基于理想弹塑性模型的复合材料结构耐撞性分析

以含外倒角薄弱环节复合材料圆管的耐撞性分析为例,提出了基于理想弹塑性模型进行耐撞性设计分析的方法,并给出了等效参数的确定策略。研究表明:利用复合材料宏观力学理论计算得到复合材料圆柱壳的等效弹性常数,并结合有限元软件计算得到的临界屈曲应力,可以将复合材料圆管等效为各向同性材料圆管进行耐撞性设计分析;采用该数值模拟策略,能够以较少的计算量来预报复合材料圆管的耐撞性能,并使得在普通的计算机上进行全尺度复合材料飞行器结构的耐撞性设计分析成为可能。     

高频宽带圆管换能器研究

压电圆管换能器是水声领域中广泛应用的换能器之一,它一般采用压电圆管的径向振动模态。利用压电圆管的径向振动模态和高阶振动模态来实现圆管的宽带发射性能。采用有限元方法对圆管换能器进行了分析,利用ANSYS软件建立圆管换能器的有限元模型,并对其进行结构优化。最终所制作换能器的径向谐振频率为47.5kHz,其工作带宽为40~80kHz,发送电压响应起伏不超过±4dB,最大发送电压响应为150dB。研究结果表明：所采用的有限元法计算结果与测试结果吻合较好,换能器实现了高频、宽带、水平无指向性的发射性能。     

金属圆管增强夹芯柱屈曲分析

金属圆管增强复合材料夹芯结构是一种新型的轻质多功能结构。考虑了横向剪切变形对临界载荷的影响,以实验方法测量出夹芯柱的横向剪切刚度,计算出临界屈曲载荷的理论值,并从实验上加以验证。利用有限元分析软件ABAQUS,模拟了夹芯柱宏观屈曲的形式,并与理论值和实验值进行了对比,结果吻合较好。采用数值模拟方法,研究了临界屈曲载荷与金属圆管排布方式之间关系。结果表明:金属圆管增强夹芯柱宏观屈曲主要以横向剪切屈曲为主;临界载荷值随着圆管在长度方向上的间距变小而增大;在圆管总重量和高度不变的情况下,临界载荷随着圆管半径和间距的变小而增大。研究结果对于在轴向压缩载荷下的金属圆管增强夹芯柱设计具有重要的指导意义。     

基于γ-Re_(θt)转捩模型的二维翼型跨音速颤振边界预测EI北大核心CSCD

转捩现象对跨音速流场和气动力有一定影响,但是目前大多数颤振问题研究主要采用全湍假设,并未对转捩现象加以考虑。基于非定常雷诺平均N-S方程(Reynold Averaged Navier-Stockes,RANS)和γ-Re_(θt)转捩模型,耦合结构运动方程,建立时域气动弹性分析方法,其中结构运动方程采用基于预估-校正技术的四阶隐式Adams线性多步法进行时域推进求解;分别对采用全湍假设和考虑转捩影响的Isogai案例A模型的跨音速颤振边界进行研究,从气动力做功的角度分析转捩对跨音速颤振特性的影响机制。结果表明转捩现象使得跨音速凹坑程度较全湍流动有所加深,凹坑范围扩大,跨音速凹坑最低点的颤振速度减小了41.6%;因此,在对表面存在转捩现象的翼型开展颤振分析时,必须在流场控制方程中添加转捩模型,从而准确分析颤振边界。     

复合材料夹芯管胶接连接结构力学特性分析

针对在航空结构中广泛应用的复合材料蜂窝夹芯圆管中的接头这一最脆弱的部分,发展了一种分析复合材料蜂窝夹芯圆管胶粘接头力学特性的解析模型.该模型根据Gibson修正公式得到了蜂窝芯子的等效弹性参数,再运用经典的复合材料壳理论和线弹性理论得到管接头的控制方程,并通过状态空间法进行求解.运用本文模型,计算了管接头在扭矩和弯矩作用下胶层内的剪应力和剥离应力;同时采用有限元法对模型进行了数值模拟,并将模拟结果与模型计算结果进行了对比,最后分析了搭接长度对胶层内应力的影响.     

