影响从涡激振动中获取能量的参数研究

涡激振动是一种常见的流固耦合现象,当结构物振幅较大时,可利用涡激振动从海洋流中或河流中获取能量。论文通过流固充分耦合数值方法,对二维弹性支撑圆柱体在均匀流中的横向涡激振动进行了模拟。用该数值方法对4种方案进行计算,分析质量比m*、阻尼比ζ、质量阻尼比m*ζ和固有频率fn对圆柱体从涡激振动中获取能量效率η的影响。结果发现m*、ζ、m*ζ和fn都会影响η,且存在一个最优的m*ζ值,使能量获取效率η最大。对基于这一想法的振动系统的设计,论文结论可作为参考。     

双圆柱尾流致涡激振动的质量比效应及其机理

双圆柱尾流激振受多种因素影响,情况复杂,质量比m*(相同体积的圆柱与流体质量的比值)对双圆柱尾流激振的影响规律尚未澄清。采用数值模拟方法,在低雷诺数下(Re=100),研究了三种质量比(m*=2,10,20)对串列双圆柱尾流致涡激振动特性和尾流流场结构的影响规律,分析了下游圆柱的升力与位移的相位差,探讨了涡激升力与能量输入的内在联系。结果表明:质量比对串列圆柱尾流致涡激振动有重要影响。随着质量比的增大,横流向最大振幅减小,并发生在较小折减速度下,振动锁定区域范围变窄;质量比越小,升力与位移之间的相位差对下游圆柱振幅的影响越显著;在较小质量比时尾流出现“2S”、不规则和平行涡街模态,而在较大质量比时只有“2S”和平行涡街模态。     

正三角形排列刚性耦合三圆柱涡激振动特性及尾涡模式

采用基于嵌入式迭代的浸入边界法对等边三角形排列的刚性耦合三圆柱涡激振动进行了数值模拟研究。其中一个圆柱在上游放置,另外两个圆柱并排放置于下游,圆柱间刚性连接,系统仅在横向自由振动。圆柱间距比L~*分别为1.0、1.6、2.5和4.0,雷诺数为Re=100,质量比为m~*=2.0,折合流速为U_r=3.0~30.0。分析了不同间距比下圆柱振幅、流体力、振动频率和脱涡模式等。研究发现,随U_r的增大,各间距比下的振动响应均可划分为初始分支(initial branch,IB)、下端分支(lower branch,LB)和非锁定区域(desynchronized region,DS)。其中,非锁定区域又可进一步分为前非锁定区域(DS1)和后非锁定区域(DS2)。随折合流速的增大,圆柱振幅整体上先增后减,而随间距比的增大,圆柱振幅则先减后增。圆柱的最大振幅(A~*=1.11)出现在L~*=1.0、U_r=8.0处。当L~*=1.0、1.6和2.5时,圆柱振动存在锁定区间,振动频率锁定在固有频率附近,而L~*=4.0时,圆柱的振动频率随折合流速增大线性增大,不存在锁定区间。当L~*=2.5时,在DS2分支上,圆柱振动出现了两个强度相当、频率不同的分量,分别为低频驰振分量与高频涡振分量,而且由于复杂的柱间流体结构使得三圆柱升力频率存在较大差异。当L~*=1.6时,在DS分支上,圆柱下游出现宽-窄尾流,导致了下游圆柱所受升阻力均值和升力均方根不相等。     

水流作用下双圆柱墩混凝土梁桥的动力响应实测与数值模拟

西藏达林大桥为一座7跨桥面连续的混凝土梁桥,下部结构采用双圆柱桥墩。2018年7月,在水流作用下达林大桥桥墩及桥面出现了显著的顺桥向振动。该文报道了水流作用下大桥的动力响应实测与数值模拟研究。实测表明:桥梁顺桥向振动表现为桥梁一阶纵向模态为主的拍振,横桥向为随机微振动;顺桥向最大加速度约为0.08 m/s2,梁端最大位移约为1.56 mm。基于一阶纵向振动模态参数,将双圆柱墩梁桥简化为单自由振动体系,在2 m/s~10 m/s流速范围内(折减流速U_r=1.69~8.45、雷诺数Re=2.6×106~1.3×107)进行了二维流固耦合数值模拟,得到了桥墩双圆柱升阻力系数以及不同结构阻尼比时的涡振响应。并对桥墩振型与水流流速剖面等三维效应进行修正,得到了墩顶位移随流速变化的关系。结果表明:上游柱尾流对下游柱的脉动涡激升力有显著增强作用,在3 m/s~6 m/s流速范围内双圆柱桥墩出现了涡激振动。在考虑三维修正后,ζ=0.01工况下墩顶位移数值模拟结果与实测值较为吻合。随着阻尼比ζ的增加,涡振最大振幅变小,锁定区间基本不变。     

