定常流下动脉狭窄局部流场的有限元分析

本文利用有限元数值模拟对定常流下动脉狭窄的局部流场进行了系统研究。在数值模拟中求解非线性轴对称Navier-Stokes方程,并对不同狭窄率、不同雷诺数、不同狭窄形状下的流场速度分布、压力降分布和壁面剪应力分布进行了全面细致的分析和讨论。     

亚临界雷诺数二维圆柱绕流边界层流场动态特性的实验研究

本文给出了关于亚临界雷诺数二维圆柱绕流动态特性的实验结果。应用热膜、热线和压力传感器测量了壁面剪切应力脉动、壁面压力脉动和流场的速度脉动,给出了壁面剪切应力脉动频率在驻点附近和分离前后变化的特征,计算了这些脉动量在圆柱面上任意两点间的相关特性。实验结果表明,在亚临界雷诺数二维圆柱绕流的边界层流场中存在着一个频率与涡脱落频率相同的整体同步脉动。     

颈动脉分支的血流动力学数值模拟

采用有限元法数值模拟颈动脉分支的血流动力学。根据在体测量的实际尺寸来构造颈动脉分支的几何模型，以保持模型的解剖精确度；利用在体测量的颈内动脉和颈外动脉流量波形以及主颈动脉的压力波形来确定数值计算的边界条件，以保持数值计算的生理真实性。关注的重点是颈动脉窦内的局部血流形态、二次流和壁面剪应力。在心脏收缩的减速期和舒张期的某些时刻，颈动脉窦中部外侧壁面附近产生了流动分离，形成了一个低速回流区。该流动分离是瞬态的，导致了壁面剪应力的振荡，其振荡范围在-2～6dyn／cm^2之间。同时，颈动脉窦中部横截面内的二次流存在于整个心动周期，最大的二次流速度为同时刻轴向速度平均值的1／3左右。     

脉动流条件下动脉狭窄血管内脂质浓度极化现象的计算机数值模拟

在脉动流条件下,用计算机数值模拟的方法对低密度脂蛋白(LDL)在动脉狭窄血管段内的质量传输进行了研究。计算结果表明．无论是在定常流还是在脉动流条件下．LDL都将聚积于血管狭窄处峰口附近的流动分离点,LDL壁面浓度在此处最高。在脉动流条件下,LDL在血流受扰动区的聚积高于定常流的值;而且．流动分离点处LDL壁面浓度峰值覆盖的区域也宽于定常流。本文所揭示出的LDL在血管狭窄处的质量传输现象可能在动脉粥样硬化的局部性和动脉狭窄的形成中起着很重要的作用。     

声门下喉气管狭窄分度的数值模拟分析

根据真实人体喉气管CT影像数据构建了声门下喉气管狭窄的气道模型，并对这些模型进行了吸气条件下的计算流体力学仿真。分析比较了不同狭窄分度模型内的流场特征、不同喉气管部段的阻力特性以及喉气管壁面的压力与切应力分布，并对这些特性与临床病患表象的关联性进行了讨论。分析表明声门下喉气管狭窄分度与狭窄导致的呼吸道阻力变化具有良好的关联性，此外，喉气管狭窄处压力及剪应力效应对病程的影响也需要关注。     

TRPIV MEASUREMENT OF DRAG-REDUCTION IN THE TURBULENT BOUNDARY LAYER OVER RIBLETS PLATE

采用高时间分辨率粒子图像测速技术对沟槽壁面平板湍流边界层速度矢量场的时间序列及其统计量进行了实验测量,讨论了在同一来流速度下沟槽壁面对平均速度剖面﹑雷诺切应力及湍流强度的影响. 用流向速度分量的多尺度空间局部平均结构函数辨识壁湍流多尺度相干结构,用条件采样和相位平均技术提取壁湍流多尺度相干结构喷射和扫掠事件的脉动速度、展向涡量的二维空间拓扑形态. 结果表明,与同材料光滑壁面对比,沟槽壁面实现了10.73%的摩阻减小量;沟槽壁面湍流边界层湍流强度及雷诺切应力皆比光滑平板湍流边界层对应统计量小,说明沟槽壁面有效降低了湍流边界层内流体的脉动. 通过比较壁湍流相干结构猝发事件各脉动速度分量与展向涡量的空间分布特征,肯定了沟槽壁面的减阻效果,发现沟槽壁面通过抑制相干结构猝发事件实现减阻.     

