圆管狭窄下游湍流场流动分离区流体力学实验模型的设计和调试

研究旨在建立和调试体外圆管狭窄模型 ,使之能够配合粒子成像流速仪 (PIV)进行狭窄下游流动分离区速度、湍流切应力的检测和压力传感器的压力检测。采用粒子成像流速仪和压力传感器 ,对模型狭窄下游定常流湍流场速度、湍流切应力和压力进行定量检测 ,初步认识流动分离区速度、切应力和压力的分布特征。实验模型能够较好地配合 PIV和压力传感器进行流动分离区速度、湍流切应力和压力的定量检测 ;圆管狭窄下游流动分离区边壁局部存在明显的低速度、低压力和低切应力分布。     

冠状动脉移植管的血流动力学数值模拟

采用有限元数值计算的方法，模拟了冠状动脉搭桥术中移植管内的生理流动。计算模型包括了冠状动脉狭窄，并考虑了由于移植管直径大于冠状动脉直径而在两者缝合时移植管的变形。计算结果分析了缝合区附近的流场、二次流、壁面切应力在心动周期内的时空分布情况。计算结果表明，在缝合前端下游，存在一个低切应力、高切应力梯度的区域，在缝合区底部存在一个高切应力、高切应力梯度的区域。这两个区域都是内皮细胞增生并造成移植管术后再阻塞的危险部位。     

锁骨下动脉、颈总动脉和椎动脉分叉处血流动力学数值模拟

目的:探讨锁骨下动脉、颈总动脉和椎动脉分叉处的血流动力学特性,分析该处发生血管狭窄引起大脑供血不足的 血流动力学原因。方法:采用内蒙古民族大学附属医院神经内科提供的CT数据,应用医学建模软件MIMICS20.0将患者 二维CT数据进行三维血管重建,经过网格划分及边界条件设置后导入计算流体力学软件FLUENT14.5中。计算和分析 不同血液入口速度的锁骨下动脉、颈总动脉和椎动脉分叉处的血流动力学特性。结果:在血液入口速度不同的情况下,锁 骨下动脉、颈总动脉和椎动脉分叉处的血液流场分布、血液压力分布和血管壁面切应力分布有显著变化。在血液入口速 度增大时,锁骨下动脉分叉处和颈总动脉分叉处的血液流速快、血管壁压力大,颈总动脉内侧血管壁面切应力大,但锁骨 下动脉分叉处和颈总动脉分叉处血管壁面切应力数值和变化幅度小,属于低切应力区。结论:通过血流动力学数值模拟 研究,分析锁骨下动脉、颈总动脉和椎动脉分叉处易发生粥样斑块病变导致大脑供血不足的血流动力学原因。     

管径比对全阻塞动脉旁路移植流场影响的数值研究

目的 研究管径比对全阻塞动脉旁路移植流场的影响,为指导动脉旁路移植手术,减少术后再狭窄提供理论依据。方法 采用数值方法研究5种不同移植管与主血管的管径比对全阻塞情况下动脉旁路移植流场的影响,分析速度、二次流、壁面切应力和壁面切应力梯度等血流动力学参数的分布及其随管径比增大的改变。同时,为表明本文所采用模型的合理性,针对目前常使用的两类模型,比较在管径比1.0情况下全阻塞完整模型（Model A）、全阻塞局部模型（Model B）和75％狭窄完整模型（Model C）之间的血流动力学差异。结果 Model A和Model C的血流动力学特性是完全不同的;移植管顶部截面内的速度分布对下游吻合处的主血管底部壁面切应力的影响是显著的,最大相差达79％。大管径比时,主血管底部的壁面低切应力区较大,但壁面切应力分布均匀,壁面切应力梯度较小。而小管径比时,主血管底部的壁面低切应力区较小,但壁面切应力梯度较大。结论 采用整体模型单独研究全阻塞情况下的管径比对流场的影响是有必要的。管径比对全阻塞动脉旁路移植的流场具有显著影响,采用大管径比进行动脉旁路移植将有助于缓解吻合口处由于再狭窄而产生的阻塞。     

附带局部突起的主动脉弓动脉瘤的血流动力学仿真

目的：为了弄清楚顶部附带局部突起的主动脉弓动脉瘤的血流动力学特征，因为针这种动脉瘤的血流动力学目前还较少有人研究。方法：建立了理想化的动脉瘤模型。利用计算流体力学的方法对模型中的生理性血液流动进行了仿真。结果：对流动情形、压力和壁面切应力分布进行了分析，以便评价血流动力学对动脉瘤的发展和破裂的影响。来自动脉的血流对下游瘤口和瘤顶局部突起的冲击较大。瘤顶局部突起区域的压力较高。在瘤口和突起口部位的局部壁面切应力比其他地方的要高。结论：下游瘤口和瘤顶局部突起部位是动脉瘤扩展和破裂的危险区域。     

