Effects of base excitation frequency on the stability of a freestanding rigid block

The behavior of a freestanding block subjected to base excitation is a nonlinear problem. In the past, many researchers have analyzed freestanding blocks subjected to base excitation and reported their findings. However, still more understanding is required to understand excitation frequency effects on the stability of a freestanding block. Detailed experiments and numerical analysis were performed on a parallelepiped blockhaving an aspect ratio of four, for different values of coefficient of friction and base amplitudes. It was observed that base excitation frequency has a predominant role on the motion and hence the stability of a freestanding block. Contrary to the present understanding of the subject, it was observed that initiation of any mode of motion (like sliding, rocking) from rest depends not only on the parameters like amplitude of base excitation,coefficient of friction and aspect ratio, but also on the base excitation frequency. A freestanding system initially at rest shall remain in rest until excited by a base excitation frequency below a particular value.We numerically determined the value of this frequency (termed as cutoff frequency) for different values of coefficient of friction (0.1–1) and base amplitude (0.1–1 g) for a constant aspect ratio of four of a parallelepiped block with square base. Using data generated from numerical analysis, a three-dimensional diagram named as initiation criteria diagram was developed that identifies various regions corresponding to sliding, rocking, slide–rock and rest (no motion) phase for a freestanding block subjected to base excitation. A novel finding was the presence of a region of no motion above a frequency value for any given combination of parameters. The ratio ofthe amplitude of acceleration obtained at the top of the block to the amplitude of applied base acceleration (amplification factor) was calculated for different experimental cases. The dependence of the amplification factor on the base excitation frequency was also studied. Hence, it was concluded that the base excitation frequency plays an important role in determining the stability of a freestanding rigid block.     

Statistics of responses of a mistuned and frictionally damped bladed disk assembly subjected to white noise and narrow band excitations

In this paper, the effects of friction dampers on the vibration of a mistuned bladed disk assembly are examined for the following types of random excitation: white noise and narrow band. To compute the statistics of nonlinear response, an analytical technique is developed based on the equivalent linearization method. The validity of the analytical technique has been established by comparison with the results from numerical simulations. The relation between the vibration mode of the system and the performance of friction dampers is investigated.     

Simultaneous optimization of force and placement of friction dampers under seismic loading

It is known that the use of passive energy-dissipation devices, such as friction dampers, reduces considerably the dynamic response of a structure subjected to earthquake ground motions. Nevertheless, the parameters of each damper and the best placement of these devices remain difficult to determine. Some articles on optimum design of tuned mass dampers and viscous dampers have been published; however, there is a lack of studies on optimization of friction dampers. The main contribution of this article is to propose a methodology to simultaneously optimize the location of friction dampers and their friction forces in structures subjected to seismic loading, to achieve a desired level of reduction in the response. For this purpose, the recently developed backtracking search optimization algorithm (BSA) is employed, which can deal with optimization problems involving mixed discrete and continuous variables. For illustration purposes, two different structures are presented. The first is a six-storey shear building and the second is a transmission line tower. In both cases, the forces and positions of friction dampers are the design variables, while the objective functions are to minimize the interstorey drift for the first case and to minimize the maximum displacement at the top of the tower for the second example. The results show that the proposed method was able to reduce the interstorey drift of the shear building by more than 65% and the maximum displacement at the top of the tower by approximately 55%, with only three friction dampers. The proposed methodology is quite general and it could be recommended as an effective tool for optimum design of friction dampers for structural response control. Thus, this article shows that friction dampers can be designed in a safe and economic way.     

Stochastic rotational response of a parametric pendulum coupled with an SDOF system

In this paper, parametric excitation of a lumped mass pendulum is investigated when having its pivot point vertically excited. Such a system is described by a sinus type nonlinear Mathieu equation and attention is drawn to the rotational motion of the mass. A single-degree-of-freedom (SDOF) system is used as a base for the pendulum, floating on ocean waves and being externally excited by them thus falling into the subclass of autoparametric systems. Due to the random nature of waves, a narrow-band stochastic process is used to model the environmental excitation. The effect of dry friction is considered and the interaction between the pendulum's rotational response and the base is studied.     

Effect of Eccentricity on a Clutch System Under a Harmonically Varying Normal Load

This paper describes a clutch model having eleven degrees of freedom and three types of nonlinearity introduced by dry friction, double-stage stiffness, and spline clearances. The Coulomb friction formulations and spline clearance functions are smoothed before they are applied to the nonlinear system. The calculation of the dynamic response of the clutch is done by the Runge Kutta method. The dynamic behavior of both a healthy clutch system and clutch system containing an eccentricity defect is compared in the time and frequency domains. The effects of a harmonically varying load on the dynamic response of the defective clutch system with nonlinear dry friction and under sinusoidal torque excitation are also reported.     

