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.     

基于格子Boltzmann方法的粘弹性流体圆柱绕流分析

将光滑界面法引入到格子Boltzmann方法中分析粘弹性流体绕流问题,分别采用单松弛模型和对流扩散模型求解运动方程和Oldroyd-B本构方程,针对圆形和椭圆内部边界条件,给出连续界面插值函数,在此基础上,运用光滑界面法将内部边界转换为作用力项施加到演化方程中。首先分析圆柱绕流问题,给出不同材料参数情况下的流场分布和阻力系数计算结果,比较发现与宏观数值模拟结果相吻合。将模型拓展到绕椭圆流动中,分析椭圆形状和材料参数对粘弹性流体绕柱流的影响,发现随着椭圆长轴与短轴比值的增加和维森伯格数的增加,阻力系数逐渐下降,并且长短轴比对迭代收敛有较大影响。     

Non‐Newtonian blood flow dynamics in a right internal carotid artery with a saccular aneurysm

Flow dynamics plays an important role in the pathogenesis and treatment of cerebral aneurysms. The temporal and spatial variations of wall shear stress in the aneurysm are hypothesized to be correlated with its growth and rupture. In addition, the assessment of the velocity field in the aneurysm dome and neck is important for the correct placement of endovascular coils. This work describes the flow dynamics in a patient‐specific model of carotid artery with a saccular aneurysm under Newtonian and non‐Newtonian fluid assumptions. The model was obtained from three‐dimensional rotational angiography image data and blood flow dynamics was studied under physiologically representative waveform of inflow. The three‐dimensional continuity and momentum equations for incompressible and unsteady laminar flow were solved with a commercial software using non‐structured fine grid with 283 115 tetrahedral elements. The intra‐aneurysmal flow shows complex vortex structure that change during one pulsatile cycle. The effect of the non‐Newtonian properties of blood on the wall shear stress was important only in the arterial regions with high velocity gradients, on the aneurysmal wall the predictions with the Newtonian and non‐Newtonian blood models were similar. Copyright © 2005 John Wiley & Sons, Ltd.     

Flow and breakup characteristics of elliptical liquid jets

This paper presents the results of an experimental study on liquid jets discharging from elliptical orifices into still ambient air. The experiments were conducted with a set of elliptical orifices of approximately same area of cross section but varying orifice aspect ratio using water and water–glycerol mixture as experimental fluids. The flow behavior of liquid jets was analyzed using their photographs captured by an imaging system. The measurements obtained for the elliptical liquid jets were compared with the circular liquid jets discharging from a circular orifice of the same area of cross section. Elliptical geometry of the orifice results in a flow process by which the emanating liquid jet periodically switches its major and minor axes as it flows downstream of the orifice. In this paper, we attempt to characterize the axis-switching process through its wavelength and amplitude. For a given elliptical orifice, the axis-switching process is dominantly seen in a particular range of flow conditions. The effects of the orifice aspect ratio and liquid viscosity on the axis-switching process are revealed through this study. The experimental results on jet breakup show that axis-switching process has a destabilizing effect on elliptical liquid jets within a particular range of flow conditions and it results in shorter breakup lengths compared to the circular jet. The extent to which axis-switching destabilizes the jet is dictated by the viscosity of liquid. An increase in orifice aspect ratio destabilizes elliptical liquid jets with low viscosity like water; however, this behavior seems to get obscured in water–glycerol mixture elliptical jets due to high viscosity.     

Blood flow dynamics and fluid–structure interaction in patient‐specific bifurcating cerebral aneurysms

Hemodynamics plays an important role in the progression and rupture of cerebral aneurysms. The current work describes the blood flow dynamics and fluid–structure interaction in seven patient‐specific models of bifurcating cerebral aneurysms located in the anterior and posterior circulation regions of the circle of Willis. The models were obtained from 3D rotational angiography image data, and blood flow dynamics and fluid–structure interaction were studied under physiologically representative waveform of inflow. The arterial wall was assumed to be elastic, isotropic and homogeneous. The flow was assumed to be laminar, non‐Newtonian and incompressible. In one case, the effects of different model suppositions and boundary conditions were reported in detail. The fully coupled fluid and structure models were solved with the finite elements package ADINA. The vortex structure, pressure, wall shear stress (WSS), effective stress and displacement of the aneurysm wall showed large variations, depending on the morphology of the artery, aneurysm size and position. The time‐averaged WSS, effective stress and displacement at the aneurysm fundus vary between 0.17 and 4.86 Pa, 4.35 and 170.2 kPa and 0.16 and 0.74 mm, respectively, for the seven patient‐specific models of bifurcating cerebral aneurysms. Copyright © 2008 John Wiley & Sons, Ltd.     