明渠中跌坎后突扩分离流数值研究

利用大涡模拟技术，对明渠中跌坎后的二维突扩分离流结构进行了数值模拟，探讨了这类典型分离流各特征区的流动性质，分析了再附区壁面上不同测点处脉动压力的统计特征，并与试验结果进行了比较．在数值模拟中，采用了弱压缩流的控制方程和非均匀网格系统．     

一个设计收缩管道的方法

本文根据不可压轴对称(及平面)势流场内存在可分离变量形式的流函数解,提出了一种新的轴向速度分布函数,在很宽的收缩比条件下有效地缩短了管道长度,并保证了管道入口附近管壁上逆压梯度很小,出口截面上的气流均匀性与平行性较优,在流场内气流不发生分离.应用本文方法计算的曲线建造了有扁八角形收缩段的模型风洞,收缩比为13.025.理论计算与实验结果符合较好.实验结果表明:在这种情况下逆压梯度很小,未观察到可见的分离区,在入口、出口及壁面上速度分布满意,出口处最大速度不均匀度小于0.4％.     

明渠流动若干特性初探

解析定量计算了光滑边壁条件下明渠内部平均流和脉动流能量间的分配关系,得到了平均流粘性耗散能量、脉动流能量、脉动流取自平均流能量之间的关系表达式.分析了能谱的结构形式,从工程应用的角度初步勾勒了恒定流动光滑边壁条件下明渠内部能量分布的轮廓.讨论了壁面剪切流层内的流场结构和特性,研究了Navier-Stokes方程的标度变换等一些定量的指标,从标度和流速分布之间的相互关系入手,对这些指标及其间的相互关系进行了讨论.分析了二维平行壁面剪切流内部流动结构的机理和特性,指出能量传递和不同阶段的不同结构有关,能量耗散和扩散与共振和锁频密切相联系.从另一个角度阐释雷诺数的物理意义,以突出雷诺数和涡之间的关系,强调雷诺数是空间点和时间的函数.     

等壁温加热微通道内气体流动换热特性的直接蒙特卡罗模拟

用直接模拟蒙特卡罗方法对压力边界条件下气体在微通道内的流动换热特性进行了研究,给出了壁面与来流存在温差时的沿程速度分布特点,以及在可压缩性与换热条件综合作用下的温度分布特点。研究结果表明:微通道内气体可压缩性作用显著,温度分布由可压缩性和换热强度的相对强弱综合决定;壁面与来流存在温差时气体沿程速度分布型线在入口段内上凸;壁温高于来流温度时,气流速度与等温流动工况下的速度的相对大小与气体稀薄性有关。     

磁场和Hall电流对狭窄动脉中血液流动的影响

A micropolar model for blood simulating magnetohydrodynamic flow through a horizontally nonsymmetric but vertically symmetric artery with a mild stenosis is presented. To estimate the effect of the stenosis shape, a suitable geometry has been considered such that the horizontal shape of the stenosis can easily be changed just by varying a parameter referred to as the shape parameter. Flow parameters, such as velocity, the resistance to flow （the resistance impedance）, the wall shear stress distribution in the stenotic region, and its magnitude at the maximum height of the stenosis （stenosis throat）, have been computed for different shape parameters, the Hartmann number and the Hall parameter. This shows that the resistance to flow decreases with the increasing values of the parameter determining the stenosis shape and the Hail parameter, while it increases with the increasing Hartmann number. The wall shear stress and the shearing stress on the wall at the maximum height of the stenosis possess an inverse characteristic to the resistance to flow with respect to any given value of the Hartmann number and the Hall parameter. Finally, the effect of the Hartmann number and the Hall parameter on the horizontal velocity is examined.     

Two-phase non-linear model for blood flow in asymmetric and axisymmetric stenosed arteries

The pulsatile flow of a two-phase model for blood flow through axisymmetric and asymmetric stenosed narrow arteries is analyzed, treating blood as a two-phase model with the suspension of all the erythrocytes in the core region as the Herschel-Bulkley material and plasma in the peripheral layer as the Newtonian fluid. The perturbation method is applied to solve the resulting non-linear implicit system of partial differential equations. The expressions for various flow quantities are obtained. It is found that the pressure drop, plug core radius, wall shear stress increase as the yield stress or stenosis height increases. It is noted that the velocity increases, longitudinal impedance decreases as the amplitude increases. For asymmetric stenosis, the wall shear stress increases non-linearly with the increase of the axial distance. The estimates of the increase in longitudinal impedance to flow of the two-phase Herschel-Bulkley material are significantly lower than those of the single-phase Herschel-Bulkley material. The results show the advantages of two-phase flow over single-phase flow in small diameter arteries with stenosis.     

Numerical investigation of effect of rolling manipulation of traditional Chinese medical massage on blood flow

The hemodynamic mechanism of rolling manipulation (RM) of traditional Chinese medical massage (TCMM) is investigated. An axisymmetrical nonlinear model and an arbitrary Lagrangian-Eulerian finite element method (ALE-FEM) with rezoning algorithm were introduced to study the viscous flow through an axisymmetrical rigid tube with axially moving stenosis to simulate the rolling manipulation. Flow rate and wall shear stress were obtained by solving complete Navier-Stokes equations numerically. The numerical results show that the stenosis moving frequency, namely the frequency of rolling manipulation, has great effect on the disturbance of flow and the wall shear stress. The stenosis coefficient, which characterizes the severity of the stenosis, another adjustable parameter in rolling manipulation, also shows the significant effect on flow rate and wall shear stress. These numerical results may provide some data that can be taken into consideration when massage is used in clinic.     

The micropolar fluid model for blood flow through a tapered artery with a stenosis

A micropolar model for axisymmetric blood flow through an axially nonsymmetreic but radially symmetric mild stenosis tapered artery is presented. To estimate the effect of the stenosis shape, a suitable geometry has been considered such that the axial shape of the stenosis can be changed easily just by varying a parameter （referred to as the shape parameter）. The model is also used to study the effect of the taper angle Ф. Flow parameters such as the velocity, the resistance to flow （the resistance impedance）, the wall shear stress distribution in the stenotic region and its magnitude at the maximum height of the stenosis （stenosis throat） have been computed for different values of the shape parameter n, the taper angle Ф, the coupling number N and the micropolar parameter m. It is shown that the resistance to flow decreases with increasing the shape parameter n and the micropolar parameter m while it increases with increasing the coupling number N. So, the magnitude of the resistance impedance is higher for a micropolar fluid than that for a Newtonian fluid model. Finally, the velocity profile, the wall shear stress distribution in the stenotic region and its magnitude at the maximum height of the stenosis are discussed for different values of the parameters involved on the problem.     

局部狭窄血管中血液振荡流的速度和切应力

本文建立一种分析局部缓慢狭窄血管中血液振荡流的数学模型，给出了血液的轴向流速，径向流速和切应力的包含压力梯度项的解析表达式，并讨论了血管内由局部狭窄引起的压力梯度沿轴向变化的规律。文章以局部余弦狭窄为例进行数值计算，详细讨论上游均匀管段压力梯度的定常部分和不同次谐波对狭窄管段内流速和切应力的影响。数值结果表明，与均匀管情况相比，在狭窄段内，血液振荡流轴向流速无论平均值还是脉动幅值均明显增大，且径向流速不再为零。但径向流速仍远小于轴向流速。同时，切应力也不再仅由轴向流速梯度提供，径向流速梯度也将产生切应力，但是在计算管壁切向上的切应力时，径向流速梯度的贡献仍相当大。与均匀管管壁切应力沿流运方向保持恒定不同。狭窄管管壁切应力（平均值和脉动值）将随着狭窄高度的增大而增大，在狭窄最大高度处达到最大，因而沿流动方向产生了较大的切应力梯度。     

Unsteady Two-Dimensional Blood Flow in Porous Artery with Multi-Irregular Stenoses

The flow characteristics of an unsteady axisymmetric two-dimensional (2D) blood flow in a diseased porous arterial segment with flexible walls are investigated. The arterial walls mimic the irregular constrictions whereas the lumen containing the thrombus, cholesterol, and fatty plaques represents the porous medium. The governing equations with appropriate initial and boundary conditions are solved numerically using MAC method. The discretization is done on staggered grid with non-uniform grid size and pressure-poisson equation is solved following SOR method. The pressure and velocity corrections are made cyclically until the steady state is achieved. It is observed that for decreasing permeability, flow is highly decelerated while pressure drop and wall shear stress increases. The separation zones and re-circulation regions are found for severe stenoses. Flow separation and re-circulation diminishes for decreasing permeability of the porous medium. Comparisons are provided with published experimental and numerical results.     

