Noise reduction analysis for a stiffened finite plate

This paper presents an analytical solution for the vibration and acoustic responses of a finite stiffened plate that is covered with decoupling layers and subjected to external excitation. The theory of elasticity is used for the decoupling layer, and the stiffened plate is modeled by the plate theory and Euler–Bernoulli beam equation. Equations are constructed by the boundary conditions at the plate/coating and coating/fluid interfaces. The problem can be solved by the proposed method in this paper. Test verification shows that a good correlation exists between theoretical and test results. Thus, the theoretical study in this paper is correct. Numerical results show that shear waves insignificantly affect the structural vibration level difference (VR) under low frequencies. The noise reduction of the stiffened plate covered with decoupling layers is greatly influenced by the decoupling layer loss factor. A failure region of the vibration level difference is present in the low frequency band of the decoupling layer. Furthermore, the thickness of the decoupling layer significantly affects noise reduction.     

一种含横向圆柱形空腔的声学覆盖层的去耦机理分析

在水下结构表面敷设去耦覆盖层是降低其声辐射的有效途径. 为了深入分析一种含横向无限长空腔的覆盖层的去耦机理, 本文将其等效为均匀介质, 建立了敷设这种覆盖层的单向基体板在线激励下的声辐射模型, 验证了计算模型的有效性, 并利用计算模型对含横向空腔覆盖层的去耦机理进行了分析. 研究结果表明: 基体板-覆盖层接触面的能量流以纵波能量为主, 而横波能量很小, 因而计算覆盖层的去耦特性时可以忽略横波的作用; 和均匀覆盖层相比, 横向空腔型覆盖层在中高频段极大地增强了基体板的力阻抗, 从而更有效地抑制了基体板的振速; 此外, 和均匀覆盖层相比, 横向空腔型覆盖层和水的阻抗失配更大, 使其在中高频具有良好的振动传递损失特性. 因此, 总体而言, 含横向空腔的覆盖层相比均匀覆盖层具有更好的中高频去耦性能.     

Quasi-resonances in sound-insulating coatings

This paper is concerned with the radiation of sound waves from a submerged cylindrical body which is coated by an imperfect elastic layer; that is, the coating only covers part of the cylinder. The focus of the study is to quantify the effect of the gap in the elastic layer on the radiated acoustic power. A finite element method is employed to determine the acoustic pressure field in the fluid and the displacement field in the coupled layer. This reveals that the effect of a modest sized gap in the coating does not markedly alter the radiated field except at distinct frequencies, at which values the coating exhibits strong fluid-coupled oscillations. We develop a simple analytical model to explain the resonance phenomenon and show that quasi-resonances arise when the wavelength of the deformation pattern ‘matches’ the azimuthal length of the surface of the coating. This resonant behaviour is conveniently captured by a single parameter Q, which is the ratio of the typical inertial fluid pressure induced by the wall oscillation to the stiffness of the elastic coating. For each choice of material parameters, there is shown to be an infinite set of values of Q corresponding to distinct quasi-resonance mode numbers. The effects on the radiated field due to variations in various physical parameters, such as acoustic wavenumber and elastic layer inertia, are also discussed.     

凹球面涂布光刻胶均匀性研究

通过对离心法在凹球面上涂布光刻胶过程进行分析,阐明了离心状态下光刻胶在凹球面基底上的流动机理,结合试验提出影响凹球面涂布光刻胶膜厚均匀性的主要因素有胶液粘度、旋涂速度、旋涂时间,列举了以上因素引起的各种现象,并进行了理论分析。引用凹球面旋涂光刻胶的膜厚公式,建立了膜厚与速度关系数学模型;利用流体力学原理解释了有限圆形空间中流体速度对膜层均匀性的影响,从而解决了大曲率凹球面上制备微细图形结构的关键工艺问题,对非球面上制备微细图形具有借鉴作用。     

Aspects of the reflexion and free wave properties of a composite panel under fluid loading

The response of a composite panel to external forcing, with inclusion of fluid loading effects, is considered. Of the two strata comprising the composite panel one, backed by a vacuum, is of the conventional thin elastic plate or membrane kind, while the other, in contact with the fluid, is more like an elastic solid and may suffer significant compression. The behaviour of the acoustic field close to grazing incidence is examined, this behaviour being determined by that of the plane wave reflexion coefficient. In the absence of the upper stratum it is well known that the reflexion coefficient has the value ?1 around grazing incidence, so that direct and reflected fields from an external source cancel and preclude the propagation of a genuine acoustic field over the surface (a situation known in optics as the “Lloyd's mirror” effect). It is shown that, at any given frequency, an impedance for the upper stratum can be prescribed which will lead to the value +1 for the grazing incidence reflexion coefficient, and will thus obviate the severe power loss which would otherwise occur in directions close to the surface. Next the free subsonic surface waves which can exist in the coupled three-part (two-layer panel plus fluid) system are examined. It is shown analytically that, if the upper layer has low impedance controlled by stiffness forces, a new surface wave can exist in the system. This wave essentially involves the stiffness of the upper layer and the mass of the fluid, and has a wavenumber much higher than that of the surface wave in a single conventional panel. It is also shown from numerical studies that two subsonic surface waves will exist over quite a wide range of parameters, though not necessarily with the wide wavenumber separation of the low impedance case. A discussion is given of the possible importance of the high wavenumber mode in the case of excitation by high wavenumber boundary layer turbulence, and of the significance of two free subsonic surface wave modes in calculations of energy transmission over composite panels of the kind modelled here.     

