基于共同作用模型的ETFE气枕力学性能研究

将ETFE气枕的内充气体假定为势流,联立内充气体波动方程和外部膜材动力方程并引入界面协调条件和边界条件得到气枕系统的共同作用有限元方程;基于该方程建立了ETFE气枕有限元模型并对其进行了形态分析和荷载分析,通过与已有测试结果对比验证了该文数值模型的有效性。此外,通过承载力分析确定了ETFE气枕的破坏模式及失效准则;分别研究了尺寸、矢跨比及膜厚等参数对气枕承载力与内压关系的影响,为气枕结构设计提供量化依据。结果表明:ETFE气枕在风荷载作用下可能出现强度破坏和失稳破坏两种破坏模式;在常见内压范围内,存在一个最优气压使得气枕具有最大承载力;尺寸、矢跨比及膜面厚度等参数变化会显著影响气枕的承载力。     

基于VOF法的平底结构自由落体入水砰击载荷模拟

应用动网格技术和VOF方法模拟了平底结构的自由落体入水过程和气-液两相流,得到了平底物体自由落体入水过程的运动特性、砰击载荷特性以及入水空泡演化过程。结果表明:整个入水过程中,平底物体在入水瞬间受到来自液面的砰击载荷最大,砰击面的载荷压力分布中间高两端低;砰击载荷随入水初速的增加而增大;平底物体入水空泡破裂后对上表面造成冲击压力。数值计算方法和结果对入水结构安全性和适应性评估具有实际意义。     

Features of the Time Shape of the Pressure Pulses of Optical Air Breakdown

The time shape and amplitude of pressure pulses initiated by surface laser air breakdown for different energies of laser pulses (1–180 mJ) has been compared to the results of numerical gasdynamic calculations of unsteady explosive motions with allowance for counterpressure at distances of 0.2 to 30 cm from the breakdown region. It has been established that the experimental pressure pulse has the character of slowly damped quasiperiodic vibrations, whereas the calculated pulse is a bipolar single pulse of a much shorter duration. Good agreement between the experimental and calculated amplitudes of a positive pressure phase has been found throughout the investigated range, whereas the agreement between the corresponding amplitudes and durations of a negative pressure phase is limited in character. The differences observed in the experimental and calculated data have been attributed to the transformation of the shockwave motion to acoustic radiation.     

The effect of compression of enclosed air on the cushioning properties of corrugated fibreboard

This work showed that the shock‐absorbing properties of corrugated fibreboard are dependent on the size of the cushion even when the static loading of the impacting product on the cushion is kept a constant. At least part of this phenomenon was found to be due to the effect of the air enclosed within the flutes of fibreboard cushions. During the dynamic compression of fibreboard pads there is a build‐up of air pressure within the cushion, which improves its cushioning performance. The greater ease with which air can escape smaller fibreboard cushions, leads to differences in their shock‐absorbing behaviour compared with larger cushions. Suggestions are made on how the design of fibreboard pads can be modified to increase the build‐up of air pressure within the flutes and improve the efficiency of the cushion. For comparison, expanded polyethylene foam was also examined and its shock absorbing properties were found to be largely independent of cushion size for constant static‐loading impacts. Copyright © 1999 John Wiley & Sons, Ltd.     

Research on Performance of a Rectangular Floatation Cushion with High Stiffness for a Pneumato-static Slideway

In this paper,a high stiffness rectangular floatation cushion with a variable-section pressure equalizing groove of an elastic membrane for a pneumato-static slideway is presented and designed in order to increase stiffness of the floatation cushion.Gas film pressure distribution and carrying capacity of the floatation cushion are studied through theoretical analysis and by using the overlapping stitching technique.The carrying capacity and stiffness were obtained by using the coupling procedure of the gas lubrication governing equation and elastic deformation of the membrane governing equation.The results of theoretical calculations can obtain a conclusion that the effect of the gas pressure makes the annular elastic membrane elastically deform that brings about changes of throttling area and pressure groove depth of the floatation cushion.It makes carrying capacity and stiffness of the new style floatation cushion markedly improve over the conventional flotation cushion.The new floatation cushion has a wider application and popularization of space.     