A novel mean square formulation of stochastic nonlinear dynamic systems based on Adomian decomposition

An Adomian decomposition based mathematical framework to derive the mean square responses of nonlinear structural systems subjected to stochastic excitation is presented. The exact mean square response estimation of certain class of nonlinear stochastic systems is achieved using Fokker–Planck–Kolmogorov (FPK) equations resulting in analytical expressions or using Monte Carlo simulations. However, for most of the nonlinear systems, the response estimation using Monte Carlo simulations is computationally expensive, and, also, obtaining solution of FPK equation is mathematically exhaustive owing to the requirement to solve a stochastic partial differential equation. In this context, the present work proposes an Adomian decomposition based formalism to derive semi-analytical expressions for the second order response statistics. Further, a derivative matching based moment approximation technique is employed to reduce the higher order moments in nonlinear systems into functions of lower order moments without resorting to any sort of linearization. Three case studies consisting of Duffing oscillator with negative stiffness, Rayleigh Van-der Pol oscillator and a Pendulum tuned mass damper inerter system with linear auxiliary spring–damper arrangement subjected to white noise excitation are undertaken. The accuracy of the closed form expressions derived using the proposed framework is established by comparing the mean square responses of the systems with the exact solutions. The results demonstrate the robustness of the proposed framework for accurate statistical analysis of nonlinear systems under stochastic excitation.     

Resonance ejection from the paul trap: a theoretical treatment incorporating a weak octapole field

In response to the growing experimental evidence of the importance of nonlinear phenomena in ion trap operation, a new theoretical model of ion ejection is developed. The pseudopotential well approximation for forced ion oscillations in an ion trap under the conditions of ion-molecule collisions is modified to include octapole perturbations on the quadrupole field. Ion ejection is investigated using the first-order Mitropol'skii asymptotic method for both infinitesimal and finite scan rates. It is shown that the combined action of collisional damping and nonlinearity distorts the resonance curve in such a way that "quenching" of oscillations takes place. As a result, with appropriate excitation and direction of scanning, the amplitude increases as if no damping exists! The main characteristics of the jump are derived as functions of scan rate and used for analytical estimation of mass resolution, mass peak width, and excitation voltage. Satisfactory agreement between calculated and experimental peak widths is demonstrated for the range of scanning rates in excess of 6 orders of magnitude.     

Hydride vapour phase epitaxy growth and characterisation of GaN layers

A vertical hydride vapour phase epitaxial reactor was applied for the growth of GaN layers on (0001) sapphire. In this paper, the growth parameters and the results of optical investigations are reported. It is seen that the layers grown at high temperature have better crystallographic properties than those grown at 600–800°C, although the latter show a smoother surface. The absorption spectra exhibited a tail from the absorption edge down to about 1 eV that may be fitted by the Lukovsky model considering the presence of a level at about 1.2 eV from the conduction band. The cathodoluminescence spectra show three main emissions: the yellow band, a blue band (BB) centred at 2.8 eV and the near band edge recombination. The BB is mostly originated close to the layer/substrate interface and practically extinguished at the layer surface. It is seen that when the intensity of the BB increases that of the fundamental recombination decreases and vice versa.     

Is high-T c superconductor an undegenerated doped semiconductor?

The model of electron spectrum of HTS is suggested. It is supposed that the main feature of the spectrum is the presence of a narrow local pair level placed near the top of the filled two-dimensional (2D) electron band. Holes in this band are the result of thermal activation of electrons from the band to the pair level. The temperature dependences of resistivity and Hall constant of HTS in this model are in agreement with experiment on YBCO. The possibility of a first-order phase transition in such a system is considered.     

随机参数刚弹耦合平面连杆的动力分析

建立考虑物理参数、几何参数及荷载均为随机变量的平面连杆机构的动力学方程,在建模中计入刚弹耦合项和运动副的粘性摩擦。利用Newmark-β逐步积分法将此随机参数机构系统的动力学方程转换为随机参数的拟静力控制方程。利用求解随机变量函数数字特征的矩法和代数综合法,导出机构动态弹性位移的均值和方差计算公式。通过算例考察了机构的杆长、截面半径、质量密度、弹性模量的随机性,刚弹耦合项和运动副摩擦对机构动力响应的影响。     

Vacuum ultraviolet optical properties of (La,Gd)PO4:RE (RE=Eu, Tb)

