张量封闭模型与三维取向分布对纤维悬浮槽流稳定性的影响

应用3种不同的纤维方向张量封闭模型,数值模拟了纤维悬浮槽流的流动稳定性问题,从而研究封闭模型和纤维的三维取向分布对稳定性分析的影响．结果发现,采用3种不同封闭模型所得到的流动稳定特性与纤维参数之间的关系是相同的,但采用三维混合封闭模型时,由于纤维的取向与流向的偏差程度较大,所以纤维对流动的不稳定性具有最强的抑制作用．而采用二维混合封闭模型时,由于纤维在平面取向条件下,其轴线整体上趋于呈流向排列,使得对流体的作用削弱,导致纤维对流动不稳定性抑制的作用最弱．     

经过修正的平面Couette流的非线性稳定性研究

本文讨论了经过修正的平面Couette流在二维扰动下的非线性稳定性性质,并同经过修正的平面Poiseuille流的非线性稳定性性质进行了比较.计算结果表明,对于有限振幅的扰动,平面Couette流比平面Poiseulle流更不稳定.     

圆管纤维悬浮湍流场中粒子运动的研究

利用从细长体理论出发得到的三维分段积分法和湍流简化方法模拟了大量纤维粒子在圆管湍流内的运动．统计了不同Re数下计算区域内的纤维的取向分布,计算结果与实验结果基本吻合,结果表明湍流的脉动速度导致纤维取向趋于无序,且随着Re数的增加,纤维取向的分布越来越趋于均匀．其后又考虑了纤维速度和角速度的脉动,二者都充分体现了流体速度脉动的影响,且纤维速度的脉动在流向上的强度大于横向,而其角速度的脉动在流向上的强度小于横向．最后统计了纤维在管道截面上的位置分布,说明Re数的增加加速了纤维在管道截面上的位置扩散．     

二元气体混合层的稳定性分析

针对氧气和氮气构成的两组分气体混合层流动,采用线性稳定性理论分析了流动的线性失稳特性.两组分气体混合层的基本流剖面由相似性解给出.首先研究了对流Mach(马赫)数对相似性解剖面的影响;然后重点考察了对流Mach数、二维和三维扰动对稳定性的影响.线性稳定性理论的结果表明,第一模态最不稳定波的最大增长率始终大于第二模态波.随着对流Mach数的增加,混合层中的第一模态和第二模态的最大增长率都受到抑制.     

悬浮固粒对二维混合层流动失稳特性的影响*

本文在不可压缩二维混合层流动方程的基础之上,通过添加固粒的作用项,推导得到了修正的瑞利方程;然后用数值计算方法解其特征方程,得到了悬浮固粒的质量密度、固粒和气流的速度比值以及Stokes数不同时二维混合层流动中扰动频率与空间增长率的关系曲线,给出了关于悬浮固粒对流场失稳特性影响的几个重要结论。     

慢扩张旋转流的稳定性分析（I）—理论研究

本研究了无粘不可压慢扩张旋转流的稳定性问题。采用多重尺度展开法对有慢扩张的旋转流的非对称扰动进行线化稳定性研究，导出了零阶及一阶扰动模所应满足的微分方程及由于慢扩张引起振幅变化的控制方程。将Ｐｌａｓｃｈｋｏ关于慢扩张喷流的结果推广到具有慢扩张的旋转流情况。     

低Reynolds数下双层液膜的空间-时间不稳定性研究

分析了黏性分层双液体薄膜在空间-时间发展扰动下不稳定的触发状况．已有的研究结果给出了在零Reynolds数极限情况下,流动在时间发展模式下不稳定的论断,而这里的空间-时间发展理论却表明,在同一极限下,液膜的流动其实是中性稳定的．该文分析了这种差异及造成差异的原因．通过对能量方程的研究还找到了一种在时间发展模式下没有发现的新不稳定机制,并将这种机制与扰动对流现象的非Galilei不变性相关联．     

慢扩张旋转流的稳定性分析（Ⅰ）—理论研究

本文研究了无粘不可压慢扩张旋转流的稳定性问题，采用多重尺度展开法对有慢扩张的旋转流的非对称扰动进行浅化稳定性研究，导出了零阶及一阶扰动模所应满足的微分方程及由于慢扩张引起振幅变化的控制方程，将Ｐｌａｓｃｈｋｏ关于慢扩张喷流的结果推广到具有慢扩张的旋转流情况。     

二维平板可压缩边界层的二次稳定性分析

本文在二维可压缩边界层中应用Floquet分析,建立了控制次谐波稳定性的方程组,研究在二维可压缩边界层转捩过程中二维有限振幅的T-S波对三维线性小扰动的作用,并计算了来流马赫数对次谐波的产生和发展情况的影响,从中可以看出二维和三维扰动波相互作用对二维可压缩流动边界层的发展过程所产生的影响.     

经过修正的层次流流动的流动稳定性问题(Ⅱ)——经过修正的平行剪切流的线性稳定性研究

本文根据文[1]给出的经过修正的层流流动的流动稳定性理论及平行剪切流中平均速度的一类修正剖面,研究了平行剪切流的线性稳定性性质,对于平面Couette流动和圆管Poiseuill流动,首次得到了二维扰动和轴对称扰动也能造成失稳的结果,并给出了这两种流动在某种定义下的中性曲线.     