Pulsatile Blood Flow in Anatomically Accurate Vessels with Multiple Aneurysms: A Medical Intervention Planning Application of Computational Haemodynamics

In the present study, we are investigating computationally the pulsatile flow in an anatomically accurate cerebral arterial segment exhibiting two saccular aneurysms. Our focus is on the haemodynamic patterns observed within the two aneurysms, in terms of inflow-outflow regions, emergence and disappearance of coherent structures and mixing throughout the cardiac cycle. The results obtained carry interesting features, important for thrombosis, pharmacokinetics and particularly for interventional planning for aneurysm treatment. For the latter, being the center-point of this study, we show that the two aneurysms behave in a dissimilar manner, since the blood inflow region oscillates significantly for one of them and practically does not oscillate at all for the second. This information can guide the medical interventionist in designing the optimal approach, particularly in cases where total obliteration of the aneurysm neck opening is impossible.     

Investigation of the supersonic flow around a pointed elliptical cone at an angle of attack

The picture of ideal gas flow around cones at zero and low angles of attack has been well studied by using approximate methods [1], and results for high angles of attack have been obtained mainly numerically [2–7]. At high angles of attack it is sensible to examine inviscid flow only up to some generator on the downwind side of the cone at which boundary-layer separation occurs. Hence, the domain where the flow can be considered inviscid yields the main contribution to the magnitude of the aerodynamic forces and the heat fluxes [5, 9]. A picture of the supersonic flow around a pointed elliptical cone is obtained in this paper by the numerical solution of the gasdynamics equations. The whole flow domain is computed at low angles of attack while the solution at high angles is obtained in a domain bounded by some surface of three-dimensional type [10]. The dependence of the flow parameters on the angle of attack is studied when the shock is attached to the cone apex. In contrast to a circular cone, at low angles of attack two spreading lines occur on the surface of an elliptical cone, to which the maximum pressure corresponds. As the angle of attack increases, these lines come together and merge at a certain time. At high angles of attack the flow picture is analogous to a circular cone with a pressure maximum in the plane of symmetry.     

Comparison of haemodynamics in cerebral aneurysms of different sizes located in the ophthalmic artery

Haemodynamics plays an important role in the progression and rupture of cerebral aneurysms. The temporal and spatial variations of the wall shear stress in the aneurysmal sac are hypothesized to be correlated with its growth and rupture. In addition, the assessment of the velocity field in the aneurysm dome and neck is important for the correct placement of endovascular coils. This work describes the flow dynamics in patient‐specific models of saccular aneurysms of different sizes located in the ophthalmic artery. The models were obtained from three‐dimensional rotational angiography image data and blood flow dynamics was studied under physiologically representative waveform of inflow. The three‐dimensional continuity and momentum equations for unsteady laminar flow were solved with commercial software using nonstructured fine grid sizes. The intra‐aneurysmal flows show complex vortex structures that change during one pulsatile cycle. A relation between the aneurysm aspect ratio and the mean wall shear stress on the aneurysmal sac is showed. Copyright © 2006 John Wiley & Sons, Ltd.     

Simulation of unsteady laminar flow in models of terminal aneurysm of the basilar artery

Blood flow dynamics play an important role in the pathogenesis and treatment of intracranial aneurysms. The evaluation of the velocity field in the aneurysm dome and neck is important for the correct placement of endovascular coils, in addition the temporal and spatial variations of wall shear stress in the aneurysm are correlated with its growth and rupture. The present numerical investigation describes the hemodynamics in two models of terminal aneurysm of the basilar artery. Aneurysm models with an aspect ratio of 1.0 and 1.67 were studied. Each model was subject to a steady, sinusoidal and physiologically representative waveform of inflow for a mean Reynolds number of 560. Symmetric and asymmetric outflow conditions in the branches were also studied.