Y型血管血流动力学边界元分析

目的：通过数值计算,判断Y型动脉血管中,血流动力学特性对分叉处粥样斑块病变产生和发展的影响。方法：利用边界元方法[4,5],计算了Y型动脉血管,主管病变前后的血液流场、血管壁切应力、压力等血液流体动力学特性,通过对计算结果的分析和比较对粥样病变产生和发展的原因做出了分析。边界元方法由于只在边界离散时作了近似,因而计算精度较高,对于象分叉血管这类复杂边界问题,有较强的适应性。结果：计算结果显示,分叉处管壁切应力明显大于主管壁切应力,说明了分叉处易产生粥样斑块的流体动力学原因;而病变的产生使血管腔变窄,病变斑块顶部血流速度、切应力变大,上、下游血流速度、切应力变小,说明了粥样斑块变厚和附壁延伸的流体动力学原因[7];另外,病变前后血管壁压力的计算结果显示,病变的产生对动脉血压有一定的影响。结论：通过对Y型分叉血管血液流体动力学特性的计算,进一步说明,边界元方法对分叉血管,以及分叉处有病变血管,这类复杂边界问题的计算,方便、快捷、精度高、节约机时,可为生物流体力学的深入研究提供一种可靠、有效的方法[8,9]。     

黏附于血管壁表面的白细胞在稳定剪切流动下发生变形的生物力学模型

白细胞与血管表面的黏附是重要的生物医学工程问题，引起了学者们的广泛研究。我们用复合液滴来模拟黏附于血管表面的白细胞，根据二维计算流体动力方法研究了流体切应力作用下白细胞黏附引起的压力分布。同时，通过引入“变形指数”的概念，研究稳定剪切流动下白细胞变形的生物力学特性，数值结果表明：（1）随着初始接触角，毛细血管数，外界流场雷诺数的增大，细胞的变形也增大，而细胞浆比细胞核更易于变形，表明细胞核更能耐受切流动；（2）当切应力增大到一定值时，细胞不能进一步变形，变形指数达到峰值；（3）压力分布曲线表明，在细胞的下游形成一个高压区，提供促使细胞受力达到平衡的升力，从而阻止了细胞的进一步变形，我们关于细胞核变形的结果有助于理解白细胞如何将外界流体作用力（如切应力）等力学信号向核内转导的生物力学机理。     

血液流体设备高剪切应力内流特性与能量熵产损失

血液流体设备普遍存在过度的剪切应力血液损伤与水力效率损失,这制约其实现安全、高效的长期稳定运行,探明高剪切应力下的内流特性与能量熵产损失可为设备研究与优化提供有力支持。对此,设计建立可模拟出高剪切应力流动的喷管模型,采用计算流体力学数值模拟与粒子图像测速可视化实验的研究方法,重点分析内流特性、剪切应力和能量熵产损失。研究产生高剪切应力和能量熵产损失的相关流动特征,解析熵产损失与剪切应力的关联特性。研究结果认为：流速变化梯度是产生高剪切应力的主要因素,局部高流速并不与剪切应力直接关联。高剪切应力流场中能量损失主要来源于湍流熵产,在主流段湍流熵产占总熵产的97.7%,主要分布在涡旋与流动剧烈变化的区域。流场能量熵产与剪切应力在分布状态与变化趋势上具有明显的关联一致性,抑制流场速度变化梯度可降低流动能量损失与剪切应力血液损伤。     

左主干分叉病变严重程度的血流动力学功能性评价

目的无保护左主干分叉病变的治疗方式选择很大程度上依赖于病变的严重程度。SYNTAX评分是基于冠脉造影图像对冠脉病变严重程度进行评价的方法,是一种完全解剖学的评价,缺少功能性的分析。本文以总灌注量作为判断病变严重程度的参考量,从血流动力学的角度对不同类型分叉病变严重程度做出评价。方法采用患者三维模型(three-dimensional,3D)耦合个性化的集中参数模型(lumped parameter model,LPM,0D)对不同的分叉病变进行多尺度数值仿真计算,提取不同分支的流量、分叉区域的壁面切应力(wall shear stress,WSS)、壁面切应力振荡指数(oscillatory shear index,OSI)3个血流动力学参数,通过对比这些参数,从血流动力学的角度对不同类型分叉病变严重程度做出分析。结果分支(回旋支)是否病变对总灌注量是有影响的,当分支存在病变时,总灌注量会降低,与SYNTAX评分中对分叉病变严重程度的分类是一致的。从恶化风险(WSS,OSI)来看,当左主干存在狭窄时,分叉区域平均WSS相对比较大,OSI并没有呈现出一定的规律。结论分支存在狭窄的左主干分叉病变比分支不存在狭窄的病变总灌注量更小,心肌缺血程度更严重,狭窄进一步恶化并无明显差异。     

