脂肪颗粒在颈动脉中的运动及其对血液动力学的影响

很多研究表明, 动脉粥样硬化通常发生在具有复杂血液动力学的区域, 比如分叉动脉和弯曲动脉. 这些地方常伴随有低壁面剪切力或震荡壁面剪切力, 这是动脉粥样硬化形成的一大诱因. 使用计算流体力学软件对2D颈动脉分叉血管进行了模拟, 研究了脂肪颗粒在颈动脉中的运动及其对血液动力学的影响. 研究表明: 1)血管狭窄对于脂肪颗粒的运动有重要影响, 同时也影响栓塞的形成; 2)脂肪颗粒可能会黏附在血管壁面, 但由于血流的冲击作用, 脂肪颗粒会随后在壁面略微铺展; 3)颈动脉狭窄区域后方是下一个血栓的可能生长位点; 4)当栓塞形成时, 速度和壁面剪切力分布将变得复杂多变, 这对于血管是有害的.     

考虑热波效应的皮肤传热过程解析解

为了研究热松弛时间及血液灌输对皮肤温度场变化影响,运用拉普拉斯方法给出了稳定的第一类条件下考虑一阶热波效应的Pennes方程的解析解.得到了指数形式的温度阶跃量,通常热松弛时间越长阶跃变化量越大,而血液灌输率越大阶跃变化量越小.若忽略血液灌输率,该解退化为普通热波方程的解析解.将皮肤内各点温度变化分为五个阶段,研究表明...     

倾斜均质表面上等径水滴聚合过程的可视化实验

对倾斜均匀表面上等直径水滴的聚合过程及特性进行了可视化实验研究,获得了水滴直径和表面倾角等参数对液滴聚合过程中液滴液桥半径、接触角和接触线变化特性的影响,分析了水滴聚合对其运动的影响.实验结果表明:表面倾角越大,下滑的临界半径越小;液滴的直径越大,液滴聚合后越容易下滑;液滴聚合可以加快液滴的运动,使下滑临界半径减小.     

颗粒物质内自旋小球运动行为的数值模拟研究

针对大耳沙蜥在沙子中的运动行为,以球形物体为研究对象,利用3维离散元数值模拟程序LIGGGHTS模拟了旋转运动模式对颗粒物质中球形物体平动和上升/下降行为的影响,定量分析了旋转速度以及颗粒间摩擦系数等因素的具体影响.研究结果表明:球形物体与颗粒物质基底颗粒间的摩擦系数以及球形物体的自转角速度对球形物体的运动有明显影响,摩擦系数越大物体运动越明显,自转角速度越大物体运动越明显.该结果比较好地解释了沙漠生物外表具有鳞片的原因.     

兴奋性作用诱发神经簇放电个数不增反降的分岔机制

非线性动力学在识别神经放电的复杂现象、机制和功能方面发挥了重要作用.不同于传统观念,本文提出了兴奋性作用可以降低而不是增加簇内放电个数的新观点.在簇放电模式休止期的适合相位施加强度合适的脉冲或自突触电流,能诱发簇内放电个数降低;电流的施加相位越早,所需的强度阈值越大,簇内放电个数越少.进一步,利用快慢变量分离获得的簇放电的动力学性质进行了理论解释.簇放电模式表现出低电位的休止期和高电位的放电的交替,存在于快子系统的鞍结分岔点和同宿轨分岔点之间;放电起始于鞍结分岔、结束于同宿轨分岔;越靠近同宿轨分岔从休止期跨越到放电所需的电流强度越大.因此,电流在休止期上的作用相位越早,就越靠近同宿轨分岔,因而从休止期跨越到放电需要的电流强度阈值越大,放电起始相位到同宿轨分岔之间的区间变小导致放电个数变少.研究结果丰富了非线性现象及机制,对兴奋性作用提出了新看法,给出了调控簇放电模式的新途径.     

煤颗粒热膨胀量与破碎特性的研究

煤颗粒的热膨胀破碎特性直接影响流化床锅炉的运行效率.本文利用热机械分析仪(TMA)测定了不同种类不同密度的型煤和原煤的热膨胀特性,并对部分破碎微观形貌进行了FSEM观察;通过旋转炉内的燃烧试验研究了热膨胀特性和破碎的关系.研究表明,煤颗粒在燃烧过程中其热膨胀破碎主要发生在挥发分析出阶段;内部挥发分的析出会使颗粒内压增大而产生膨胀,进而产生细小裂纹并破碎;挥发分越高,颗粒密度越大,其热膨胀形变率越大,越容易发生破碎现象;主要挥发分析出后热膨胀引起的破碎可以忽略.     

脉动流血液通栓的晶格玻尔兹曼模型

本文把血浆看为水,红细胞当作密度为水的悬浮刚性小球,模拟脉动流在锥形管的中的通栓.首先在一定的压积和压差下,在锥形管中产生稳定的栓塞,然后用脉动流来通栓.改变脉动流的频率研究脉动流通栓的条件.     