The three-dimensional continuity and the Navier-Stokes equations for incompressible, unsteady laminar flow with Newtonian properties were solved with a commercial software using non structured grids with 61334 and 65961 cells for models 1 and 2, respectively. The grids were primarily composed of tetrahedral elements.

The intra-aneurysmal flow was unsteady for all input conditions and in both models, the flow always showed a complex vortex structure. The inflow and outflow zones in the aneurysm neck were determined. The wall shear stress on the aneurysm showed large temporal and spatial variations. The asymmetric outflow increased the wall shear stress in both models.     

在椭圆横截面弹体正侵彻下有限厚铝靶的破坏模式及响应特性

基于30 mm口径弹道炮平台,开展了3种不同椭圆横截面弹体在200~600 m/s撞击速度范围内正侵彻2A12铝靶的实验,获得了2A12铝靶的破坏形貌及弹体的剩余速度。在此基础上,建立了相应的数值模型,结合实验结果验证了所建模型的有效性,并系统分析了弹体横截面长短轴长度比对靶体的破坏情况及响应特性的影响。研究结果表明:弹体最大横截面面积是影响弹体剩余速度的主要因素,而弹体横截面长短轴长度比对弹体剩余速度的影响较弱;在圆形横截面弹体侵彻下靶体背部形成的花瓣大小和形状一致,空间分布均匀,而在椭圆横截面弹体侵彻下,随着弹体横截面长短轴长度比的增大,靶体背部形成的花瓣数量增加、尺寸变小,且在短轴方向的花瓣数量和靶体表面隆起高度均大于长轴方向的;靶体在圆形横截面弹体侵彻下的径向位移、径向应力和切向应力与其在椭圆横截面弹体侵彻下的显著不同,前者沿周向方向各点的变化规律基本一致,靶体处于简单的压缩状态,切向应力为零,而后者各点的应力状态与弹体横截面长短轴长度比和周向角密切相关,靶体受到压缩和剪切应力的耦合作用。     

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。     

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.     

Characteristics of air-water flow in an emptying tank under different conditions

This paper experimentally studied the features of air-water flow during the emptying of a water-filled prismatic tank with a bottom orifice under different conditions. The experiments were conducted with both circular and elliptical orifices, with and without ventilation. The evolution of bubbles, water pressure variation, and water level change with time were recorded in the experiments and analyzed. Based on the results, the evolution of bubbles could be mainly divided into three stages of formation, deformation, and decomposition. Ventilation was found important to the emptying process, with which the drainage efficiency was much higher than that under the unventilated condition. Additionally, under the unventilated condition, the drainage efficiency with the circular orifice was slightly higher than that with the elliptical orifice.     

Elliptical and circular FRP-confined concrete sections: A Mohr–Coulomb analytical model

An analytical stress–strain model is developed for predicting the compressive behavior of elliptical and circular fiber reinforced polymer (FRP)-confined concrete members. The model is based on a diagonal Poisson’s ratio formulation expressed as a function of the mechanical properties of the unconfined concrete and confining FRP jacket, the geometry of the concrete section, and the extent of internal damage in the confined concrete core. A Mohr–Coulomb yield criterion is introduced for analysis of the compressive behavior of confined concrete. Equilibrium and strain compatibility are used to obtain the ultimate compressive strength and strain of elliptical and circular FRP-confined concrete sections as a function of the effective confining stiffness of the FRP jacket. A simplified expression is derived for the FRP reinforcement ratio which precludes strain softening in elliptical and circular FRP-confined concrete sections.     

