复杂约束三维功能梯度输流管道稳定性和临界流速分析

针对输流管道在恶劣的动力学环境所受到多方向载荷所引起的管道结构失效或破坏等问题,提出一种新型三维功能梯度材料构造输流管道来提升管道的载荷忍耐力。基于欧拉伯努利梁理论,考虑流体和管道的耦合关系,利用哈密顿变分原理建立复杂约束下三维功能梯度输流管道的运动微分方程。利用微分求积法求解,分析流体流速提升所引起的三维功能梯度输流管道振动的固有频率变化,当第一阶固有频率首次降低为0系统失稳,所对应的流体流速被确定为系统的临界流速。研究复杂约束线性和扭转弹簧刚度、轴向、径向和环向功能梯度指数等物理参数对输流管道振动频率和临界流速的影响。研究结果表明：当流速较小时,增加轴向功能梯度指数和降低径向和环向功能梯度指数会降低系统的基频和提高系统的临界流速;而当流速较大时,系统的基频随着三维功能梯度指数的变化会展现相反的趋势。增大三维功能梯度指数能降低系统的第二阶固有频率。这说明通过调节复杂约束和三维功能梯度参数能够实现对输流管道稳定性的调控。     

硅薄膜导热系数微尺度效应的临界尺寸

基于灰介质假设及拟合的色散关系,采用蒙特卡洛方法对微纳米硅薄膜内的声子热输运特性进行了模拟.将3μm厚的硅薄膜导热系数的计算结果与文献结果进行了对比,二者吻合较好.分析了厚度对薄膜内温度场及导热系数的影响.结果表明,当厚度小于某一尺度时,薄膜导热系数会随着厚度的减小而降低,呈现尺度效应;薄膜内温度场也不再是线性分布,而是在边界处呈现阶跃特性,且随着厚度减小,温度阶跃增大.基于该方法求得了100~400 K温度区间内,硅薄膜法向导热系数出现明显尺度效应的临界尺寸.计算发现,随着温度的升高,临界尺寸变化范围很大,平均温度为100 K时,临界尺寸约为50μm,平均温度为400 K时,临界尺寸约为2.5μm,前后相差了一个数量级.     

管材参数对输液管流固耦合振动的影响

采用特征线法对一蓄水池-管道-阀门(RPV)系统的流固耦合振动响应进行了数值计算,研究了管道结构阻尼、管材泊松比以及管道壁厚管道材料对系统振动响应的影响。结果表明：随着管道结构阻尼的增大,管壁本身的振动受到抑制;当管道结构阻尼大于某一临界值时,系统的振动能量主要集中在液体里,造成液体压力能的升高;随着泊松比的增大或管道壁厚的增大,液体的压能增大,管壁的轴向振动强度降低。     

TBM液压管道等效固有频率及稳定性研究

针对TBM掘进过程中产生的基础振动对液压长管道稳定性的影响,选取长管道相邻两固定支承及其之间的管道作为一个单元,建立基础振动下管道单元的非线性微分方程,并通过实验验证振动微分方程的正确性,运用等价线性化推导出管道的等效固有频率的数学模型。研究输流管道系统固有频率与稳定性的相关性,分别讨论流体参数和结构参数对等效固有频率的影响规律。结果表明:利用等效固有频率能很好地反映系统的稳定性,两者相关系数大于0.85。随着流速、压力及单元管长、内径的增大或刚度、壁厚的减小,管道系统的等效固有频率降低,系统稳定性降低;随着基础振动的增大,系统等效固有频率越低,越容易失稳。     

分析弹性地基一般支承输流管道的动力学特性

研究Pasternak双参数地基一般支承输流管道的线性固有频率及非线性动力学特性。综合考虑管道黏弹性系数、地基的剪切效应、线性刚度的影响,建立了系统运动微分方程。根据两端一般支承的边界条件推导出线性系统固有频率方程,分析了基础激励与脉动流作用下,流速对系统非线性动力学特性的影响。数值结果表明,管道一阶临界流速随弹性系数的增大呈现先增大后减小的趋势,当弹性系数足够大时,管道随流速的增加发生一阶、二阶模态耦合现象;系统响应随流速变化呈现由倍周期分岔过渡到混沌运动的特性;当管内流体流速足够大时,系统响应保持混沌运动状态。     