Vacuum ultraviolet excitation spectra of phosphors (La,Gd)PO₄:RE³⁺ (RE=Eu or Tb) and X-ray photoelectron spectra of LaPO₄ and GdPO₄ are investigated. The vacuum ultraviolet excitation intensity of (La,Gd)PO₄:RE³⁺ is enhanced with the increasing of Gd³⁺ content, which implies that Gd³⁺ plays an intermediate role in energy transfer from host absorption band to RE³⁺. When Gd³⁺ is doped into LaPO₄:Eu³⁺, charge transfer band (CT band) begins to shift to higher energy region and the overlap degree of CT band and the host absorption band gets greater with more Gd³⁺ doped into LaPO₄. These results suggest that the dopant (Gd³⁺) gives an important influence on energy transfer efficiency. The top of LaPO₄ valance band is formed by the 2p level of O²⁻, whereas that of GdPO₄ valance band is formed by the 2p level of O²⁻ and the 4f level of Gd³⁺, showing the differences in band structures between LaPO₄ and GdPO₄.     

Laser-induced fluorescence of formaldehyde hot bands in flames

Laser-induced fluorescence and excitation spectra of formaldehyde in the A-X 4(1)(0) band at 370 nm are recorded in the primary flame front of a Bunsen flame. An examination of partition functions shows that this excitation can minimize temperature bias for formaldehyde in situ diagnostic measurements.     

Experimental and numerical results of dynamics behavior of a nonlinear piezoelectric beam

This article presents a numerical formulation and experimental implementation for the dynamics behavior verification of the nonlinear piezoelectric beam through harmonic excitation. The nonlinear piezoelectric beam dynamic analysis program is developed with MATLAB software. To verify the nonlinear piezoelectric beam dynamic analysis results, the experimental results are used for the vibration analysis of a piezoelectric beam to the harmonic excitation of the base of the beam. Then, the piezoelectric effect on the output voltage, velocity, acceleration values, and the time response are obtained. Afterwards, the effects of the excitation velocity and the position of concentrated mass on the output voltage are verified.     

Optimized algorithm for the spatial nonuniformity correction of an imaging system based on a charge-coupled device color camera

We present an optimized linear algorithm for the spatial nonuniformity correction of a CCD color camera's imaging system and the experimental methodology developed for its implementation. We assess the influence of the algorithm's variables on the quality of the correction, that is, the dark image, the base correction image, and the reference level, and the range of application of the correction using a uniform radiance field provided by an integrator cube. The best spatial nonuniformity correction is achieved by having a nonzero dark image, by using an image with a mean digital level placed in the linear response range of the camera as the base correction image and taking the mean digital level of the image as the reference digital level. The response of the CCD color camera's imaging system to the uniform radiance field shows a high level of spatial uniformity after the optimized algorithm has been applied, which also allows us to achieve a high-quality spatial nonuniformity correction of captured images under different exposure conditions.     

Resonance Raman spectroscopy of photolabile transition metal carbonyls: controlled photoalteration with continuous wave lasers to record spectra of reactants or transient species

A method has been developed that enables resonance Raman spectra of photolabile species in solution to be recorded under conditions where the level of photoalteration is controlled: a low level enables reactant spectra to be recorded, whereas a high level enables the spectra of short-lived transient species to be recorded in real time using continuous-wave (CW) lasers and standard Raman detection equipment. The design includes a sealed flow system, enabling air-sensitive species to be studied under an inert atmosphere. A simple theoretical model has been developed to aid the interpretation of experimental results, and its applicability is demonstrated. Controlled photoalteration and its theory are demonstrated with 413.1-nm excitation of carbonmonoxymyoglobin (MbCO), which generates deoxymyoglobin (deoxy-Mb) on photolysis, and for which the spectra of both species are well established. The methods have also been applied to two air-sensitive, photolabile transition metal carbonyls using 514.5-nm wavelength excitation: for Cp2Mo2(CO)6 (Cp = eta5-C5H5), increasing levels of photoalteration result only in a decrease in the parent band intensities, relative to the solvent bands; for Cp2Fe2(CO)4, increasing levels of photoalteration result in the appearance of additional bands that are assigned to the transient species CpFe(mu-CO)3FeCp, formed following the loss of a CO ligand.     

Direct measurement of resonance Raman spectra and cross sections by a polarization difference technique

Resonant Raman (RR) spectroscopy, despite its many promising applications in analytical chemistry and biology, remains an experimental challenge (compared to standard Raman) primarily because of the presence of large fluorescence backgrounds overwhelming the RR signals. The observation of RR spectra of fluorophores therefore requires the use of specialized, picosecond-time-resolved setups. Here, we present and demonstrate a method, based on polarization-difference, by which RR spectra and cross sections can be measured using the most standard Raman setup with continuous wave excitation and CCD-based detection. The method is applied to the dyes Nile Blue and rhodamine 6G under resonant excitation. This work should open a new era in RR spectroscopy, where RR spectra can be routinely measured and studied with conventional Raman systems.     