TA2圆管辊弯成型边波缺陷及机理研究

目的 为消除TA2薄壁圆管在辊弯成型过程中产生的边波缺陷,研究分析不同成型方法中TA2薄壁圆管焊缝处纵向应力及应变大小的变化规律。方法 运用专业型钢软件COPRA RF和有限元MSC.MARC商业软件建立符合生产标准的三维薄壁圆管模型,展开多机架连续辊弯成型过程的有限元仿真,并进行辊弯成型试验验证。结果 试验结果表明,成型方式不合理是引起TA2薄壁圆管辊弯成型边波的重要原因。随着道次间成型结束后塑性应力与纵向应力的累积,TA2薄壁圆管焊缝处出现褶皱现象,即薄壁圆管产生边波缺陷。通过有限元分析及TA2纯钛薄壁圆管生产试验验证发现,基于普通成型法加工的薄壁圆管焊缝处失稳区辊弯纵向应变波动峰值由0.6%降低到基于上山法成型的0.3%。结论 使用上山法可以消除TA2纯钛薄壁圆管在辊弯加工工艺中产生的边波缺陷,提高了产品精度并为后续装配提供了保障,研究结果为消除工业纯钛圆管边波缺陷奠定了良好的理论基础。     

飞轮扰动作用下卫星结构响应能量预示方法

为计算卫星框式结构在飞轮扰动下中高频域的响应,用能量有限元法对其进行研究。通过分析产生扰动的主因及飞轮扰动模型特性,采用正弦扫频激励,计算结构振动的能量密度响应,得到卫星框式结构能量密度的空间域及频率域分布,并与波动方法的结果进行对比。结果表明,飞轮位置处扰动响应的能量密度最高,能量有限元法是对波动方法结果的平均,对激励与边界模型的要求较低,更适用于高频振动领域及工程实际,为航天器结构的扰动响应计算提供一种新思路。     

非圆管管外降膜流动和换热特性分析

利用VOF方法,对圆管、椭圆管和扁管进行水平管外降膜流动二维数值模拟分析,研究Re数在1 278~3 406范围内3种管形对管外降膜传热和流动的影响。计算结果表明:在顶部冲击区和尾流区,管的曲率及其变化对流动和换热的影响很复杂,曲率较小会导致顶部冲击区的扰动面积增加和液膜厚度增加;相比圆管,椭圆管和扁管平均液膜厚度更小,传热性能更优;从平均液膜厚度上看,椭圆管最小,扁管次之;从平均传热系数上看,扁管最大,椭圆管次之。之所以椭圆管和扁管的平均液膜厚度和平均传热系数存在不一致,本文认为是由顶部冲击区对扁管的扰动影响要远大于椭圆管,且Re数取值较大,椭圆管的平均液膜厚度较小的优势无法凸显导致的。     

基于γ-Re_(θt)转捩模型的二维翼型跨音速颤振边界预测

转捩现象对跨音速流场和气动力有一定影响,但是目前大多数颤振问题研究主要采用全湍假设,并未对转捩现象加以考虑。基于非定常雷诺平均N-S方程(Reynold Averaged Navier-Stockes,RANS)和γ-Re_(θt)转捩模型,耦合结构运动方程,建立时域气动弹性分析方法,其中结构运动方程采用基于预估-校正技术的四阶隐式Adams线性多步法进行时域推进求解;分别对采用全湍假设和考虑转捩影响的Isogai案例A模型的跨音速颤振边界进行研究,从气动力做功的角度分析转捩对跨音速颤振特性的影响机制。结果表明转捩现象使得跨音速凹坑程度较全湍流动有所加深,凹坑范围扩大,跨音速凹坑最低点的颤振速度减小了41.6%;因此,在对表面存在转捩现象的翼型开展颤振分析时,必须在流场控制方程中添加转捩模型,从而准确分析颤振边界。     

超声圆管换能器中过盈配合的应力分析

介绍一种利用过盈配合的连接形式装配而成的大功率圆管换能器,分别通过理论计算和有限元热应力分析对这种圆管换能器所受的预应力进行计算分析,分析数据表明两种计算结果吻合一致。同时,根据多次实践经验提出了此换能器在工程实现中需要完善的问题。     

Two-phase bubbly flows in microgravity: Some open questions

Several studies on gas-liquid pipe flows in micro gravity have been performed. They were motivated by the technical problems arising in the design of the thermohydraulic loops for the space applications. Most of the studies were focused on the determination of the flow pattern, wall shear stress, heat transfer and phase fraction and provided many empirical correlations. Unfortunately some basic mechanism are not yet well understood in micro gravity. For example the transition from bubbly to slug flow is well predicted by a critical value of the void fraction depending on an Ohnesorge number, but the criteria of transition cannot take into account the pipe length and the bubble size at the pipe inlet. To improve this criteria, a physical model of bubble coalescence in turbulent flow is used to predict the bubble size evolution along the pipe in micro gravity, but it is still limited to bubble smaller than the pipe diameter and should be extended to larger bubbles to predict the transition to slug flow.     