滚振试验台液压管道系统的振动特性分析

液压管道系统是跨座式单轨车辆滚动振动试验台的重要组成部分,在设计过程中需要对管道的振动特性进行重点分析。为此,基于单向和双向流固耦合的模态分析方法,采用k-ε 的湍流模型,对比分析管道在不同分析方法下的固有频率和振型;在双向流固耦合基础上分析流体速度和压强对管道的影响。研究表明：单向流固耦合下管道的固有频率均大于双向流固耦合作用下管道的固有频率,相差率达2 %～10 %,相同阶次下的振型形态基本一致;流体压强对管道的固有频率的影响比流体速度大,在一定的压强范围内,管道的固有频率随着流体的压强增加而增加,流体的管道总变形量和等效应力随流体压强和支管流速增加而增加。     

滚振试验台液压管道系统的振动特性分析

液压管道系统是跨座式单轨车辆滚动振动试验台的重要组成部分,在设计过程中需要对管道的振动特性进行重点分析。为此,基于单向和双向流固耦合的模态分析方法,采用k-ε 的湍流模型,对比分析管道在不同分析方法下的固有频率和振型;在双向流固耦合基础上分析流体速度和压强对管道的影响。研究表明：单向流固耦合下管道的固有频率均大于双向流固耦合作用下管道的固有频率,相差率达2 %～10 %,相同阶次下的振型形态基本一致;流体压强对管道的固有频率的影响比流体速度大,在一定的压强范围内,管道的固有频率随着流体的压强增加而增加,流体的管道总变形量和等效应力随流体压强和支管流速增加而增加。     

轴向运动压电纳米板的非局部热-力-电耦合振动

基于非局部理论结合基尔霍夫压电薄板模型,研究了轴向运动压电纳米板的热-力-电耦合振动响应。考虑压电纳米板受到双向轴力、外部电压和温度变化等作用并做轴向运动,应用哈密顿原理推导了系统的控制方程组,利用伽辽金法数值求解轴向运动压电纳米板横向振动固有频率,进一步讨论小尺度参数、轴力、外部电压以及温度变化等因素对固有频率及亚临界区域的影响。结果表明:双轴压力、外部正电压、温度升高以及小尺度参数的存在使得压电纳米板的等效刚度降低,从而导致固有频率和亚临界区域的减小,而双轴拉力、外部负电压、温度降低则引起纳米结构等效刚度提高。     

基于分层模型的功能梯度输流管道耦合振动

基于Timoshenko梁模型,研究了材料属性沿管道壁厚方向呈梯度变化的输流管道的耦合振动。沿厚度方向将功能梯度材料离散为若干层均质材料;考虑流体在轴向的可压缩性及管壁与流体间的摩擦,通过Hamilton原理及流体动量方程、连续方程建立了功能梯度输流管道的耦合振动模型;采用动刚度法对管道单元进行求解,并通过算例分析了功能梯度材料的梯度指数变化对管道动态特性的影响。     

微尺度悬臂管的空间弯曲振动-非线性运动方程及尺度效应EI北大核心CSCD

具有圆环形横截面的微尺度悬臂输液管可以同等地向空间各方向产生弯曲振动。按照欧拉-伯努利梁理论,在分析管道上点的位移及相关几何关系的基础上,考虑Lagrange应变张量所给出的几何非线性,基于修正的偶应力理论计算了管的应变能,运用Hamilton原理建立了微尺度悬臂输液管的空间弯曲振动的非线性动力学方程。研究了无量纲材料长度尺寸参数对系统动力学性质的影响,结果表明,尺度效应增大管道的临界流速,并使得稳定的平面周期运动(空间周期运动)在整个质量比区间上占的比例越大(小)。     

流体诱导弹性管束振动响应数值分析

基于流固耦合问题的弱耦合法,研究弹性管束不同流速的壳程或/和管程流体诱导下的振动响应。研究表明,流体诱导振动幅值随壳程或/和管程流速的增加而增加。与相同管程流速条件相比,壳程流体引起的振幅较大。随壳程流速增加监测点振动频率增加;随管程流速增加监测点振动频率基本不变。壳、管程流体耦合诱导的振动位移曲线与仅壳程流体诱导的振动位移曲线类似,说明弹性管束工作过程中的振动主要由壳程流体诱导。流体诱导的振动频率接近管束第一阶固有频率时,监测点在y、z方向振幅逐渐趋于峰值。流体诱导弹性管束的振动主要表现为面内振动。     

