简支Mexwell模型粘弹性输流管道的稳定性分析

在弹性输流管道研究的基础上,采用递推格式的有限差分法,对简支Maxwell模型粘弹性输流管道（回转守恒系统）,探讨了其动力特性和稳定性问题,具体分析了材料的松弛时间对无量纲流速与前三阶模态的无量纲频率的实部及虚部之间的变化曲线的影响。发现发散临界流速随松弛时间的减小而降低,随后发生的耦合模态颤振临界流速随松弛时间的减小而增大;甚至在质量比较大时,随着松弛时间的减小,可推迟乃至不发生耦合模态颤振。当无量纲松弛时间达到10^3量级以上时,即可将其按弹性管道处理。甚至在H为10^2量级时,按弹性管道处理也不会带来太大的误差。     

轴向运动粘弹性板的横向振动特性

研究了轴向运动粘弹性矩形薄板的动力特性和稳定性问题．从二维粘弹性微分型本构关系出发,建立了轴向运动粘弹性板的运动微分方程．采用微分求积法,对四边简支、一对边简支一对边固支两种边界条件下粘弹性板的无量纲复频率进行了数值计算．分析了薄板的长宽比、无量纲运动速度及材料的无量纲延滞时间对其横向振动及稳定性的影响．     

输液管道流固耦合非线性动力稳定分析

将弱约束输流管道非定常流液固耦合运动按波-流-振动系统建模成由4个非线性微分方程组成的分析模型,按模态进行分解研究系统在多种耦合状态下具有的运动稳定特性.以悬臂梁管道为例分析了耦合系统奇点的属性,得到了前四阶模态运动的相图.结果说明,多种耦合条件下输流管道的稳定性变得更为复杂,各阶模态运动具有不同的稳定特性.     

关于分数阶Maxwell淤泥模型及其在水波衰减问题中的应用

分数阶Maxwell模型可用来模拟粘弹性的海床淤泥．与传统的有理分式模型相比,分数阶Maxwell模型可以用更少的自由参数,较好地描述某些真实淤泥的流变特性．将该分数阶Maxwell模型用于研究淤泥与自由表面水波的相互作用,并得到了线形单色波的衰减率．从水波衰减率曲线中可观测到淤泥层的共振现象,共振时衰减率将达到峰值．对于线形单色波,其衰减率还可表示为各模态衰减率之和,从而可研究某一模态的运动对水波衰减的影响．模态分析表明,当某一模态运动引发共振时,总衰减率由该模态的模态衰减率决定．     

流体诱发水平悬臂输液管的内共振和模态转换(Ⅰ)

运用牛顿法导出水平悬臂刚性输液管的非线性动力学数学模型．为了对该模型进行理论分析,通过对各个相关实际物理量的量级定性分析,给出了模型中各个物理参数的量级．在此基础上,应用多尺度法首先得到输液管自由振动模态的特征函数,利用悬臂管的边界条件给出了特征值满足的特征方程, 发现管内流体速度可以诱发第一阶模态和第二阶模态3种形式的内共振分别是3∶1、2∶1和1∶1内共振, 从理论上解释了流速诱发水平悬臂输液管系统内共振的机理．由于3∶1内共振所对应的流速最小,因此这种形式的内共振是最先出现的,也是最重要的．     

空间分数阶非Newton流体本构及圆管流动规律研究

对非Newton流体的本构及流动规律进行研究是分析、预测和控制非Newton流体在管道中流动的关键.实验表明非Newton流体在流动过程中具有历史记忆性,基于空间分数阶微积分方法,建立了分数阶非Newton流体本构模型;并推导了该模型在圆管中的流速分布、流量、平均流速、压降、平均Reynolds数等管道流动参数;提出了分数阶非Newton流体圆管流态判别准则.研究表明非Newton流体的圆管流层间的切应力可以通过流速的轴向分布大小来描述.对于不含屈服切应力的分数阶非Newton流体,分数阶的阶数越大,断面流速分布越均匀,记忆能力越强.分数阶的阶数大小反映了流体对全域空间的记忆性强弱;而对于含有屈服切应力的分数阶非Newton流体,分数阶的阶数越大,速梯区流速分布越均匀,流核区速度越小.分数阶的阶数大小反映了局部空间记忆性强弱.该研究为非Newton流体的记忆特征提供了一种新的建模方法.     