Pressure and flow in fluid films constrained between translating and vibrating flexible surfaces

In this paper, the dynamic pressure and flow developed in a two-dimensional, viscous fluid film constrained between flexible surfaces are analyzed. The problem formulation assumes that the response of the flexible surface is governed by linear equations of motion, and the fluid motion is governed by linearized momentum equations including the unsteady inertia. Three states of the model are developed to describe the coupled fluid-structural response problem. The fluid dynamic pressure is derived in the frequency domain as a function of the fluid impedances and the surface transverse vibrations. The perturbed, coupled problem is described by an integral equation (in state vector form) that governs the coupled responses of the flexible surfaces. The integral equation is solved by a discretization method. The analysis is applied to a rigid slider bearing with a flexible, translating plate surface under the excitation of a harmonic point load. The accuracy of the discretization method is evaluated, and numerical results for the dynamic pressure and the plate response are presented.     

Effect of pulsations on two-layer channel flow

The effect of vertical wall vibrations on two-phase channel flow is examined. The basic flow consists of two superposed fluid layers in a channel whose walls oscillate perpendicular to themselves in a prescribed, time-periodic manner. The solution for the basic flow is presented in closed form for Stokes flow, and its stability to small periodic perturbations is assessed by means of a Floquet analysis. It is found that the pulsations have a generally destabilizing influence on the flow. They tend to worsen the Rayleigh–Taylor instability present for unstably stratified fluids; the larger the amplitude of the pulsations, the greater the range of unstable wave numbers. For stably stratified fluids, the pulsations raise the growth rate of small perturbations, but are not sufficient to destabilize the flow. In the latter part of the paper, the basic flow for arbitrary Reynolds number is computed numerically assuming a flat interface, and the motion of the interface in time is predicted. The existence of a time-periodic flow is demonstrated in which the ratio of the layer thicknesses remains constant throughout the motion.     

The flow regime during the crystallization state and convection state on a vibrating granular bed

It is possible to utilize the large particle distribution on the free surface and inside a vibrating granular bed to understand the segregation phenomenon and the granular motion states in the vibrating bed. In this study we strive to analyze different flow regimes in a binary mixture in a granular vibrating bed. The granular temperature of large particles on the surface of the vibrating bed can be used to define the motion states, the crystallization state or the convection state. When the dimensionless vibration amplitude increases from 0.25 to 1.0, the granular motion transforms from the slowly stabilizing crystallization state to the strong convection state. When the amplitude of the dimensionless vibration increases from 1.25 to 2.5, the granular motion transforms from the fast stabilizing crystallization state to the unstable crystallization state. The percentage of large particles occupying the free surface and the concentration of these large particles are analyzed to understand the motion states.     

基于谱元法的车辆-轨道结构频域振动特性研究

基于谱元法建立车辆-轨道结构频域振动模型,其中轨道结构模拟为三层铁木辛柯梁,车辆部分考虑为整车模型,运用Lagrange方程实现车辆与轨道结构的耦合,并采用虚拟激励法将轨道不平顺模拟为虚拟荷载,通过求解车辆-轨道整体结构的谱元法方程,得到车辆-轨道结构在频域内的振动响应。结果表明:钢轨、轨道板和底座板的第一、二、四阶振动峰值分别由车体、转向架、车轮自振引起,其他振动峰值由轨道结构系统自振引起;钢轨、轨道板和底座板的振动能量分布在较宽的频率范围;在离开车辆一侧且距离端轮对2.5 m处,1~800 Hz内钢轨振动迅速衰减,当大于800 Hz时,钢轨振动衰减缓慢;在距离端轮对18 m处,25~1171 Hz内钢轨振动衰减基本稳定;在距离端轮对20.5 m处,小于25 Hz时,钢轨振动随着离开端轮对距离的增加迅速衰减,当大于1171 Hz时,钢轨振动则衰减较小。     

不同空气侧风速下微通道蒸发器换热特性实验研究

搭建微通道蒸发器性能实验台,采用控制变量法研究不同空气侧风速下微通道蒸发器表面温度分布、制冷剂进出口压力的变化规律,计算换热量和换热系数,从而分析空气侧风速对微通道蒸发器的流量分配特性和换热效果的影响。结果表明,随着风速增大,微通道蒸发器制冷剂流量分配不均匀性增大,进出口压力波动振幅和周期增加,压降增大,风速2 m/s时微通道蒸发器换热效果最佳。     