Flow induced vibrations of a triangular tube array in various arrangements of orientation and natural frequency

Abstract

Crossflow induced vibrations of a triangular tube array with a pitch ratio 1.33 were investigated experimentally. The streamwise and cross‐stream displacements of a monitored tube in the array were simultaneously measured by two accelerometers to examine the tube response to the cross flow in a water tunnel. The experiment was aimed to study the effects of the array orientation, and the tube's natural frequency on the flow induced vibration of the tube array. It is shown by amplitude diagrams that fluid elastic vibrations exist when the reduced velocity is above a critical value. The critical reduced velocity is found to be sensitive to the orientation of the test array. Based on the measured data of critical reduced velocity, the tube array in a triangular pattern (at a 30‐deg orientation with respect to the flow direction in the experiment) is found to be more stable than when in a rotated triangular pattern (0‐deg orientation). Furthermore, it is illustrated that the discrepancy in natural frequency of the tubes delays the occurrence of the fluid elastic vibrations of the tube array. With all the tubes in the test array having the same natural frequency, the orbits of the tube that exhibits fluid elastic vibrations are an organized, elliptic shape. The corresponding spectra are line‐dominated with peaks at the natural frequency and its harmonics, suggesting that the tube vibration is an organized oscillator. Without the same natural frequency as the surrounding tubes, the monitored tube exhibits fluid elastic vibration at larger reduced velocity, vibrating in a relatively random orbit.     

The effect of frequency of fluctuating driving force on basic gaseous micro-flows

Summary The effect of frequency of fluctuation of the driving force on velocity slip and temperature jump at the wall(s) is theoretically investigated in four cases of basic gas micro-flow problems. The cases considered are the transient Couette flow, the pulsating Poiseuille flow, the Stoke's second problem flow and the transient natural convection flow. The formulation of the problem revealed that the controlling parameter of the problem is a combination of Knudsen number (Kn) and frequency of fluctuation (ω) in the form of an effective Knudsen number, that replaced the usual Knudsen number (Kn), and consequently the slip flow regime is found to be valid when 10^-3≤ Kn_eff≤ 10^-1. It is found that when the frequency is small the velocity and temperature profiles are similar to the corresponding classical macro-flow profiles at zero frequency. Also, the slip in velocity and the jump in temperature increase as the Knudsen number and/or the frequency of the driving force increases. In addition, the slip in velocity and the jump in temperature are found to be negligible when the frequency and/or Kn are sufficiently small.     

Surface Effect on Phase Transformation of Single Crystal NiTi Shape Memory Alloys Studied by Molecular Dynamics Simulation

Surface effect on phase transformation of single crystal (SC) NiTi shape memory alloys (SMAs) with various thicknesses and various Ni contents is studied by molecular dynamics simulation. For the SMAs with various thicknesses considering surface effect, the residual strain, the average atomic potential energy, and the four characteristic phase transformation temperatures all increase, and fewer twinned martensite boundaries are left after cooling compared to the sample without considering surface effect. Moreover, with increasing thickness of the sample, the surface effect is gradually weakened, and there is no obvious regularity in phase transformation characteristics. For the SMAs with various Ni contents considering surface effect, segmental martensite phase transformation, phase transformation fluctuation, and wider phase transformation temperature ranges are observed, whereas these phenomena do not occur in the sample without considering surface effect. With the increasing of Ni contents, for both cases of considering surface effect or not, the critical stress of phase transformation, average atomic potential energy, the modulus of detwinned martensite increase, but the phase transformation temperature and modulus of twinned martensite decrease.     

变厚度环板在面内变温下的固有频率

本文采用薄板线性振动理论分析了厚度沿径向变化和受面内约束的各向同性环形薄板在均匀变温下的自由振动和热弹性稳定性问题。分别针对厚度按指数函数和线性函数变化的两种情况，用有限差分法计算了变厚度环板在内、外周边横向夹紧和简支条件下的线性固有频率和临界温度。由数值结果表明，固有频率的平方与面内变温成线性关系。厚度变化参数、内外半径的比值均对环板的固有频率和临界温度有显著影响。文中给出了丰富的数值结果，可供工程设计参考。     

Thermoelectric power of tellurium thin films and its thickness and temperature dependence

Tellurium thin films of thicknesses between 25 and 200 nm have been vacuum-deposited on glass substrates at room temperature in a vacuum of 5×10^–5torr. The thermoelectric power measurements on these films have been carried out, after annealing, in the temperature range from 300 to about 500 K. It is found from the study that thermoelectric power is independent of temperature and is also, apparently, independent of thickness, over the range of temperatures and thicknesses investigated. The results are discussed on the basis of size effect and thermoelectric effect theories.     