双滑移转动率模型对椭圆散粒体的适用性

首先基于椭圆颗粒接触点的运动关系推导出新的平均纯转动率（APR）表达式,并将其引入到已有的描述圆形散粒体流动特性的双滑移转动率模型（DSR²模型）中;而后采用改进的NS2D离散元程序对长短轴比例分别为1.4和1.7的椭圆颗粒堆积体进行一系列不排水单剪试验,验证以椭圆颗粒为基础的离散元方法模拟砂土流动特性的可行性及DSR²模型的合理性。数值试验及已有成果表明,以椭圆颗粒为基础的NS2D程序能够模拟砂土的流动特性,对圆形和椭圆形颗粒体系,DSR²模型均能很好地预测运动模型中转动率参量的变化情况;APR是联系连续介质力学和离散介质力学的重要参数,它将二者有机结合成一个整体。     

Lattice Boltzmann simulation of a fluid flow around a triangular unit of three isothermal cylinders

The lattice Boltzmann method is employed to simulate heat transfer in the flow past three arrangements of elliptical and circular cylinders under an isothermal boundary condition. The lattice Boltzmann equations and the Bhatnagar–Gross–Krook model are used to simulate two-dimensional forced convection at 30 ≤ Re ≤ 100 and Pr = 0.71. Pressure distributions, isotherms, and streamlines are obtained. Vortex shedding maps are observed in detail for several cases. The present results are in good agreement with available experimental and numerical data.     

Theoretical investigation of an elliptical crack in thermopiezoelectric material. Part II: Crack propagation

Propagation behavior of an elliptical crack in thermopiezoelectric material subjected to a uniform temperature is investigated in this paper. The three-dimensional strain energy density formulation is used to determine the direction of crack propagation and the shape of the initial fracture increment. It is found that the elliptical crack grows coplanarly under this particular load case but not normal to the crack front. The elliptical crack tends to become a circular one when thermal loading is applied.     

Studies on freejets from nozzles for high-speed mixing applications

In this study four different supersonic nozzles – circular, elliptical, tabbed, and radially lobed nozzles are compared experimentally for their freejet mixing performance. With the background of studies by various groups conducted on elliptical, tabbed, and radially lobed nozzles, the present paper aims at a comparative experimental study to compare their mixing performance with that of a conventional circular nozzle under identical operating conditions. The investigation is performed non-intrusively, using digital image processing of the planar Mie scattering images of the flow field. The results of these investigations reveal the superiority of the mixing performance of the lobed nozzles over conventional circular and other non-conventional nozzles. Received: 29 July 1998/Accepted: 5 November 1999     

Fluid–solid interaction simulation of flow and stress pattern in thoracoabdominal aneurysms: A patient-specific study

Thoracoabdominal aneurysm (TA) is a pathology that involves the enlargement of the aortic diameter in the inferior descending thoracic aorta and has risk factors including aortic dissection, aortitis or connective tissue disorders. Abnormal flow patterns and haemodynamic stress on the diseased aortic wall are thought to play an important role in the development of this pathology and the internal wall stress has proved to be more reliable as a predictor of rupture than the maximum diameter for abdominal aortic aneurysms; but this assumption has not been validated yet for aneurysms involving the thoracic aorta. In the present study, three patients with TAs of different maximum diameters were scanned using magnetic resonance imaging (MRI) techniques. Realistic models of the aneurysms were reconstructed from the in vivo MRI data acquired from the patients, and subject-specific flow conditions were applied as boundary conditions. The wall and thrombus were modelled as hyperelastic materials and their properties were derived from the literature. A normal descending aorta was also simulated to provide data for comparison. Fully coupled fluid–solid interaction (FSI) simulations as well as solid static simulations were performed using ADINA 8.2. The results show that the wall stress distribution and its magnitude are strongly dependent on the 3-D shape of the aneurysm and the distribution of thrombus. Maximum wall stresses in all TA models are higher than in the normal aorta, and values of maximum wall stress are not directly related to the maximum aneurysm diameter. Comparisons between the FSI and solid static simulation results showed no significant difference in maximum wall stress, supporting those previous studies which found that FSI simulations were not necessary for wall stress prediction.     

弯曲波在含非圆孔洞无限压电薄板中的散射

基于线性压电动力学理论,采用波函数展开法、保角映射以及复变函数,对含非圆孔洞无限大压电薄板弹性波的散射及动应力集中问题进行了分析,给出了其动弯矩集中系数（DMCF）的解析表达式。为说明问题,以PZT-4为例,讨论了外加电场、椭圆孔长短半轴比、椭圆孔倾角以及入射波频率对含圆孔和椭圆孔无限大压电薄板弹性波散射的影响,并分别给出了无限压电薄板开圆孔和椭圆孔动弯矩集中系数的数值结果。