非饱和多孔介质内毛细驱动流动分析

将非饱和含湿多孔介质层内液体饱和度当量为无量纲的液层厚度,在多孔层内发生相变时其分布规律直观地反映相变传热能力从而预示临界热流(CHF)的发生.计算结果显示,多孔层中心处最容易出现干涸而导致CHF;残余饱和度和孔隙率越大,越不容易出现CHF;饱和度分布形状(液层形状)主要受输入热流密度和孔隙率的影响,热流越小、孔隙率越大就越平缓.     

激光空泡在文丘里管中运动的动力学特性

以水为工作介质,在不同文丘里管入口压力下,利用YAG激光器产生的激光轰击水中的金属靶材产生空泡,借助高速摄像系统记录激光空泡在文丘里管中的运动过程,并采用流体动力学模拟对文丘里管中的流场特性和空泡的溃灭特性进行分析.结果表明:激光空泡在文丘里管中的运动,其形状变化可分为产生阶段、挤压阶段、溃灭初始阶段和溃灭阶段等四个阶段.空泡的溃灭取决于流场状态,当流动为层流时,空泡不发生溃灭;当流动为湍流时,空泡发生溃灭,且湍流程度越剧烈,溃灭现象越显著.搭建的激光空泡生成与运动系统能够实现空泡的定点溃灭.     

组合式吸液芯内液汽相变过程的数值模拟

本文对高温热管吸液芯多孔介质内金属钠液的液汽相变过程进行了模拟研究,并采用流体体积分数模型和传质模型对此过程进行了数值分析.研究表明孔隙率和渗透率越大的组合式吸液芯多孔介质,热量在液态金属中的传递速率越快,进而可以增加高温热管的传热量,提高传热效率;但是孔隙率和渗透率越大,组合式吸液芯多孔介质加热壁面的温度分布越不均匀,产生沸腾传热极限的可能性亦越大.     

脉动流在分叉管中通栓效果的晶格玻尔兹曼方法研究

血栓引发的各种心血管疾病一直威胁着人们的健康. 在已经产生血栓的血管中, 脉动对于疏通血管有良好的作用. 由于血液的黏滞作用以及红细胞的惯性, 脉动流的频率会影响血管通栓的效果. 在分叉管模型中, 低压差的条件下, 由于另一畅通管子的导通作用减少了回流, 导致通栓效果不理想. 通过增大压差和提高脉动流的振幅, 降低畅通管子导通作用的影响, 研究脉动流在分叉管中的通栓效果. 研究发现, 脉动低频通栓效果好, 但是通栓需要的时间较长; 高频通栓时间短, 但是当频率高于一定值, 则通栓效果不明显. 细胞和管壁的摩察系数对通栓效果也有影响.     

三角波脉动流通栓的晶格玻尔兹曼方法模型

血液栓塞作为心血管疾病的一大诱因, 其形成机理及外部因素一直是医学、生物物理等领域专家关心的问题. 血栓的形成及其结构复杂多样, 大大增加了治愈血栓的难度. 脉动对于疏通血液栓塞有良好的作用, 而由于血液的黏滞作用以及红细胞的惯性, 脉动流的波形、振幅和频率都会影响通栓的效果. 本文主要基于晶格玻尔兹曼方法, 在栓塞的锥形管中, 用三角波脉动流进行通栓计算, 探索三角波脉动流的波形、压差、频率对血管通栓效果的影响. 计算发现, 低频低压条件下三角波脉动流通栓效果不明显, 而高频条件下通栓效果良好; 适当增加压差, 可以提高能通栓的三角波脉动流的频率.     

Automatic detection and branch switching methods for steady bifurcation in fluid mechanics

This paper deals with the computation of steady bifurcations in the framework of 2D incompressible Navier–Stokes flow. We first propose a numerical method to accurately detect the critical Reynolds number where this kind of bifurcation appears. From this singular value, we introduce a numerical tool to compute all the steady bifurcated branches. All these algorithms are based on the Asymptotic Numerical Method [1], [2]. The critical values are determined by using a bifurcation indicator [3], [4], [5] and the bifurcated branches are computed by using an augmented system which was first introduced in solid mechanics [4], [6]. Several numerical examples from 2D Navier–Stokes show the reliability and the efficiency of the proposed methods.     

Simulation of Blood Flow at Vessel Bifurcation by Lattice Boltzmann Method

The application of the lattice Boltzmann method to the large vessel bifurcation blood flow is investigated in a wide range of Reynolds numbers. The velocity, shear stress and pressure distributions at the bifurcation are presented in detail The flow separation zones revealed with increase of Reynolds number are located in the areas of the daughter branches distal to the outer corners of the bifurcation where some deposition of particular blood components might occur to form arteriosclerosis. The results also demonstrate that the lattice Boltzmann method is adaptive to simulating the flow in larger vessels under a high Reynolds number.     