Agglomeration process of dry ice particles produced by expanding liquid carbon dioxide

Formation of dry ice particles and their agglomeration process have been studied experimentally. The dry ice particles were produced by expanding liquid carbon dioxide at room temperature and pressure, and then introduced into an additional tube acting as an agglomeration chamber. In the experiments, the temperatures of the jet flow and the tube wall were measured by thermocouples, and dry ice particles in the jet flow were observed by a high speed camera with a zoom lens. It was found that two stages of temperature reduction occurred in the jet flow, corresponding to the agglomeration process. It was also found that the particle size of the agglomerates increased and the particle velocity decreased with increasing tube diameter. The agglomeration process of dry ice particles can be explained by the particle deposition and reentrainment, i.e. dry ice particles of several micrometers are deposited on the tube wall and form a deposition layer; then, agglomerates are reentrained from the layer into the jet flow.     

Intermittent Switching Between Potential Flow and Turbulence in Superfluid Helium at mK Temperatures

Superfluid flow around an oscillating microsphere is investigated at temperatures down to 25 mK. Stable laminar flow below a critical velocity and turbulence at large drives are found to be separated below 0.5 K by an intermediate range of driving forces where the flow is unstable, intermittently switching between laminar and turbulent phases. We have recorded time series of this switching phenomenon and have made a statistical analysis of the switching probability. The mean lifetime of the turbulent phases grows with increasing drive and becomes infinite at a critical value. Stability of the laminar phases above the critical velocity is limited by natural background radioactivity or cosmic rays.     

Experimental study of the gravity effect on the distribution of refrigerant flow in a multi-pass condenser

Gravity in multi-pass condensers affects the refrigerant flow rate distribution, owing to the gravitational pressure drop that occurs mainly in the U-bend tubes in fin and tube condensers with horizontal tubes. This effect was studied using an experimental approach. A condenser with two ‘nU’ circuits was selected, and the temperature variation of the refrigerant side was measured and compared along each circuit. The critical air velocity, which indicated the initiation of the gravity effect, was found for a given refrigerant flow rate. As the air velocity increased beyond the critical air velocity, the gravity effect (or mal-distribution of the refrigerant flow) developed further. Similarly, the critical refrigerant flow rate was also determined for a given air velocity. As the refrigerant flow rate decreased below the critical refrigerant flow rate, the gravity effect also developed further. The gravity-affected region was shown in the table with rows of air velocities and columns of refrigerant flow rates, and expressed using a single parameter for a given refrigerant flow circuit.     

The Frequency Dependence of Critical-Velocity Behavior in the Oscillatory Flow of Superfluid 4He Through a 2 × 2-μm Aperture in a Thin Foil

The critical-velocity behavior of oscillatory superfluid ⁴He flow through a 2 × 2-μm aperture in a 0.1-μm-thick metal foil has been studied from 0.36 to 2.10 K at frequencies from less than 50 Hz up to above 1880 Hz. The pressure remained less than 0.5 bar. In early runs during which the frequency remained below 400 Hz, the critical velocity was a nearly-linearly decreasing function of increasing temperature throughout the region of temperature studied. In runs at the lowest frequencies, isolated 2π phase slips could be observed at the onset of dissipation. In runs with frequencies higher than 400 Hz, downward curvature was observed in the decrease of critical velocity with increasing temperature. In addition, above 500 Hz an alteration in supercritical behavior was seen at the lower temperatures, involving the appearance of large-energy-loss events. These irregular events typically lasted a few tens of half-cycles of oscillation and could involve hundreds of times more energy loss than would have occurred in a single complete 2π phase slip at maximum flow. The temperatures at which this altered behavior was observed rose with frequency, from ~ 0.6 K and below at 500 Hz, to ~ 1.0 K and below at 1880 Hz.