高温壁面润湿性对气层稳定性及其壁面滑移性能的分子动力学研究

针对流体在纳米通道的小尺度效应,采用分子动力学方法模拟了传热效应以及流体流动行为,研究在壁面温度影响下,不同润湿性壁面上方气层生成状态以及流体流动时气层的稳定特性和相应的减阻性能.结果表明:当壁面为纯疏水壁面时,不能形成气层;疏水基底+亲水组合壁面形成不规则气层;纯亲水壁面和亲水基底+疏水组合壁面能形成规则气层.当流体流动时,疏水基底+亲水组合壁面气层消失,而纯亲水壁面和亲水基底+疏水组合壁面气层较为稳定.纯疏水壁面主流区域速度较大,而纯亲水壁面主流区域最低.对于壁面滑移速度,存在气层的壁面滑移速度与纯疏水表面相对接近,甚至稍优于纯属疏水表面,而疏水基底+亲水组合壁面滑移速度最小.     

Positioning of small particles by an ultrasound field excited by surface waves

A method for the controlled positioning of small particles in one or two dimensions by an ultrasound field excited by a surface wave is presented. Particles of a diameter between 10 and 100 microm placed on a surface can be concentrated at certain locations and moved over the surface. In other approaches it is possible to let the particle levitate freely in the fluid. However for the use of ultrasonic positioning in for example microassembling it is necessary to move particles over a surface as well as to let them levitate over the surface. Physical principle: A two- or three-dimensional ultrasound field is excited in a fluid filled gap between a rigid surface at the bottom and a vibrating surface of a solid at the top. The height of the gap varies between 0.1 and 2 mm. A one-dimensional sinusoidal vibration of the upper surface excites a two-dimensional ultrasound field in the fluid. Particles that are arbitrarily distributed on the lower surface will be concentrated in lines by the ultrasound field. First the calculation of the field of forces on particles in the fluid layer is presented. Then the dispersion relation of a vibrating plate which is in contact with a fluid on one side is derived. The technical setup will be introduced. Finally the experiments are shown and compared to the theoretical results.     

Radiation from modes of a rectangular panel into a coupled fluid layer

Modal radiation efficiencies are evaluation for a rectangular panel which is simply supported in an infinite baffle and coupled to a fluid layer. The analysis is based on the calculation of the acoustic power radiated into the layer by the panel vibrating in one of its in vacuo natural modes. At low frequencies, the efficiency is inversely proportional to the layer depth; at high frequencies, it exhibits a complex, multiple peak characteristic, associated with the acoustic field of the layer. Comparison with the modal radiation efficiencies of a panel coupled to a fluid half-space shows a similar dependence on mode order and panel dimensions.     

Comparisons of predicted steady-state levels in rooms with extended- and local-reaction bounding surfaces

A combined beam-tracing and transfer-matrix model for predicting steady-state sound-pressure levels in rooms with multilayer bounding surfaces was used to compare the effect of extended- and local-reaction surfaces, and the accuracy of the local-reaction approximation. Three rooms—an office, a corridor and a workshop—with one or more multilayer test surfaces were considered. The test surfaces were a single-glass panel, a double-drywall panel, a carpeted floor, a suspended-acoustical ceiling, a double-steel panel, and glass fibre on a hard backing. Each test surface was modeled as of extended or of local reaction. Sound-pressure levels were predicted and compared to determine the significance of the surface-reaction assumption. The main conclusions were that the difference between modeling a room surface as of extended or of local reaction is not significant when the surface is a single plate or a single layer of material (solid or porous) with a hard backing. The difference is significant when the surface consists of multilayers of solid or porous material and includes a layer of fluid with a large thickness relative to the other layers. The results are partially explained by considering the surface-reflection coefficients at the first-reflection angles.     