多界面干摩擦系统的减振特性及设计方法研究

作为一类非线性阻尼, 干摩擦阻尼器的减振效果通常与激振力水平直接相关, 且往往只在一个激振力水平下具有最佳振动抑制效果。为改善其“激振力-减振效果”特性, 提出了设计多界面干摩擦系统的思路。从一类可描述扭转振动的集总参数模型出发, 探讨了此类系统的可行性, 并给出了无量纲的动力学方程组。其次, 提出了一种改进的时/频转换方法, 用以分析包含任意干摩擦界面系统的频域响应, 考虑了每个干摩擦界面上可能出现的周期内“滑移”、“滑移-阻滞”和“阻滞”运动状态。同时, 用相应的数值积分方法验证了该方法的正确性和在计算速度上的优势。在此基础上, 分析了干摩擦界面的个数、临界摩擦力和质量对系统特性的影响, 以二界面和三界面干摩擦系统为算例, 阐明了采用多个干摩擦界面改善系统减振性能的机理和设计方法。     

A panel method for modelling level ice actions on moored ships. Part 1: Local ice force formulation

Model tests of a moored ship in level ice have been performed. The experimental setup made it possible to measure interaction forces between the ice and the ship's bow area. These kinds of measurements are for the first time reported in the literature.Methods for constructing local ice forces based on observations from the model tests are introduced. The ice-hull interaction is separated into ice breaking, rotation and sliding phases. It is assumed that breaking and rotation occur in the vicinity of the waterline and that the main sliding phase occurs under water. A simplified formulation for forces near the waterline caused by breaking and rotation of ice floes is based on four random variables. An algorithm to extract these variables from the measured data is given. Probability distributions are fitted to the random variables and the distributions are analysed with respect to effects of the mooring stiffness and the ice drift speed. The difference between the constructed and the measured ice forces in terms of mean values and standard deviations are small.A Coulomb friction model is assumed for the friction forces on the wet part of the bow. The friction coefficient is assumed to be stochastic and is extracted from the measurements. Even though the assumed friction model is simplified, it closely resembles the measured forces.The model is applied to study the dynamic response of a moored realistic hull in a separate paper.     

Aspects of emission variation in CdSeTe/ZnS quantum dots conjugated to antibodies

CdSeTe/ZnS quantum dots (QDs) with the emission peak at 705 nm have been studied comparatively in the non-conjugated state and after bioconjugation to anti-pseudo rabies virus antibodies (ABs) by means of photoluminescence (PL) and Raman scattering methods. It is revealed that PL spectra of QDs vary significantly after conjugating to ABs. In PL spectra of non-conjugated QDs only one PL band of Gaussian shape peaked at 1.76–1.78 eV and related to exciton emission in the CdSeTe core has been detected. The PL spectra of bio-conjugated QDs demonstrate the high energy spectral shift and asymmetric shape of PL bands. The study of Raman scattering spectra permits to estimate the CdSeTe alloy composition and to detect the surface enhanced Raman scattering (SERS) effect for bioconjugated QDs. The last fact testifies on the interaction of excitation light electromagnetic field with the electric dipoles excited in ABs. The optical band gap in CdSeTe core has been calculated numerically versus core radius on the base of the effective mass approximation model. Then the energy band diagrams for non-conjugated and bio-conjugated states of CdSeTe/ZnS QDs have been designed. It is revealed the type II quantum well in CdSeTe core that explains the optical transition at 705 nm in the wide band gap CdSeTe alloy. The analysis has shown that AB dipoles excited in bio-conjugated QDs stimulate changing the profile of QD energy band diagram that manifests itself in the mentioned PL spectrum transformations. Actually, the study of PL spectrum varying in CdSeTe/ZnS QDs conjugated to specific antibodies can be an informative tool in biology and medicine for early medical diagnostics.     

广义小量分解法在载荷识别中的应用

采用小量分解法处理载荷识别计算过程中频响函数矩阵秩缺的问题，推导出了广义小量分解法及进行误差分析和小量选取的乘积误差项。最后将小量分解法应用到钢悬臂梁逆虚拟激励法随机振动的载荷识别实验过程中，在一定程度上克服了频响函数在某些频率处的病态程度，取得了，较好的试验结果。