Large-amplitude vibrations of nanowires with dissipative surface stress effects

In this study, nonlinear free vibrations of doubly clamped Euler–Bernoulli nanowires (NWs) have been considered. The von Kármán strain–displacement relationships along with the classic Zener model are implemented to derive the nonlinear differential equation of the flexural motion of NW. Nonlinear natural frequencies are calculated using the computer package Mathematica. The effects of size-dependent surface dissipation, mode numbers, and amplitude of vibrations on the nonlinear natural frequencies are investigated. It is shown that the surface dissipation effect on the normalized nonlinear natural frequencies depends on the amplitudes of vibrations. Also, comparisons are made with the results published in previous studies.     

浸没式参数激励摆波能转换装置能量俘获特性研究

引入参数激励摆的非线性机制,应用于波浪能发电装置,并根据实际波浪低频、低幅值激励特性,提出浸没式单摆波能转换装置,研究其在规则波中的能量俘获情况。基于势流理论,针对垂荡运动,建立非线性运动方程,进行数值求解,分析摆球质量比、激励频率、幅值和系统阻尼对能量俘获效率的影响,并与空气中单摆的能量俘获效率进行对比。结果表明:合理选取质量比,可降低浸没式参数激励摆固有频率;与空气中单摆相比,浸没式参数激励摆在低频、小振幅的波浪激励环境下,俘获能量频谱带宽更宽,发电功率更高。     

空气垫静态缓冲性能的数值模拟

目的采用有限元数值模拟方法研究空气垫在静态压缩时的力学性能,以便更直观、系统地了解空气垫的静态压缩力学行为。方法以柱状空气垫为对象,建立空气垫的静态压缩物理模型和力学模型,并基于该有限元模型模拟该结构的静态压缩过程,分析结构尺寸和充气压力对其静态缓冲性能的影响,得到空气垫在不同压缩量下的应力分布规律和变形特点。同时,将试验结果与模拟结果进行对比。结果空气垫的变形是从接触钢板的地方开始的,上下位置同时被压缩,有限元分析结果与试验结果吻合良好。结论所建立的有限元模型和数值模拟方法结果准确、合理。     

On the solution of nonlinear vibration of truncated conical shells covered by functionally graded coatings

This paper deals with the linear and nonlinear vibrations of a truncated conical shell; both internal and external surfaces are covered by functionally graded coatings (FGCs). The theoretical formulation is based on the von Karman–Donnell-type nonlinear kinematics. The material properties of FGCs are assumed to be graded in the thickness direction according to a simple power-law distribution in terms of the volume fractions of the constituents. The fundamental relations, the modified Donnell-type nonlinear motion, and compatibility equations of the truncated conical shell with FGCs are derived. The basic equations are reduced to the ordinary differential equation depending on time with geometric nonlinearity using the Superposition and Galerkin methods. By applying the homotopy perturbation method to the foregoing equation, the relation between nonlinear frequency parameters with the dimensionless amplitude of a truncated conical shell with FGCs is obtained. Parametric studies are performed to illustrate the effect of different values of thickness and material composition of the FGCs on the frequency-amplitude relationships. The validity of the present solution is demonstrated by comparison with solutions available in the literature.     