低雷诺数下串联双圆柱涡激振动机理的数值研究

利用有限元数值方法求解不可压缩粘性流体的Navier-Stokes方程,结合任意拉格朗日-欧拉（ALE）动网格方法,对低雷诺数下（Re = 150）等直径串联圆柱的涡激振动问题进行了数值模拟研究。其中上游圆柱固定,下游圆柱在弹簧和阻尼作用下允许同时发生顺流向和横流向的运动。在约化速度Ur = 3.0 ~ 12.0的范围内（阻尼比ξ = 0.007）研究了两圆柱圆心间距比（LX / D = 3.0、5.0、8.0）及圆柱质量比（m* = 5.0、10.0、20.0）对下游圆柱的运动响应及受力的影响。数值结果表明,圆柱间距比的变化会导致锁定区间的变化,进而影响到圆柱涡激振动的位移响应和受力特性。这些方面都与尾流区涡旋脱落模式密切相关,体现了双圆柱间干涉作用对涡激振动的影响。进一步的研究表明,圆柱质量比的变化对以约化速度表征的锁定区间、运动响应和尾流模式等都有一定的影响作用。     

刚性耦合三圆柱流致振动特性和机制EI北大核心CSCD

采用迭代式浸入边界法对刚性耦合三圆柱的流致振动进行了数值模拟研究。三圆柱按照等边三角形排列,上游两个并排圆柱,下游一个圆柱。圆柱间距比为P/D=1.0~4.0,雷诺数为Re=100,质量比为m=2,折合流速为U_(r)=3~30。通过研究圆柱的振幅、频率和流体力随折合流速的变化规律,发现了两种不同的振动模式,即小间距比条件(P/D=1.0)下的驰振模式和中、大间距比条件(P/D=1.6~4.0)下的涡激振动模式。而涡激振动模式在不同的间距比条件下又具有单锁定区间(P/D=1.6)和双锁定区间(P/D=2.5~4.0)两种不同振动特征。进一步分析尾流模式,发现第一锁定区间(含单锁定区间)内的振动响应由剪切层重附着机制激发,而第二锁定区间内的振动响应由交替尾涡泄放机制激发。     

质量比对圆柱涡激特性的影响研究

采用有限体积法对不同质量比圆柱在限制流向及不限制流向下的涡激振动进行了研究。圆柱涡激振动系统简化为质量-弹簧-阻尼模型,引入雷诺平均应力模型求解不可压缩粘性Navier-Stokes方程,结合SST  湍流模型对限制流向和不限制流向下圆柱涡激振动进行了数值模拟。研究发现：限制流向和不限制流向时圆柱涡激振动横向振幅均出现了初始激励分支和下端分支, 不限制流向质量比2.0时还出现了超上端分支,其横向振幅最大值为1.05D,是限制流向工况的1.81倍,质量比越大两者相差越小;限制流向和不限制流向两种工况下圆柱涡激振动均发现频率锁定现象,但锁定区间不同;质量比大小对圆柱涡激振动锁定区间也有影响;最后对不同质量比下圆柱涡激振动轨迹进行了讨论分析。     

带防撞栏杆扁平箱梁高阶模态涡激振动的CFD研究

为研究大跨度桥梁常用扁平箱梁带防撞栏杆时的高阶模态涡激振动(VIV)特性,基于雷诺时均Navier-Stokes(RANS)方程、SST k-ω湍流模型和动网格技术求解扁平箱梁的绕流场并获得气动力,将Newmark-β算法代码嵌入到用户自定义函数(UDF)求解该气动力作用下的桥梁动力响应,开展了大带东桥主桥加劲梁断面高阶模态涡激振动响应的预测,模拟雷诺数为3.18×10~4～6.10×10~4。获得了扁平箱梁断面随折减风速变化的高阶涡振响应振幅根方差(RMS)曲线和加速度时程,预测了与文献较一致的高阶模态涡激振动锁定区间。研究了阻尼比和来流风攻角对扁平箱梁高阶模态涡振响应幅值和加速度的影响,表明随着阻尼比的增大高阶模态涡激振动响应逐渐变小甚至消失,而来流风攻角只有大于2°时才会发生显著的高阶模态涡激振动。     

