force amplitude. The proposed section model is applied to a footbridge benchmark subject to the pedestrian harmonic load and results are compared with those obtained through a nonlinear dynamic analysis on a 3D Finite Element model of the bridge.     

The Wigner function for two dimensional tori: Uniform approximation and projections

The Wigner representation of a quantum state, corresponding to a classically integrable Hamiltonian, has been shown to be intimately tied to a classical phase space torus of the same energy. The fact that the semiclassical approximation of the Wigner function there derived turns out to be singular on the torus, as well as on the “Wigner caustic” which contains it, is due to well known limitations of the stationary phase method. The uniform approximation, here derived, does indeed ascribe to the Wigner function a high amplitude along the Wigner caustic, but this is modulated by rapid oscillations except at the torus itself. Asymptotic expansion away from the torus leads back to the semiclassical approximation. Close to the torus the Wigner function is described by a simple transitional approximation which can be resolved into a product of Wigner functions corresponding to one dimensional tori. These results permit one to explicitly project the Wigner function onto any (Lagrangian) coordinate plane so as to obtain the corresponding wave intensity.     

Lumped-element technique for the measurement of complex density.

A novel lumped-element technique is employed to measure the complex density of a gas in circular pores. The complex density expresses the geometry-dependent viscous coupling between the gas and the pore walls and is related to the thermoacoustic function f(mu), or equivalently, F(lambdaV). The acoustic impedance of a compliant region coupled to a pore (or pore-array) is measured and the impedance of the compliant region is subtracted to yield the impedance of the pore(s) alone, which is directly related to F(lambdaV). Pores of different lengths are measured in order to eliminate end effects. Working down to very low frequencies achieves a wide range of values for the ratio of the viscous penetration depth to the mean pore size. The results agree very well with analytical solutions for circular pores. The technique is also applied to two porous foam materials. Comparing the results to previous measurements of the complex compressibility, it is shown that two different shape factors (or equivalently, characteristic dimensions) are required to account for the data.     

浅谈光电实验室的教学管理改革

实验教学是帮助学生将理论知识转化为实践技能的重要途径。因此在重视理论教学的同时,必须加强实验环节。针对光电实验室的现状,有必要进行教学方法和管理制度的改革。强化实验室仪器管理,创造好的硬件条件为实验教学服务;改革创新教学方法,使实验效果在传统实验教学的基础上更上一层楼;因材施教,满足各层次学生的不同需求;全方位展开改革实践,建立适合科技发展的教学新体系,培养理论知识扎实,实践技能过硬的复合型人才,以适应社会的发展。     

Fluctuation spectroscopy of granularity in superconducting structures

We suggest to use "fluctuation spectroscopy" as a method to detect granularity in a disordered metal close to a superconducting transition. We show that with lowering temperature T the resistance R(T) of a system of relatively large grains initially grows due to the fluctuation suppression of the one-electron tunneling but decreases with further lowering T due to the coherent charge transfer of the fluctuation Cooper pairs. Under certain conditions, such a maximum in R(T) turns out to be sensitive to weak magnetic fields due to a novel Maki-Thompson-type mechanism.     

Correlations in alpha-alpha scattering and semi-classical optical models

We show the equivalence of semi-classical solutions to optical model coupled-channel equations derived from Watson's form of the nucleus-nucleus multiple-scattering series to the Glauber multiple-scattering series. A second-order solution to the semi-classical coupled-channel elastic amplitude is shown to be nearly equivalent to a second-order optical-phase-shift approximation to the Glauber amplitude if the densities of all nuclear excited states are approximated by the ground-state density. Using the Jastrow method to model the two-body density we find an average excited-state density to be of negligible importance in the double-scattering region of alpha-alpha scattering.     

Exciton emissions of CdS nanowire array fabricated on Cd foil by the solvothermal method

Nanowires have recently attracted more attention because of their low-dimensional structure, tunable optical and electrical properties for next-generation nanoscale optoelectronic devices. Cd S nanowire array, which is(002)-orientation growth and approximately perpendicular to Cd foil substrate, has been fabricated by the solvothermal method. In the temperature-dependent photoluminescence, from short wavelength to long wavelength, four peaks can be ascribed to the emissions from the bandgap, the transition from the holes being bound to the donors or the electrons being bound to the acceptors, the transition from Cd interstitials to Cd vacancies, and the transition from S vacancies to the valence band,respectively. In the photoluminescence of 10 K, the emission originated from the bandgap appears in the form of multiple peaks. Two stronger peaks and five weaker peaks can be observed. The energy differences of the adjacent peaks are close to 38 me V, which is ascribed to the LO phonon energy of Cd S. For the multiple peaks of bandgap emission, from low energy to high energy, the first, second, and third peaks are contributed to the third-order, second-order, and first-order phonon replica of the free exciton A, respectively;the fourth peak is originated from the free exciton A;the fifth peak is contributed to the first-order phonon replica of the excitons bound to neutral donors;the sixth and seventh peaks are originated from the excitons bound to neutral donors and the light polarization parallel to the c axis of hexagonal Cd S, respectively.