Diffraction of femtosecond pulses; nonparaxial regime

We present a systematic study of linear propagation of ultrashort laser pulses in media with dispersion, dispersionless media, and vacuum. The applied method of amplitude envelopes makes it possible to estimate the limits of the slowly varying amplitude approximation and to describe an amplitude integrodifferential equation governing propagation of optical pulses in the single-cycle regime in solids. The well-known slowly varying amplitude equation and the amplitude equation for the vacuum case are written in dimensionless form. Three parameters are obtained defining different linear regimes of optical pulse evolution. In contrast to previous studies we demonstrate that in the femtosecond region the nonparaxial terms are not small and can dominate over the transverse Laplacian. The normalized amplitude nonparaxial equations are solved using the method of Fourier transforms. Fundamental solutions with spectral kernels different from those according to Fresnel are found. Exact unidirectional analytical solution of the nonparaxial amplitude equations and the 3D wave equations with initial conditions compatible with Gaussian light bullets are obtained also. One unexpected new result is the relative stability of light bullets (pulses with spherical and spheroidal spatial form) when we compare their transverse enlargement with paraxial diffraction of light beams in air. It is important to emphasize here the case of light disks, i.e., pulses whose longitudinal size is small with respect to the transverse one, which in some partial cases are practically diffractionless over distances of a thousand kilometers. A new formula that calculates the diffraction length of optical pulses is suggested. Finally, propagation of single-cycle pulses in air and vacuum was investigated, and a coronal (semispherical) form of diffraction at short distances was observed.     

柱形空气衬垫振动传递特性的试验研究

目的 研究柱形空气衬垫的振动传递特性,以获取振动传递率曲线。方法 在频率为3~100 Hz区间对柱形空气衬垫和确定载荷质量形成的振动系统进行正弦扫频振动试验,分析初始内压和厚度对其振动传递特性的影响规律。结果 初始内压对空气衬垫的隔振能力没有显著影响,3种初始内压下振动系统的共振频率均在35 Hz左右,传递率峰值集中在11左右。当厚度增大时,共振频率和传递率峰值同时减小,空气衬垫隔振能力增强。结论 在满足缓冲性能的前提下,选择合适的初始内压,以及采用多层空气衬垫可显著提高其隔振能力。研究结果可为柱形空气衬垫的缓冲隔振包装设计提供基本依据。     

基于Fluent的气液两相流喷嘴内部流动特性仿真

目的探究气液两相流喷嘴内部流动特性及工作参数对流动特性的影响。方法测量得到气液两相流喷嘴的结构图,利用Fluent软件建立喷嘴模型,并选择流体体积(VOF)两相流模型和RNG(重整化群)k-?湍流模型,以常温状态下液态水和空气为研究介质,并以气压和液压为变量,进行多参数的流动特性分析,并引入气液比的概念。结果得到了不同时刻喷嘴内部的压力、速度及液相分布云图。其中最大压力为827 kPa,出现在出口段和进气段交叉的壁面上,由于喷嘴内部出现缩口,故出口段存在负压(?1.53 MPa);喷嘴内部最高速度出现在气液两相交汇处,为134 m/s;液相在最初迅速充满喷嘴后,逐渐与气相混合,最终出口段中心液相体积占比为0.543,混合情况良好。还得到了多参数对喷嘴内部压力、速度及液相分布的影响。结论使用软件仿真的方法得到了喷嘴内部的流动特性和多参数对流动特性的影响规律,并为进一步研究优化喷嘴结构及喷雾提供了建议和参考。     

粗糙度模式对硬盘气膜承载特性的影响

当前硬盘中磁头磁盘之间气膜的厚度已经接近或低于分子平均自由程,在这种情况下表面粗糙度及气体稀薄效应的影响不容忽视.应用最小二乘有限差分法,求解考虑气体稀薄效应并引入压力流因子和剪切流因子的量纲一雷诺方程,研究了不同粗糙度模式下硬盘超低飞高(1nm~2nm)气膜的承载特性.数值结果表明:粗糙度模式对超低飞高气膜压力分布及承载力的影响较大,而对压力中心的影响较小;在各种粗糙度模式情形下,磁头取横向粗糙度模式时,可使气膜压力分布升高,并提高气膜的承载力.     

啤酒灌装过程数值模拟

针对啤酒灌装过程,将酒液与空气的运动归结为具有自由表面的VOF两相流模型问题.以640mL啤酒灌装为例,应用Fluent软件计算了不同灌装速度下瓶内流场的速度、压力、紊流损失等,分析了瓶子形状和灌装速度对液体运动状态的影响,宏观显示了啤酒灌装过程中瓶内气液两相流的运动状态,计算结果与前人计算结果相符.模拟计算结果将对调整啤酒灌装工艺、设计和改进啤酒瓶形状提供依据,从而对提高瓶装啤酒质量等具有重要意义.