在有一级化学反应时粘弹性流体流经无限竖直多孔平板时的边界层流动

在有一级化学反应时,研究不可压缩的粘弹性流体,在竖直多孔连续运动平板上的不稳定自然对流.控制方程用隐式有限差分法进行数值求解.与解析解的结果比较,证明所选用的数值方法有效.详细图示了速度分布的数值结果.研究了粘弹性参数、无量纲化学反应参数和平板运动速度,对稳定的速度分布、与时间相关的摩擦因数、Nusselt数和Sherwood数的影响.     

方柱/板结合部马蹄涡流动结构的动力学模态分解

方柱/板结合部区域的马蹄涡系统存在多频流动现象.为了研究各频率所对应的振荡规律及其潜在的动力学信息,对方柱/板结合部处于周期振荡流动状态的马蹄涡系流动结构进行数值模拟,发现处于周期振荡流动状态的马蹄涡系为倍频流动现象.运用动力学模态分解(DMD)技术对方柱体上游对称面上的速度场进行模态分解,将所得到的第1、2、3阶模态分别叠加到平均流模态进行模态重构并在时域上进行推进演化分析.结果表明:周期振荡马蹄涡系以不同尺度马蹄涡间的相互卷并为主,发现了马蹄涡间不同的卷并方式.     

轴向变速运动粘弹性弦线横向振动的复模态Galerkin方法

在考虑初始张力和轴向速度简谐涨落的情况下,利用含预应力三维变形体的运动方程,建立了轴向变速运动弦线横向振动的非线性控制方程,材料的粘弹性行为由Kelvin模型描述．利用匀速运动线性弦线的模态函数构造了变速运动非线性弦线复模态Galerkin方法的基底函数,并借助构造出来的基底函数研究了复模态Galerkin方法在轴向变速运动粘弹性弦线非线性振动分析中的应用．数值结果表明,复模态Galerkin方法相比实模态Galerkin方法对变系数陀螺系统有较高的收敛速度．     

粘弹性板动力稳定性分析中的两模态Galerkin逼近

利用最大Liapunov指数分析法以及其它数值和解析的动力学方法,研究了大挠度粘弹性薄板的动力稳定性。材料的行为由Boltzmann叠加原理描述。采用Galerkin方法将原积分-偏微分模型简化为两模态的近似积分模型,而通过引进新变量,该近似积分模型可进一步化为一个常微分模型。数值比较了1-模态和2-模态截断系统的动力学性质,讨论了面内周期激励下材料的粘弹性性质、加载的幅度和初值对板动力学行为的影响。     

Shifted Legendre Polynomials algorithm used for the dynamic analysis of viscoelastic pipes conveying fluid with variable fractional order model

The dynamic analysis of viscoelastic pipes conveying fluid is investigated by the variable fractional order model in this article. The nonlinear variable fractional order integral-differential equation is established by introducing the model into the governing equation. Then the Shifted Legendre Polynomials algorithm is first presented for dealing with this kind of equations. The convergence analysis and numerical example verify that the algorithm is an effective and accurate technique for addressing this type complicated equation. Numerical results for dynamic analysis of viscoelastic pipes conveying fluid show the effect of parameters on displacement, acceleration, strain and stress. It also indicates that how dynamic properties are affected by the variable fractional order and fluid velocity varying. Most of all, the proposed algorithm has enormous potentials for the problem of high precision dynamics under the variable fractional order model.     

旋转输液管动力稳定性理论分析北大核心CSCD

基于Lagrange原理和假设模态法建立了旋转输液管的动力学模型.通过降阶升维的方法求解系统的特征值问题,并分析了旋转输液管自由振动特性.得到了不同端部集中质量和转速下,系统特征值随流速升高的演变轨迹.揭示了临界流速随系统参数的变化规律.研究发现,内部流体的流动对旋转输液管动力学特性存在显著影响.在某些参数组合下,系统低阶模态能够形成不同形式的内共振关系.预示了旋转输液管模型蕴含丰富的动力学现象.     