Two-phase bubbly flows in microgravity: Some open questions

Several studies on gas-liquid pipe flows in micro gravity have been performed. They were motivated by the technical problems arising in the design of the thermohydraulic loops for the space applications. Most of the studies were focused on the determination of the flow pattern, wall shear stress, heat transfer and phase fraction and provided many empirical correlations. Unfortunately some basic mechanism are not yet well understood in micro gravity. For example the transition from bubbly to slug flow is well predicted by a critical value of the void fraction depending on an Ohnesorge number, but the criteria of transition cannot take into account the pipe length and the bubble size at the pipe inlet. To improve this criteria, a physical model of bubble coalescence in turbulent flow is used to predict the bubble size evolution along the pipe in micro gravity, but it is still limited to bubble smaller than the pipe diameter and should be extended to larger bubbles to predict the transition to slug flow. Another example concerns the radial distribution of the bubbles in pipe flow, which control the wall heat and momentum transfers. This distribution is very sensitive to gravity. On earth it is mainly controlled by the action of the lift force due to the bubble drift velocity. In micro gravity in absence of bubble drift, the bubbles are dispersed by the turbulence of the liquid and the classical model fails in the prediction of the bubble distribution. The first results of experiments and numerical simulations on isolated bubbles in normal and micro gravity conditions are presented. They should allow in the future improving the modelling of the turbulent bubbly flow in micro gravity but also on earth.     

Gas–Liquid Interfacial Friction Factor for the Transition from Stratified to Slug Flow

Calculations based on the slug stability model and simplified stratified flow model provide predictions of the critical liquid height and the critical superficial velocities of a stratified flow for the transition to a slug flow in a horizontal pipe. Since slug flow derives from different interfacial waves patterns, previous interfacial waves model in stratified gas–liquid flows brings about the discrepancy between theoretical prediction and experimental data. A partial analysis for this behavior is given, which recognizes that the values of gas–liquid interfacial friction factor at the onset of slug flow have been underestimated, especially at high gas flows, and they should be obtained indirectly from other measured variables. Modified correlations for the interfacial friction factor are presented and better agreement between predicted and measured critical superficial velocities has been obtained.     

The flow structure of a puff

From time-resolved stereoscopic particle image velocimetry measurements over the entire circular cross section of a pipe, a first-of-its-kind quasi-instantaneous three-dimensional velocity field of a turbulent puff at a low Reynolds number is reconstructed. At the trailing edge of the puff, where the laminar flow undergoes transition to turbulence, pairs of counterrotating streamwise vortices are observed that form the legs of large hairpin vortices. At the upstream end of the puff, a quasi-periodic regeneration of streamwise vortices takes place. Initially, the vortex structure resembles a travelling wave solution, but as the vortices propagate into the turbulent region of the puff, they continue to develop into strong hairpin vortices. These hairpin vortices extract so much energy from the mean flow that they cannot be sustained. This structure provides a possible explanation for the intermittent character of the puffs in pipe flow at low Reynolds numbers.     

Frequency characteristics of the noise of R600a refrigerant flowing in a pipe with intermittent flow pattern

The acoustic characteristics of a long-shaped cylindrical bubble for slug or churn flow in a pipe are different from those of a freely rising spherical bubble in infinite liquid. In this research, the theoretical estimation of the natural frequency of the long-shaped cylindrical bubble was derived using the energy conservation law for a single bubble in a pipe. The acoustic characteristics of bubbles in a pipe were also investigated with the R600a refrigerant, which is widely used in refrigerators when the flow pattern in a pipe is slug or churn flow. In order to make slug and churn flow artificially, refrigerant-supplying equipment was designed and developed. Using this test equipment, the frequency characteristics of the long-shaped cylindrical bubble in 2-phase flow were investigated experimentally.     

Aspects of linear and nonlinear instabilities leading to transition in pipe and channel flows

The failure of normal-mode linear stability analysis to predict a transition Reynolds number (Retr) in pipe flow and subcritical transition in plane Poiseuille flow (PPF) has led to the search of other scenarios to explain transition to turbulence in both flows. In this work, various results associated with linear and nonlinear mechanisms of both flows are presented. The results that combine analytical and experimental approaches indicate the strong link between the mechanisms governing the transition of both flows. It is demonstrated that the linear transient growth mechanism is based on the existence of a pair of least stable nearly parallel modes (having opposite phases and almost identical amplitude distributions). The analysis that has been applied previously to pipe flow is extended here to a fully developed channel flow predicting the shape of the optimized initial disturbance (a pair of counter-rotating vortices, CVP), time for maximum energy amplification and the dependence of the latter on Re. The results agree with previous predictions based on many modes. Furthermore, the shape of the optimized initial disturbance is similar in both flows and has been visualized experimentally. The analysis reveals that in pipe flow, the transient growth is a consequence of two opposite running modes decaying with an equal decay rate whereas in PPF it is due to two stationary modes decaying with different decay rates. In the first nonlinear scenario, the breakdown of the CVPs (produced by the linear transient growth mechanism) into hairpin vortices is followed experimentally. The associated scaling laws, relating the minimal disturbance amplitude required for the initiation of hairpins and the Re, are found experimentally for both PPF and pipe flow. The scaling law associated with PPF agrees well with the previous predictions of Chapman, whereas the scaling of the pipe flow is the same as the one previously obtained by Hof et al. indicating transition to a turbulent state. In the second nonlinear scenario, the base flow of pipe when it is mildly deviated from the Poiseuille profile by an axisymmetric distortion is examined. The nonlinear features reveal a Retr of approximately 2000 associated with the bifurcation between two deviation solutions.     