Influence of thickness and permeability of endothelial surface layer on transmission of shear stress in capillaries

The molecular coating on the surface of microvascular endothelium has been identified as a barrier to transvascular exchange of solutes. With a thickness of hundreds of nanometers, this endothelial surface layer(ESL) has been treated as a porous domain within which fluid shear stresses are dissipated and transmitted to the solid matrix to initiate mechanotransduction events. The present study aims to examine the effects of the ESL thickness and permeability on the transmission of shear stress throughout the ESL. Our results indicate that fluid shear stresses rapidly decrease to insignificant levels within a thin transition layer near the outer boundary of the ESL with a thickness on the order of ten nanometers. The thickness of the transition zone between free fluid and the porous layer was found to be proportional to the square root of the Darcy permeability. As the permeability is reduced ten-fold, the interfacial fluid and solid matrix shear stress gradients increase exponentially two-fold. While the interfacial fluid shear stress is positively related to the ESL thickness, the transmitted matrix stress is reduced by about 50% as the ESL thickness is decreased from 500 to 100 nm, which may occur under pathological conditions. Thus, thickness and permeability of the ESL are two main factors that determine flow features and the apportionment of shear stresses between the fluid and solid phases of the ESL. These results may shed light on the mechanisms of force transmission through the ESL and the pathological events caused by alterations in thickness and permeability of the ESL.     

Effect of tangential electric field on the evolution of the Rayleigh-Taylor instability of a dielectric liquid film

The effect of surface ponderomotive forces produced by a uniform tangential electric field on the evolution of the Rayleigh-Taylor instability of an isothermal film of a homogeneous incompressible dielectric liquid coating the lower surface of a horizontal insulating plate is studied in the long-wavelength approximation for the hydrodynamic equations and a simplified system of electrostatics equations. The lower boundary of the liquid is the interface with a stationary gas. It is shown in the framework of the linear theory that during the disruption of the continuous film, ponderomotive forces induce the formation of liquid billows extended along the applied field lines.     

Study on vibro-acoustical characteristics of damped composite boxlike shells in water

Based on the structural FEM and the acoustic BEM, a numerical model of coupled elastic layer and viscoelastic layer and outside sound field is established and the vibro-acoustical characteristics of damped composite boxlike shells are studied systematically. It can be concluded that the structural vibration responses and the sound radiation are reduced significantly due to the viscoelastic layer and its effects are dependent on the geometric, physical parameters of the layer and the excitation frequency. It is also shown that compared with the bare elastic shells, the influence of the fluid compressibility on the vibration responses of shells covered with a damping layer is not evident and the effects of the free surface and the rigid plane are weakened.     

低超声速飞行器瞬态热状况解耦算法

针对低超声速飞行器非稳态飞行条件下内外流固耦合一体化计算的复杂性,将飞行器外部流场的实时气动热转化为浮动的第三类边界条件进行解耦.以加速俯冲的超声速三维头锥体为例,分别采用浮动温差法和辐射平衡法提取表面对流换热系数进行解耦计算,并与直接耦合计算结果进行比较,验证两种解耦算法的可靠性.结果表明,将非稳态飞行过程离散为不同飞行状态点,通过提取对流换热系数解耦计算得到的不同状态点的锥体表面温度分布与直接耦合计算得到的结果吻合较好.两种解耦算法在计算效率方面均要优于耦合计算方法;在外界气动环境发生剧烈变化的过程中,最大相对误差均不超过2%.     

Optical fiber humidity sensor based on evanescent-wave scattering

The phenomenon of evanescent-wave scattering (EWS) is used to design an optical-fiber humidity sensor. Porous solgel silica (PSGS) coated on the surface of a silica optical-fiber core scatters evanescent waves that penetrate the coating layer. Water molecules in the gas phase surrounding the optical fiber can be absorbed into the inner surface of the pores of the porous silica. The absorbed water molecules form a thin layer of liquid water on the inner surface of the porous silica and enhance the EWS. The amount of water absorbed into the PSGS coating is in dynamic equilibrium with the water-vapor pressure in the gas phase. Therefore the humidity in the air can be quantitatively determined with fiber-optic EWS caused by the PSGS coating. The humidity sensor reported here is fast in response, reversible, and has a wide dynamic range. The possible interference caused by EWS to an optical-fiber gas sensor with a reagent-doped PSGS coating as a transducer is also discussed.     

负偏压离子鞘及气体压强影响表面波放电过程的粒子模拟

由于表面电磁波沿着介质-等离子体分界面传播,而很难通过对传统的表面波等离子体(SWP)源施加负偏压实现金属材料溅射,因此限制了SWP源的使用范围.近期,一种基于负偏压离子鞘导波的SWP源克服了这个问题,且其加热机理是表面等离激元(SPP)的局域增强电场激励气体放电产生.但是该SWP源放电过程的影响因素并未研究清晰,导致其最佳放电条件没有获得.本文采用粒子(PIC)和蒙特卡罗碰撞(MCC)相结合的模拟方法,探讨了负偏压离子鞘及气体压强影响SWP电离发展过程的放电机理.模拟结果表明,负偏压和气体压强的大小影响了离子鞘的厚度、SPP的激励和波模的时空转化,从而表现出不同的放电形貌.进一步分析确定,在负偏压200 V左右和气体压强40 Pa附近,该SWP源的放电效果最佳.     