广义二阶流体管内轴向流动

在流体的本构关系中引入分数阶导数运算，对于介于粘性与弹性之间的流体的描述更具有合理性。本文将这种关系引入二阶流体，研究其管内轴向流动。我们先求出了１／２阶导数的解析解，用以验证Ｌａｐｌａｃｅ数值反演的ＣＲＵＭＰ方法的有效性。然后用ＣＲＵＭＰ法分析二阶流体管内轴向流动的特征。分析表明粘弹性特征越明显的流体，其速度与应力对分数导数的阶数越具有敏感性。     

弹性地基输流管道的耦合模态颤振分析

推导出了弹性地基输流管道的固-液耦合振动微分方程,用幂级数法计算了Winkler模型地基和双参数模型地基输流管道的临界流速和复频率,分析了弹性地基对输流管道静力稳定性与动力稳定性的影响.结果表明,与不考虑弹性地基的情况相比较,弹性地基的作用可使管道发生静力失稳和动力失稳的临界流速增大,并且增大弹性地基参数可提高静力失稳和动力失稳的临界流速,从而推迟发散与颤振的发生.当质量比β较大时,管道会在某个地基参数组合下,在发生静力失稳后,会在较高流速下出现再稳定和再发散现象,然后发生耦合模态颤振.     

Vibration analysis of conveying fluid pipe via He’s variational iteration method

In this paper, a recently new semi-analytical method, i.e., He’s variational iteration method is developed to apply to free vibration analysis of conveying fluid pipe. The critical flow velocity and frequency of pipe conveying fluid are obtained with considering the various boundary conditions. The results are compared with the ones of different transform method, and prove VIM that has the same precision and efficient with DTM. The mode shapes of cantilevered pipe and both ends with elastic support pipe are shown under different flow velocity.     

Modeling water hammer in viscoelastic pipes using the wave characteristic method

For the first time, the wave characteristic method (WCM) is used to simulate transient flow in viscoelastic pipes. The WCM is based on Newton's second law equating the change in momentum and net pressure forces applied on the liquid, which leads to the Joukowsky equation. The friction head loss within a pipe segment is replaced by an imaginary friction orifice with the same head loss. Here the Joukowsky equation is rederived for elastic pipes on the basis of the conservation of energy principle. Then the same method is used to derive a quadratic equation for the conservation of energy in viscoelastic pipes. Under a relaxation assumption, the square root of the energy equation yields a linear equation that is applicable with the superposition principle. The method developed is studied numerically and verified with experimental data from the literature. The water hammer in a simple pipe system consisting of a reservoir, a pipe, and a valve is demonstrated. Parameters calibrated with the method of characteristics are taken from the literature and used as essential input data for the proposed WCM. A constant friction coefficient of the pipe is considered. Even if a small number of friction orifices are selected, good agreement is found between the experimental data and the simulation results especially for the first pressure head cycles. Finally, the numerical results obtained with both the WCM and the method of characteristics are compared to investigate the effectiveness of the WCM. The WCM shows superior computational efficiency in determining the maximum pressure.     

Nonlinear Frequency Analysis of FGM Pipes Based on the Homotopy Method北大核心CSCD

Based on the homotopy analysis method, the nonlinear vibration of porous functionally graded material (FGM) conveying pipes under generalized boundary conditions was studied. Based on the power-law distribution of the FGM and the Voigt model, the physical properties of the porous pipe material were described. Under the Euler-Bernoulli beam theory and the von Kármán nonlinear theory, and by means of Hamilton’s variational principle, the dynamic control equations and generalized boundary conditions for porous FGM conveying pipes were established. The homotopy analysis method was used to solve the nonlinear vibration characteristics of the porous FGM conveying pipe under generalized boundary conditions. The numerical results show that, the translation spring has little effect on the critical velocity of instability, while the rotation spring increases the critical velocity of instability, making the system more stable; in the nonlinear system, the viscoelastic coefficient does not change the critical velocity; the pipe length, the power-law exponent and the porosity all influence the nonlinear free vibration of the porous FGM conveying pipe. © 2023 Editorial Office of Applied Mathematics and Mechanics. All rights reserved.