小直径附加圆柱对圆柱涡激振动抑制的参数优化

采用基于迭代嵌入式浸入边界法对后方对称布置两个小直径圆柱的单圆柱涡激振动进行了数值模拟研究,对涡激振动抑制进行了参数优化。雷诺数和圆柱直径比分别为Re=100和d/D=0.125,其中D和d分别为大、小圆柱直径。通过改变控制角度(θ)、主圆柱与小圆柱的间隙比(G/D)、小圆柱旋转角速度和旋转方向(α)和阻尼比(ζ)确定的最优控制参数组合为θ=25°、G/D=0.125、α=-2.2和ζ=1.02。小圆柱的旋转角速度和旋转方向对圆柱振幅有一定的影响,其中内向反转会进一步抑制圆柱的振动,外向反转则恰好相反。随着阻尼比的增加,圆柱振幅先增后减,但影响程度较小。     

低雷诺数下两类串列圆柱的涡激振动

为了研究上游圆柱的运动状况对下游圆柱涡激振动的影响,针对两类串列双圆柱(上游圆柱固定、下游圆柱可作两自由度振动,上下游圆柱均可作两自由度振动),在低雷诺数下(Re=100),采用数值模拟方法,研究了下游圆柱在不同尾流干扰下的振幅、振动频率、相位差等振动特性随折减速度的变化规律,从能量输入和尾流模态角度探讨了上游圆柱的振动状况对下游圆柱涡激振动的影响规律及其流场机理。研究表明:上游圆柱的运动状况对下游圆柱涡激振动有显著影响;与上游圆柱固定情况相比,上游圆柱振动时下游圆柱存在明显的振动锁定区间;下游圆柱横流向振幅更大且最大振幅发生在更高的折减风速下。振动过程中两类串列圆柱的顺流向间距比明显不同。上游圆柱固定时,顺流向圆柱间距大于上游圆柱振动情况。两类双圆柱的尾流模态也有明显不同,上游圆柱固定、下游圆柱振动时的串列双圆柱为平行涡街模态,而上下游圆柱均能振动时的尾流为"2S"模态。     

一种改进的大涡模拟入口湍流生成方法研究

大涡模拟中的入口湍流的生成方法研究,是当前计算风工程领域国内外研究的热点问题。该文在NSRFG(narrowband synthesis random flow generation)方法的基础上,对其中重要参数无量纲长度尺度\begin{document}$\beta$\end{document}、空间相关性\begin{document}$R$\end{document}和调谐因子\begin{document}${\gamma _j}$\end{document}进行深入理论分析,推导了调谐因子\begin{document}${\gamma _j}$\end{document}与无量纲长度尺度\begin{document}$\beta $\end{document}的函数关系,建议了一种改进的入口湍流合成技术——INSRFG(improved NSRFG)方法。利用该方法进行了与规范相对应的4类标准地貌湍流风场的大涡模拟数值仿真;通过对比分析,表明INSRFG方法模拟的大气边界层湍流风场,能较好满足脉动风速功率谱、空间相关性等湍流风场基本特性,并较好实现大气边界层风场模拟中的平衡态基本要求。研究表明,这种新的INSRFG湍流合成方法具有参数取值明确、数学模型简洁、计算效率相对较高的优点,是一种进行建筑结构大涡模拟研究的具有较好前景的通用入口湍流生成方法。     

运动方程自适应步长求解的高性能Galerkin时程单元初探

该文以一阶运动方程为例,利用其非自伴随性质,构建了新型的凝聚检验函数,进而提出了一套高性能Galerkin有限单元——凝聚单元.该单元为无条件稳定的单步法单元,对于(m)次多项式单元,其端结点位移和速度均可达到O(h2(m)+2)阶的超高收敛性,比常规Galerkin单元的结点精度高2阶.采用此单元,该文进而实现了无需...     

Numerical integration of structural dynamics equations including natural damping and periodic forcing terms

This paper shows that it is comparatively simple to analyse algorithms for the numerical integration of the Space discretized equations from structural dynamics when applied to \documentclass{article}\pagestyle{empty}\begin{document}$\ddot x + \mu \dot x\omega ^2 x = p{\rm e}^{ist} $\end{document}, instead of the usual \documentclass{article}\pagestyle{empty}\begin{document}$ \ddot x + \omega ^2 x = 0 $\end{document}, and suggests that this should be done in order to gain some insight into the effect with natural damping and/or a periodic forcing term. The method is illustrated on some three- and four-time-level schemes.     

Exponential fitting using integral equations

The fitting of a function y =\documentclass{article}\pagestyle{empty}\begin{document}$ \sum\nolimits_{i = 1}^n {A_i {\rm{e}}^{\lambda ix}} $\end{document} to experimental data is considered. Integral equations are developed which have the functions \documentclass{article}\pagestyle{empty}\begin{document}$ \sum\nolimits_{i = 1}^n {A_i {\rm{e}}^{\lambda ix}} $\end{document} as their solutions for n = 1, 2, 3, These integral equations are used to find the frequencies λ_i. Examples are worked out to illustrate the method. The method is shown to be capable of extension to other functions.     