Softening of magnetorotons in a semi-infinite Fibonacci superlattice

Calculations of interlayer and intralayer screening of the Coulomb interaction on the softening of bulk and surface magnetoroton modes are presented for density and position modulations of the two-dimensional (2D) electron gas (EG) layers of a semi-infinite quasiperiodic superlattice. It is shown that the softening of these modes is due to an increase in the screening by all other layers of the effective intralayer Coulomb interaction. Numerical results are obtained for variable thickness of a 2DEG layer, the separation between layers and the distance between the surface layer and the top metal gate. The critical values of the structure parameters, determining the interlayer and intralayer screening of the Coulomb interaction, are obtained and used in constructing the phase diagrams showing the separation between the quantum fluid and charge-density wave phases.     

Strain induced novel quantum magnetotransport properties of topological insulators

Recent theoretical and experimental researches have revealed that the strained bulk HgTe can be regarded as a three-dimensional topological insulator (TI). Motivated by this, we explore the strain effects on the transport properties of the HgTe surface states, which are modulated by a weak 1D in-plane electrostatic periodic potential in the presence of a perpendicular magnetic field. We analytically derive the zero frequency (dc) diffusion conductivity for the case of quasielastic scattering in the Kubo formalism, and find that, in strong magnetic field regime, the Shubnikov–de Haas oscillations are superimposed on top of the Weiss oscillations due to the electric modulation for null and finite strain. Furthermore, the strain is shown to remove the degeneracy in inversion symmetric Dirac cones on the top and bottom surfaces. This accordingly gives rise to the splitting and mixture of Landau levels, and the asymmetric spectrum of the dc conductivity. These phenomena, not known in a conventional 2D electron gas and even in a strainless TI and graphene, are a consequence of the anomalous spectrum of surface states in a fully stained TI. These results should be valuable for electronic and spintronic applications of TIs, and thus we fully expect to see them in the further experiment.     

Influence of binder viscosity on the control of infrared emissivity in low emissivity coating

Low emissivity is the complex system and polymer binder is one of the most important factors that affect optical and mechanical properties of the coating. Low infrared emissivity coatings were prepared by using flake aluminum particles and three types of polymer resins as fillers and binders, respectively. The influence of polymer binder viscosity on pigment particles distribution, surface morphology and infrared emissivity of the coating was systematically investigated. The results indicate that infrared emissivity of the coating can be strongly affected by the resin viscosity at the same preparation condition, which induces different aluminum particles distribution and surface morphology of the coating. Low resin viscosity is helpful for aggregating pigments and reducing the top polymer layer thickness near the surface, thus the infrared emissivity is reduced. If the resin viscosity value is decreased by two orders of magnitude, the infrared emissivity values would be reduced as much as 0.2. Additionally, a theoretical model is proposed to account for this mechanism, which indicates that sedimentation, evaporation and diffusion play important roles in forming different aluminum particles distribution during the drying process of the coating.     

Characteristics of Photonic Bandgap Fibres with Hollow Core＇s Inner Surface Coated by a Layer Material

Hollow core＇s inner surface coating in a photonic bandgap fibre （PBCF） is investigated by means of finite element method. The coat material and thickness-dependence dispersion curve and group velocity dispersion are numerically studied. The coating with materials of low index or small thickness will rise up the dispersion curve but will not induce surface modes. However, coating with materials of high index or big coat thickness will induce surface modes and avoided-crossings. By varying coat material＇s refractive index and thickness, the appearances of surface modes and avoided-crossings can be changed. It is found that the avoided-crossing can enormously enlarge the negative dispersion which can find applications in dispersion compensation. We numerically achieve a negative dispersion as large as -21416.15ps/nm/km. The results give a physical insight into the propagation properties of PBGFs with the hollow core coated by a layer of material and are of crucial significance in the applications of PBGF coating.     

Reentrant layering in rare gas adsorption: preroughening or premelting?

The reentrant layering transition found in rare gas adsorption on solid substrates has conflictually been explained in terms either of preroughening (PR) or of top layer melting-solidification phenomena. We obtain adsorption isotherms of Lennard-Jones particles on an attractive substrate by off lattice grand canonical Monte Carlo simulation, and reproduce reentrant layering. Microscopic analysis confirms a transformation of the top surface layer from solid to quasiliquid across the transition. At the same time, however, the surface coverage is found to switch from close to one to close to half, the latter indicating a disordered flat surface and establishing PR as the underlying mechanism. We conclude that top layer melting can trigger PR. In turn, PR appears to act as the threshold transition for surface melting in rare gas solids.