Turbulent flow in smooth and rough pipes

Recent experiments at Princeton University have revealed aspects of smooth pipe flow behaviour that suggest a more complex scaling than previously noted. In particular, the pressure gradient results yield a new friction factor relationship for smooth pipes, and the velocity profiles indicate the presence of a power-law region near the wall and, for Reynolds numbers greater than about 400x103 (R+>9x103), a logarithmic region further out. New experiments on a rough pipe with a honed surface finish with krms/D=19.4x10-6, over a Reynolds number range of 57x103-21x106, show that in the transitionally rough regime this surface follows an inflectional friction factor relationship rather than the monotonic relationship given in the Moody diagram. Outer-layer scaling of the mean velocity data and streamwise turbulence intensities for the rough pipe show excellent collapse and provide strong support for Townsend's outer-layer similarity hypothesis for rough-walled flows. The streamwise rough-wall spectra also agree well with the corresponding smooth-wall data. The pipe exhibited smooth behaviour for ks+ < or =3.5, which supports the suggestion that the original smooth pipe was indeed hydraulically smooth for ReD< or =24x106. The relationship between the velocity shift, DeltaU/utau, and the roughness Reynolds number, ks+, has been used to generalize the form of the transition from smooth to fully rough flow for an arbitrary relative roughness krms/D. These predictions apply for honed pipes when the separation of pipe diameter to roughness height is large, and they differ significantly from the traditional Moody curves.     

Structure and dynamics of low Reynolds number turbulent pipe flow

Using large-scale numerical calculations, we explore the proper orthogonal decomposition of low Reynolds number turbulent pipe flow, using both the translational invariant (Fourier) method and the method of snapshots. Each method has benefits and drawbacks, making the 'best' choice dependent on the purpose of the analysis. Owing to its construction, the Fourier method includes all the flow fields that are translational invariants of the simulated flow fields. Thus, the Fourier method converges to an estimate of the dimension of the chaotic attractor in less total simulation time than the method of snapshots. The converse is that for a given simulation, the method of snapshots yields a basis set that is more optimal because it does not include all of the translational invariants that were not a part of the simulation. Using the Fourier method yields smooth structures with definable subclasses based upon Fourier wavenumber pairs, and results in a new dynamical systems insight into turbulent pipe flow. These subclasses include a set of modes that propagate with a nearly constant phase speed, act together as a wave packet and transfer energy from streamwise rolls. It is these interactions that are responsible for bursting events and Reynolds stress generation. These structures and dynamics are similar to those found in turbulent channel flow. A comparison of structures and dynamics in turbulent pipe and channel flows is reported to emphasize the similarities and differences.     

Dynamical systems and the transition to turbulence in linearly stable shear flows

Plane Couette flow and pressure-driven pipe flow are two examples of flows where turbulence sets in while the laminar profile is still linearly stable. Experiments and numerical studies have shown that the transition has features compatible with the formation of a strange saddle rather than an attractor. In particular, the transition depends sensitively on initial conditions and the turbulent state is not persistent but has an exponential distribution of lifetimes. Embedded within the turbulent dynamics are coherent structures, which transiently show up in the temporal evolution of the turbulent flow. Here we summarize the evidence for this transition scenario in these two flows, with an emphasis on lifetime studies in the case of plane Couette flow and on the coherent structures in pipe flow.     

Experimental and theoretical progress in pipe flow transition

There have been many investigations of the stability of Hagen-Poiseuille flow in the 125 years since Osborne Reynolds' famous experiments on the transition to turbulence in a pipe, and yet the pipe problem remains the focus of attention of much research. Here, we discuss recent results from experimental and numerical investigations obtained in this new century. Progress has been made on three fundamental issues: the threshold amplitude of disturbances required to trigger a transition to turbulence from the laminar state; the threshold Reynolds number flow below which a disturbance decays from turbulence to the laminar state, with quantitative agreement between experimental and numerical results; and understanding the relevance of recently discovered families of unstable travelling wave solutions to transitional and turbulent pipe flow.