An iterative technique for fitting exponentials

An interative method to fit the function \documentclass{article}\pagestyle{empty}\begin{document}$\mathop \[y = \sum\nolimits_i^n { = 1} a_i e^{lix}\] $\end{document} to data is considered. The technique used is that of inversion of a linear differential operator with constant coefficients. This method reproduces the parameters for mathematically precise data and gives satisfactory results when the data are affected by random errors.     

A Subspace-Projected Approximate Matrix Method for Systems of Linear Equations

Given two $n×n$ matrices $A$ and $A_0$ and a sequence of subspaces$\{0\}=\mathscr{V}_0⊂···⊂\mathscr{V}_n=\mathbb{R}^n$with dim$(\mathscr{V}_k)=\mathscr{k}$, the $k$-th subspace-projected approximated matrix $A_k$ is defined as $A_k=A+Π_k(A_0−A)Π_k$, where $Π_k$ is the orthogonal projection on $\mathscr{V}_{k}^⊥$. Consequently, $A_{k}v=Av$ and $v^{∗}A_{k}=v^{∗}A$ for all $v∈\mathscr{V}_{k}$. Thus $(A_{k})^{n}_{k≥0}$ is a sequence of matrices that gradually changes from $A_0$ into $A_n=A$. In principle, the definition of $\mathscr{V}_{k+1}$may depend on properties of $A_k$,which can be exploited to try to force $A_{k+1}$ to be closer to $A$ in some specific sense. By choosing $A_0$ as a simple approximation of $A$, this turns the subspace-approximated matrices into interesting preconditioners for linear algebra problems involving $A$. In the context of eigenvalue problems, they appeared in this role in Shepard et al. (2001), resulting in their Subspace Projected Approximate Matrix method. In this article, we investigate their use in solving linear systems of equations $Ax=b$. In particular, we seek conditions under which the solutions $x_k$ of the approximate systems $A_kx_k=b$ are computable at low computational cost, so the efficiency of the corresponding method is competitive with existing methods such as the Conjugate Gradient and the Minimal Residual methods. We also consider how well the sequence $(x_k)_{k≥0}$ approximates $x$, by performing some illustrative numerical tests.     

Preconditioning of the second‐kind boundary integral equations for 3D eddy current problems

Starting from the time‐harmonic Maxwell equations at low‐frequency eddy current approximation the H–? formulation is presented. An equivalent system of boundary integral equations of the second kind on the conductor surface (resp. the conductor/dielectric) is derived. Discretizing these equations with a boundary element method (BEM) yields a block linear equation system \[ \left[\begin{array}{cc} A_{1} & B_{1} \\ \tfrac{\mu}{\mu_{0}}\tilde{B}_{2} & A_{2}\end{array} \right] \left( \begin{array}{c} j \\ \sigma \end{array} \right)=\left(\begin{array}{c} b_{1} \\ b_{2}\end{array} \right) \] with a fully populated stiffness matrix. This system is non‐symmetric and, for large μ/μ₀ (of interest in practice) ill‐conditioned. Iterative solvers like GMRES converge very slowly, in general. We propose a new preconditioner which depends on μ. We present numerical results with a serial and parallel version of this preconditioner also on large industrial eddy current problems with compex geometry. The performance of preconditioned GMRES is found to be practically independent of μ/μ₀. Copyright © 2001 John Wiley & Sons, Ltd.     

Probleme der zerstörenden Prüfung von Proben mit sehr geringer Verformung

Problems of Destructive Testing of Samples with Very Low Deformation In the case of materials with very low deformation at break, for example ceramics and graphite, the stiffness of the testing machine has to fulfill requirements which must be clearly defined and capable of being checked, in order to avoid errors in the test result. Besides sensitive test factors there are also some which remain constant regardless of the spring stiffness of the testing apparatus. The effect on the commonly used loading methods (v_o = constant; \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm F}\limits^. = {\rm constant} $\end{document}) is investigated. Possibilities for corrections are discussed an appropriate algorithms are given.     

Mass transport of cobalt and nickel in Incoloy-800

Lattice diffusion of cobalt and nickel in Incoloy-800 has been studied in the temperature range 1070 to 1500K by serial sectioning and residual activity techniques using radioactive tracers⁶⁰Co and⁶³Ni. The lattice diffusion coefficient can be expressed by the relation: