动脉瘤内流场以及瘤体尺寸的影响的数值研究

采用计算流体动力学(CFD)数值模拟的方法,在周期性脉动速度入流条件下,建立刚性动脉瘤模型并研究了动脉瘤模型中流场的特征(速度、压力、壁面剪切应力)。得到了脉动入流一个周期内流场特征的变化规律,发现动脉瘤的后端有相当高的压力和壁面剪切应力,而且高压力和壁面剪切应力分布的位置几乎是固定的。探讨了不同动脉瘤尺寸对内部流场的影响,动脉瘤的直径与瘤体长度之比越大,瘤壁承受的剪切应力就越大,动脉瘤破裂的危险性就越高。     

A comparison between different asymmetric abdominal aortic aneurysm morphologies employing computational fluid–structure interaction analysis

Considering representative asymmetric aneurysms in the abdominal aorta, the transient 3-D blood flow and pressure distributions as well as aneurysm wall stresses were numerically analyzed. To obtain more realistic and accurate results for blood flow fields and wall stress distributions, a coupled fluid-flow and solid–structure solver was employed. Geometric abdominal aortic aneurysm (AAA) variations studied included the degree of asymmetry, neck angle and bifurcation angle, and hence their impacts on the hemodynamics and biomechanics. The simulation results indicated that the assumption of symmetric AAA geometry may underestimate AAA-wall stress considerably. The neck angle influences the blood flow field substantially. A large neck angle, resulting in strong wall curvatures near the proximal neck, can produce aggravating blood flow patterns and elevated wall stresses (Von Mises). The iliac bifurcation angle affects blood flow patterns insignificantly but plays an important role in wall-stress concentrations. The wall stress of lateral asymmetric AAAs is higher than for the anterior-posterior asymmetric types. The maximum wall stress-site is located near the anterior distal side for the anterior-posterior asymmetric AAA and the distal side towards the asymmetric bulge in the lateral asymmetric AAA.     

夹层动脉瘤及覆膜支架模型的流固耦合分析

腹主动脉瘤腹主动脉瘤(Abdominal Aortic Aneurysm,AAA)是危及病人生命的严重疾病,若主动脉瘤不加以治疗,瘤腔将不断长大而有破裂的危险,尽管目前对主动脉瘤的研究较多,但对主动脉夹层动脉瘤(Aortic Dissecting Aneruysm,ADA)的研究较少,本文在周期性脉动速度和压力条件下,对DNA内部流场及植入覆膜支架进行了流固耦合的数值模拟.得到了夹层动脉瘤内部流场的速度矢量分布,瘤壁上的位移、应力分布.同时对夹层动脉瘤植入覆膜支架前后进行了对比分析,覆膜支架的植入使得瘤壁上的最大应力和最大位移均从瘤腔壁面转移到管状动脉壁上,且数值大大下降,说明植入覆膜支架能很好的防止夹层动脉瘤破裂.     

动脉分岔血管内膜增生过程的数值模拟

内膜增生从发生到阻塞血管是一个复杂的变化过程,在这个过程中,内膜的增生、血管腔体形状的改变和血流动力学之间是相互影响的。为了研究这些变化,本文提出一种单元填充方法数值模拟了三维颈动脉分岔血管在低切应力作用下血管内膜增生的过程。该方法既可以克服节点移动方法所不可避免的内膜增生的不连续性,也可以避免网格重划分的困难。结果发现,如果单纯以切应力阈值作为内膜增生的判据,低切应力的作用将无法导致血管完全阻塞,但内膜增生和血流动力学之间的相互影响是可以通过数值方法进行模拟的。在本数值模拟中,内膜增生的过程分为"增厚"(先)和"扩展"(后)两个阶段,最大狭窄率为34.4%,发生在距血管分岔5mm处动脉窦的外侧壁面。其发生位置和形状与临床观察吻合。     

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.     

Rheology of shear thickening suspensions and the effects of wall slip in torsional flow

The rheological characterisation of concentrated shear thickening materials suspensions is challenging, as complicated and occasionally discontinuous rheograms are produced. Wall slip is often apparent and when combined with a shear thickening fluid the usual means of calculating rim shear stress in torsional flow is inaccurate due to a more complex flow field. As the flow is no longer “controlled”, a rheological model must be assumed and the wall boundary conditions are redefined to allow for slip. A technique is described where, by examining the angular velocity response in very low torque experiments, it is possible to indirectly measure the wall slip velocity. The suspension is then tested at higher applied torques and different rheometer gaps. The results are integrated numerically to produce shear stress and shear rate values. This enables the measurement of true suspension bulk flow properties and wall slip velocity, with simple rheological models describing the observed complex rheograms.     

The interactions between bloodstream and vascular structure on aortic dissecting aneurysmal model: A numerical study

Stent-graft implantation is an important means of clinical treatment for aortic dissecting aneurysm （ADA）. However, researches on fluid dynamics effects of stent were rare. Computer simulation was used to investigate the interactions between bloodstream and vascular structure in a stented ADA, which endures the periodic pulse velocity and pressure. We obtained and analyzed the flow velocity distribution, the wall displacement and wall stress in the ADA. By comparing the different results between a non-stented and a stented ADA, we found that the insertion of a vascular graft can make the location of maximum stress and displacement move from the aneurysm lumen wall to the artery wall, accompanied with a greatly decrease in value. These results imply that the placement of a stent-graft of any kind to oc-clude ADA will result in a decreased chance of rupture.     

Intra-aneurysmal blood flow based on patient-specific CT angiogram

To discuss the validity of the hemodynamic hypothesis of aneurysm rupture, we used a patient-specific, realistic aneurysm model to reveal the flow structure and wall shear stress distribution in two cases: one with an unruptured aneurysm and the other with a ruptured aneurysm. We used particle imaging velocimetry and laser Doppler velocimetry to measure velocity profiles of intra-aneurysmal flow. Both cases had a circulating flow along the aneurysm wall, although the second case had a recirculating zone only in the minimum phase. Differences in the wall shear stress profile may identify aneurysm rupture.     

Friction relaxation model for fast transient flows application to water hammer in two-phase flow – The WAHA code

The paper deals with the problem of the wall shear stress during rapid transient 1D flows in a piping system caused by water hammers in two-phase flow induced by a fast valve closure. The evolution of the transient wall shear stress is interpreted in terms of two steps. The first step is a sudden and dramatic change of the wall shear stress due to the passage of the pressure wave. The second step is a relaxation process of the shear stress which is modeled from the Extended Irreversible Thermodynamics theory. The friction relaxation model (FRM) presented in the first part of this paper describes both steps of the evolution of the wall shear stress during water hammers. The second part of the paper deals with the application of the FRM model as a closure law in the WAHA code. The main purpose of the WAHA code is to predict various situations relative to single- and two-phase water hammer transients in piping systems. The last part of the paper deals with the simulation of several cases from the UMSICHT databank using the adapted WAHA computer code with the FRM model. The results of these simulations are systematically compared with the experimental data. It is concluded that the new FRM model has a clear effect on water hammer pressure wave damping and on the pressure wave propagation velocity.     

Viscoelastic blood flow through arterial stenosis—Effect of variable viscosity

Current theoretical investigation deals with mathematical model of unsteady non-Newtonian flow of blood through a stenosed artery. The flowing blood is considered as a viscoelastic fluid having shear-thinning rheology and characterized by generalised Oldroyd-B model. The arterial wall is considered to be rigid having cosine shaped stenosis in its lumen. The governing equations of motion accompanied by appropriate choice of the initial and boundary conditions are solved numerically by MAC (Marker and Cell) method and the results are checked for numerical stability with desired degree of accuracy. The quantitative analysis has been carried out finally which includes the respective profiles of the flow-field. The key factors like the wall shear stress and flow separation are also examined for further qualitative insight into the flow through arterial stenosis. The present results show quite consistency with several existing results in the literature which substantiate sufficiently to validate the applicability of the model under consideration.     

窦部对称狭窄对颈动脉内流场影响的数值研究

以颈动脉分岔血管为例，采用数值方法研究了窦部环缩狭窄之后的流场分布情况，并和正 常血管情况下的流场分布进行了比较. 结果表明，采用环缩方式给颈动脉分岔血管施加对称 的狭窄改变了颈动脉窦内流场，特别是壁面剪应力的分布规律. 低剪应力区出现在狭窄段之 后的窦内，并且沿整个周向均匀分布. 根据低剪应力和动脉粥样硬化的关系，指出： 若人为地给颈动脉窦内施加对称狭窄，则脂质沉积将在狭窄下游的窦内沿周向轴对称 发展. 为了更真实地反映颈动脉窦内的狭窄，建议根据动脉血管中的实际狭窄情况，采用非 对称的狭窄分布模式.     

Hemodynamic analysis of intracranial aneurysms using phase-contrast magnetic resonance imaging and computational fluid dynamics

Additional hemodynamic parameters are highly desirable in the clinical management of intracranial aneurysm rupture as static medical images cannot demonstrate the blood flow within aneurysms. There are two ways of obtaining the hemodynamic information—by phase-contrast magnetic resonance imaging(PCMRI) and computational fluid dynamics(CFD). In this paper, we compared PCMRI and CFD in the analysis of a stable patient's specific aneurysm.The results showed that PCMRI and CFD are in good agreement with each other. An additional CFD study of two stable and two ruptured aneurysms revealed that ruptured aneurysms have a higher statistical average blood velocity,wall shear stress, and oscillatory shear index(OSI) within the aneurysm sac compared to those of stable aneurysms.Furthermore, for ruptured aneurysms, the OSI divides the positive and negative wall shear stress divergence at the aneurysm sac.     

Three‐dimensional numerical simulation of compound meandering open channel flow by the Reynolds stress model

Turbulent flow in a compound meandering open channel with seminatural cross sections is one of the most complicated turbulent flows as the flow pattern is influenced by the combined action of various forces, such as centrifugal force, pressure, and shear stresses. In this paper, a three‐dimensional (3D) Reynolds stress model (RSM) is adopted to simulate the compound meandering channel flows. Governing equations of the flow are solved numerically with finite‐volume method. The velocity fields, wall shear stresses, and Reynolds stresses are calculated for a range of input conditions. Good agreement between the simulated results and measurements indicates that RSM can successfully predict the complicated flow phenomenon. Copyright © 2008 John Wiley & Sons, Ltd.     

Computational Modelling of Non-Newtonian Effects on Flow in Channels with Moving Wall Indentations

Non-Newtonian effects in a channel with moving wall indentations are assessed numerically by a finite volume method for solving the unsteady incompressible Navier-Stokes equations and using a power-law model exhibiting shear thinning viscosity and Casson's model as the constitutive equations for the non-Newtonian fluid. The computations show that for a non-Newtonian fluid, there are differences in the velocity profiles and in the structure and size of the reversed flow regions as compared with the corresponding Newtonian fluid. The comparison of non-Newtonian and Newtonian wall shear stress reveals a slight decrease in the magnitude on the average for the non-Newtonian case, eventually resulting in the strength of the “wave train” being slightly weaker than those corresponding to a Newtonian fluid.     

Pulsatory Turbulent Compressible Fluid Flow and Propagation of Pressure Waves in a Channel

The properties of the damping coefficient and phase velocity of propagation of small-amplitude pressure waves as functions of the oscillation frequency are investigated for the turbulent flow of a weakly compressible fluid in a circular pipe. The wall friction is found by solving numerically the equation of motion and the relaxation equations for the turbulent shear stress and viscosity which provide the basis for a turbulent transfer model developed for unsteady conditions. The properties are explained in terms of an analysis of the calculated data on turbulent transfer. The results obtained are compared with experiments.     

Turbulent flow structure and heat transfer in an inclined bubbly flow. Experimental and numerical investigation

The effect of channel inclination on the variation in the wall shear stress and the heat transfer in a two-phase bubbly flow in a rectangular channel is experimentally and numerically investigated. The wall friction was measured using the electrodiffusion method and the temperature was measured by tiny platinum resistance thermometers. The model is based on the system of RANS equations with account for the back influence of the bubbles on the flow characteristics. Flow turbulence is calculated according to the model of transport of the Reynolds stress tensor components. It is shown that in the gas-liquid flow the angle of the channel inclination to the horizon can have a considerable effect on the friction and the heat transfer. The greatest friction and heat transfer values correspond to the angles of channel inclination ranging from 30 to 50°. In the inclined two-phase bubbly flow the shear stress enhancement on the wall amounts to 30% and that of the heat transfer to 15%. A friction and heat transfer reduction to 10 and 25%, respectively, is noticed in near-horizontal flows.     

动脉中非平稳流动壁面剪切力及其Hilbert-Huang变换

本文通过数值方法求解均匀动脉中的非平稳脉动流,给出了通过测量非平稳脉动血流量确定壁面切应力的方法.作为算例,采用实测的大鼠颈总动脉流量信号,求出了均匀动脉壁面切应力波形.进一步对求得的切应力波形进行经验模态分解(EMD),得到了切应力波形的各内在模态(IMF),以及Hilbert幅值谱.从切应力波形经Hilbert-Huang变换得到的IMF和Hilbert谱图可以明显地看出切应力各频率成分的物理意义.所得结果为进一步深入研究非平稳脉动切应力与血管重建的关系提供了一种方法学基础.     

Mathematical modeling and parametric investigation of blood flow through a stenosis artery

In this study, a mathematical model is formulated to examine the blood flow through a cylindrical stenosed blood vessel. The stenosis disease is caused because of the abnormal narrowing of flow in the body. This narrowing causes serious health issues like heart attack and may decrease blood flow in the blood vessel. Mathematical modeling helps us analyze such issues. A mathematical model is considered in this study to explore the blood flow in a stenosis artery and is solved numerically with the finite difference method. The artery is an elastic cylindrical tube containing blood defined as a viscoelastic fluid. A complete parametric analysis has been done for the flow velocity to clarify the applicability of the defined problem. Moreover, the flow characteristics such as the impedance, the wall shear stress in the stenotic region, the shear stresses in the throat of the stenosis and at the critical stenosis height are discussed. The obtained results show that the intensity of the stenosis occurs mostly at the highest narrowing areas compared with all other areas of the vessel, which has a direct impact on the wall shear stress. It is also observed that the resistive impedance and wall shear pressure get the maximum values at the critical height of the stenosis.

     

Comparison between computational fluid dynamics,fluid–structure interaction and computational structural dynamics predictions of flow-induced wall mechanics in an anatomically realistic cerebral aneurysm model

Haemodynamically induced stress plays an important role in the progression and rupture of cerebral aneurysms. The current work describes computational fluid dynamics (CFD), fluid–structure interaction (FSI) and computational structural dynamics (CSD) simulations in an anatomically realistic model of a carotid artery with two saccular cerebral aneurysms in the ophthalmic region. The model was obtained from three-dimensional (3D) rotational angiographic imaging data. CFD and FSI were studied under a physiologically representative waveform of inflow. The arterial wall was assumed elastic or hyperelastic, as a 3D solid or as a shell depending on the type of modelling used. The flow was assumed to be laminar, non-Newtonian and incompressible. The CFD, FSI and CSD models were solved with the finite elements package ADINA. Predictions of velocity field and wall shear stress (WSS) on the aneurysms made using CFD and FSI were compared. The CSD model of the aneurysms using complete geometry was compared with isolated aneurysm models. Additionally, the effects of hypertensive pressure on CSD aneurysm models are also reported. The vortex structure, WSS, effective stress, strain and displacement of the aneurysm walls showed differences, depending on the type of modelling used.     

Numerical simulation for the simple shear test in amorphous glassy polymers

In this paper, a driving stress finite element method of elastic-plastic large deformation based on implicit time integrating algorithm and an eight-chain molecular network model is used for the numerical simulation of the simple shear test of polycarbonate (PC) materials. The simulated results are compared with experimental ones. The strain localization propagation for the shear band deformation for simple shear deformation is investigated numerically. The effects of microstructure parameters in the model on strain softening and orientation hardening of the PC are discussed in detail. Supported by the National Natural Science Foundation of China.