The second Hopf bifurcation in lid-driven square cavity

To date, there are very few studies on the second Hopf bifurcation in a driven square cavity, although there are intensive investigations focused on the first Hopf bifurcation in literature, due to the difficulties of theoretical analyses and numerical simulations. In this paper, we study the characteristics of the second Hopf bifurcation in a driven square cavity by applying a consistent fourth-order compact finite difference scheme recently developed by us. We numerically identify the critical Reynolds number of the second Hopf bifurcation located in the interval of(11093.75, 11094.3604) by bisection. In addition, we find that there are two dominant frequencies in its spectral diagram when the flow is in the status of the second Hopf bifurcation, while only one dominant frequency is identified if the flow is in the first Hopf bifurcation via the Fourier analysis. More interestingly, the flow phase portrait of velocity components is found to make transition from a regular elliptical closed form for the first Hopf bifurcation to a non-elliptical closed form with self-intersection for the second Hopf bifurcation. Such characteristics disclose flow in a quasi-periodic state when the second Hopf bifurcation occurs.     

Flow-induced oscillations of a cantilevered pipe conveying fluid with base excitation

It is known that a plain cantilevered pipe conveying fluid loses its stability by a Hopf bifurcation, leading to either planar or non-planar flutter for flow velocities beyond the critical flow velocity for Hopf bifurcation. If an external mass is attached to the end of the pipe (an end-mass), the resulting dynamics become much richer, showing 2D and 3D quasiperiodic and chaotic oscillations at high flow velocities. In this paper, a cantilevered pipe, with and without an end-mass, subjected to a small-amplitude periodic base excitation is considered. A set of three-dimensional nonlinear equations is used to analyze the pipe?s response at various flow velocities and with different amplitudes and frequencies of base excitation. The nonlinear equations are discretized using the Galerkin technique and the resulting set of equations is solved using Houbolt?s finite difference method. It is shown that for a plain pipe (with no end-mass), non-planar post-instability oscillations can be reduced to planar periodic oscillations for a range of base excitation frequencies and amplitudes. For a pipe with an end-mass, similarly to a plain pipe, three-dimensional period oscillations can be reduced to planar ones. At flow velocities beyond the critical flow velocity for torus instability, the three-dimensional quasiperiodic oscillations can be reduced to two-dimensional quasiperiodic or periodic oscillations, depending on the frequency of base excitation. In all these cases, a low-amplitude base excitation results in reducing the three-dimensional oscillations of the pipe to purely two-dimensional oscillations, over a range of excitation frequencies. These numerical results are in agreement with the previous experimental work.     

螺旋管内气液固三相流颗粒相分布规律

为了深入认识螺旋管多相流相分离现象,并为新型螺旋管除砂器设计提供指导,本文应用马尔文粒度仪,测量了螺旋管气水砂三相流底部水平段液膜中的颗粒浓度和粒度分布。研究表明:在泡状流和分层流条件下,螺旋管底部水平段可形成稳定的连续液膜流动;在宽广的气速范围内,液膜中的颗粒浓度分布规律均为内弯侧较低、外弯侧较高,说明螺旋管除砂器对于实际生产中流动工况的变化具有良好的适应性;泡状流中提高气速有利于分离;分层流中在中等气速条件下外弯侧颗粒浓度最大,中等气速是相分离的最佳操作工况。     

Laser Doppler velocimetry analysis of transitional pipe flow

The objective of this study has been to experimentally analyze the correlation structure of the strong temporal intermittency which characterizes pipe flow close to the transition to turbulence. In doing so transitional pipe flow has been analyzed by Laser Doppler velocimetry and the Reynolds number dependence of the covariance function has been studied. The range which has been analyzed covers the transition to turbulence and moderately developed turbulence (Reynolds number from 1 500 to 5 000). The correlation structure which has been evidenced is generally in agreement with the deterministic, dynamical, interpretation of temporal intermittency which explains the intermittent behavior as a result of a saddle node bifurcation. However, the analysis has evidenced fluctuations even before the onset of turbulence. The structure of these fluctuations is perfectly autoregressive which leads us to conclude that the transition to turbulence can be viewed as a transition from linear randomness to (non-linear) homogeneity. Received 29 March 1999 and Received in final form 6 September 1999     

Nonlinear dynamics of cantilevered extensible pipes conveying fluid

In this paper the nonlinear planar dynamics of a fluid-conveying cantilevered pipe is investigated. The centreline of the pipe is considered to be extensible; i.e., coupled longitudinal and transverse displacements are considered. The extended version of the Lagrange equations for systems containing non-material volumes is employed to derive the equations of motion, resulting directly in a set of coupled nonlinear ordinary differential equations. The pseudo-arclength continuation technique along with direct time integration are used to solve these equations. Bifurcation diagrams of the system are constructed as the flow velocity is increased; these diagrams are supplemented by time traces, phase-plane portraits, and fast Fourier transforms for some sets of system parameters. As opposed to the case of an inextensible pipe, an extensible pipe elongates in the axial direction as the flow velocity is increased from zero; depending on the system parameters, this static elongation can be considerable. At the critical flow velocity, the system loses stability via a supercritical Hopf bifurcation, emerging from the trivial solution for the transverse displacement and leading to a flutter.