Expansions at small Reynolds numbers for the flow past a porous circular cylinder

The purpose of this article is to use the method of matched asymptotic expansions (MMAE) in order to study the two-dimensional steady low Reynolds number flow of a viscous incompressible fluid past a porous circular cylinder. We assume that the flow inside the porous body is described by the continuity and Brinkman equations, and the velocity and boundary traction fields are continuous across the interface between the fluid and porous media. Formal expansions for the corresponding stream functions are used. We show that the force exerted by the exterior flow on the porous cylinder admits an asymptotic expansion with respect to low Reynolds numbers, whose terms depend on the characteristics of the porous cylinder. In addition, by considering Darcy's law for the flow inside the porous circular cylinder, an asymptotic formula for the force on the cylinder is obtained. Also, a porous circular cylinder with a rigid core inside is considered with Brinkman equation inside the porous region. Stress jump condition is used at the porous–liquid interface together with the continuity of velocity components and continuity of normal stress. Some particular cases, which refer to the low Reynolds number flow past a solid circular cylinder, have also been investigated.     

Analytical solutions of laminar swirl decay in a straight pipe

In this work, the laminar swirl flow in a straight pipe is revisited and solved analytically by using prescribed axial flow velocity profiles. Based on two axial velocity profiles, namely a slug flow and a developed parabolic velocity profiles, the swirl velocity equation is solved by the separation of variable technique for a rather general inlet swirl velocity distribution, which includes a forced vortex in the core and a free vortex near the wall. The solutions are expressed by the Bessel function for the slug flow and by the generalized Laguerre function for the developed parabolic velocity. Numerical examples are calculated and plotted for different combinations of influential parameters. The effects of the Reynolds number, the pipe axial distance, and the inlet swirl profiles on the swirl velocity distribution and the swirl decay are analyzed. The current results offer analytical equations to estimate the decay rate and the outlet swirl intensity and velocity distribution for the design of swirl flow devices.     

二阶动态亚格子尺度应力模型

提出了一个基于亚格子尺度应力与速度梯度张量之间关系的二阶动态模型.然后利用在高雷诺数的流场直接数值解的结果对此二阶模型进行检验.直接数值解的流场包括均匀各向同性强迫湍流,衰减湍流以及均匀旋转湍流.数值检验结果发现与一阶动态模型相比,二阶模型的相关系数提高.     

Experiments on the flow stability in a double-lid-driven cavity

The stability of the two-dimensional, steady, incompressible flow in a rectangular square cavity is investigated experimentally for the parallel motion of two facing walls. The critical Reynolds numbers for the onset of three-dimensional steady flow, its structure, and the bifurcation diagram of the velocity field, measured by LDV, agree with numerical predictions. It is observed that the wavelength of the selected pattern increases with the Reynolds number. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

低Mach数翼型绕流数值模拟的低耗散预处理格式

低Mach数流动中,基于可压缩流动的数值模拟算法存在严重的刚性问题,预处理方法可以有效地解决这一问题,但其计算结果不稳定．基于原有的预处理Roe格式,引入可调节参数,得到一种新的低耗散格式．该格式可以减弱边界层以及极低速区域的过度耗散,使得整个流场计算稳定．低Mach数、低Reynolds数定常圆柱绕流和低Mach数、高Reynolds数翼型(NACA0012和S809)绕流3个验证算例表明,带可调节参数的低耗散预处理方法正确可靠,是低速流动数值模拟的有效方法．     

Normal Feedback Stabilization of Periodic Flows in a Two-Dimensional Channel

We consider a two-dimensional incompressible channel flow with periodic condition along one axis. We stabilize the linearized system by a boundary feedback controller with vertical velocity observation, which acts on the normal component of the velocity only. The stability is achieved without any a priori condition on the viscosity coefficient, that is, on the Reynolds number.     

柱状固粒两相边界层流场稳定性研究

从运动方程和本构方程出发,推导得到了含柱状粒子两相流场的修正Orr-Sommerfeld方程,然后在边界层流场中,采用数值计算方法,得到了含柱状粒子流场的稳定性中性曲线,给出了流场失稳的临界雷诺数.结果表明在所述情况下,柱状粒子对流场起着抑制失稳的作用,而且抑制的程度随着柱状粒子体积分数和长径比的增加而提高.     

Navier-Stokes方程的脉动速度方程的最优动力系统建模和动力学分析

研究了基于Navier-Stokes方程的脉动速度方程的最优低维动力系统建模理论.最优目标泛函为脉动速度基函数的不可压缩性和正交性.数值计算了充分发展的并排双方柱绕流问题,并基于双尺度全局最优化方法,建立了它的脉动速度的最优动力系统模型.对其相空间轨道、Poincaré截面、分岔特性、功率谱和Lyapunov指数集等动力学特性进行了分析.随着Reynolds数的增加,双方柱绕流的脉动速度方程最优动力系统具有复杂的类倍周期分岔行为.     

Mathematical modeling of turbulent fiber suspension and successive iteration solution in the channel flow

The modified Reynolds mean motion equation of turbulent fiber suspension and the equation of probability distribution function for mean fiber orientation are firstly derived. A new successive iteration method is developed to calculate the mean orientation distribution of fiber, and the mean and fluctuation-correlated quantities of suspension in a turbulent channel flow. The derived equations and successive iteration method are verified by comparing the computational results with the experimental ones. The obtained results show that the flow rate of the fiber suspension is large under the same pressure drop in comparison with the rate of Newtonian fluid in the absence of fiber suspension. Fibers play a significant role in the drag reduction. The amount of drag reduction augments with increasing of the fiber mass concentration. The relative turbulent intensity and the Reynolds stress in the fiber suspension are smaller than those in the Newtonian flow, which illustrates that the fibers have an effect on suppressing the turbulence. The amount of suppression is also directly proportional to the fiber mass concentration.