Emeritus Professor Shan-Fu Shen,Cornell University

A numerical analysis of blood flow in a rigid artery has been performed to observe the variations in flow pattern and haemodynamic parameters under the influence of multiple (double) stenoses. The interspacing distance, degree of stenotic severity and Reynolds number of flow have been varied. It is found that two stenoses interact when the interspacing distance is lower than a critical value that corresponds to the redevelopment length of fully developed flow after the proximal stenosis. In the case of non-interacting stenoses the haemodynamic parameters, like the peak centreline velocity (representing the peak systolic velocity ratio in medical term), the maximum wall shear stress, region of low shear stress and the irrecoverable pressure drop at the stenosis site, repeat themselves at each individual stenosis. However, when the stenoses interact hydrodynamically because of their proximity, the parameters behave differently, which can lead to a different haemodynamic profile in the arteries and pose difficulties in their pathological interpretation. This work evaluates the influence of double stenoses on the variations in the key haemodynamic features under different degree of stenoses and interspacing distance between the stenoses.     

Pressure effects on viscosity and flow stability of polyethylene melts during extrusion

In the present work, the effects of pressure on the viscosity and flow stability of four commercial grade polyethylenes (PEs) have been studied: linear-low-density polyethylene copolymer, high-density polyethylene, metallocene polyethylenes with short-chain branches (mPE-SCB), and metallocene polyethylenes with long chain branching (mPE-LCB). The range of shear rates considered covers both stable and unstable flow regimes. “Enhanced exit-pressure” experiments have been performed attaining pressures of the order of 500×10⁵ Pa at the die exit. The necessary experimental conditions have been clearly defined so that dissipative heating can be neglected and pressure effects isolated. The results obtained show an exponential increase in both shear and entrance-flow pressure drop with mean pressure when shear rate is fixed and as long as flow is stable. These pressure effects are described by two pressure coefficients, β_S under shear and, β_E under elongation, that are calculated using time–pressure superposition and that are independent of mean pressure and flow rate. For three out of four PE, pressure coefficient values can be considered equal under shear and under elongation. However, for the mPE-LCB, the pressure coefficient under elongation is found to be about 30% lower than under shear. Flow instabilities in the form of oscillating flows or of upstream instabilities appear at lower shear rates as mean pressure increases. Nevertheless, the critical shear stress at which they are triggered remains independent of mean pressure. Moreover, it is found that the β_S values obtained for stable flows do not differ much from the values obtained during upstream instability regimes, and differ really from pressure effects observed under oscillating flow and slip conditions.     

Wall functions for numerical modeling of laminar MHD flows

A general wall function treatment is presented for the numerical modeling of laminar magnetohydrodynamic (MHD) flows. The wall function expressions are derived analytically from the steady-state momentum and electric potential equations, making use only of local variables of the numerical solution. No assumptions are made regarding the orientation of the magnetic field relative to the wall, nor of the magnitude of the Hartmann number, or the wall conductivity. The wall functions are used for defining implicit boundary conditions for velocity and electric potential, and for computing mass flow and electrical currents in near wall-cells. The wall function treatment was validated in a finite volume formulation, and compared with an analytic solution for a fully developed channel flow in a transverse magnetic field. For the case with insulating walls, a uniform 20×20 grid, and Hartmann numbers Ha={10,30,100}, the accuracy of pressure drop and wall shear stress predictions was {1.1%,1.6%,0.5%}, respectively. Comparable results were obtained also with conducting Hartmann walls. The accuracy of predicted pressure drop and wall shear stress was essentially independent of the resolution of the Hartmann layers. When applied also to the parallel walls, the wall functions reduced the errors by a factor two to three. The wall functions can be implemented in any general flow solver, to allow accurate predictions at reasonable cost even for complex geometries and nonuniform magnetic fields.