双分叉动脉血流动力学特性的边界元分析

目的通过数值计算判断血流动力学特性的突变对双分叉动脉血管病变发展的影响。方法利用边界元方法计算了一类双分叉侧支型动脉血管主支管病变前后的血液流场、血管壁切应力、压力等血液流体动力学特性,并作了分析和比较,对病变产生、发展和加剧的流体动力学原因做出了合理的解释;通过对病变前后分支管壁切应力和压力的计算结果分析,对主支管的病变是否会影响分支管的血液流体动力学特性做出了判断。结果主支管病变处由于血管腔变窄血液流速明显变大、切应力变大,主支管病变或病变加剧对分支管血液流场分布及切应力并无明显影响,而使分支管壁血压升高。结论血液的流体动力学特性对动脉粥样斑块的形成、发展、加剧、破裂有一定的影响,双分叉动脉主支管病变的加重极易导致分支管分叉处粥样斑块病变的出现。     

Importance of collateral vessels in aortic coarctation: computer simulation at rest and exercise using transmission line elements

Coarctation of the aorta causes arterial hypertension in the upper body and a low blood pressure downstream. Collateral blood vessels compensate by reducing the downstream pressure drop. To study the effect of various coarctation and collateral properties, we designed a computer model of the arterial circulation. The model contains a flow source and a library of subroutines for the lines and connectors. Distributed friction and wall viscoelasticity effects are included. Computer simulation was performed, using published values for vessel dimensions, in an arterial model with a coarctation and one lumped collateral. Rest and two levels of exercise (by increased heart rate) were studied. Without a collateral, we found the downstream pressure of the model was extremely dependent on the size of the coarctation. A collateral vessel reduced the pressure difference between the up- and downstream circulations. For a severe coarctation, the length and the diameter of the collateral were the main factors determining the downstream pressure and flow, whereas wall stiffness of the collateral had little influence. The relationship between mean pressure drop and cardiac output in coarctation was also dependent on the peripheral resistance in different flow beds, especially during exercise.     

Importance of collateral vessels in aortic coarctation: computer simulation at rest and exercise using transmission line elements

Coarctation of the aorta causes arterial hypertension in the upper body and a low blood pressure downstream. Collateral blood vessels compensate by reducing the downstream pressure drop. To study the effect of various coarctation and collateral properties, we designed a computer model of the arterial circulation. The model contains a flow source and a library of subroutines for the lines and connectors. Distributed friction and wall viscoelasticity effects are included. Computer simulation was performed, using published values for vessel dimensions, in an arterial model with a coarctation and one lumped collateral. Rest and two levels of exercise (by increased heart rate) were studied. Without a collateral, we found the downstream pressure of the model was extremely dependent on the size of the coarctation. A collateral vessel reduced the pressure difference between the up- and downstream circulations. For a severe coarctation, the length and the diameter of the collateral were the main factors determining the downstream pressure and flow, whereas wall stiffness of the collateral had little influence. The relationship between mean pressure drop and cardiac output in coarctation was also dependent on the peripheral resistance in different flow beds, especially during exercise.     

中央分流术中血管弹性壁和刚性壁对血管中血流动力学的影响

目的 通过数值模拟仿真研究中央分流手术（central shunt, CS）的血流动力学环境,并分别研究弹性与刚性血管壁条件对其血管内血流动力学参数分布的影响。方法 建立两个理想化的CS搭桥模型,其中一个假设为刚性血管壁,另一个为弹性血管壁。利用有限元方法进行数值计算,其中弹性血管壁模型采用流固耦合方法。结果 两个模型中的流速和压力分布总体大致相同。刚性血管壁模型中大约有68.9%血液从主动脉分流进入肺动脉中,弹性血管壁模型中该值增加到了70%。弹性模型和刚性模型中搭桥血管两端的压降分别为7.668 8 kPa和7.222 3 kPa。弹性模型中搭桥管各处的横截面积有一定变化,最大变化率约为2.2%,出现在近心端吻合口处。提取两个模型中的5个关键区域进行壁面切应力比较,其数值差别最多约为16.1%。结论 总体来说两个模型的血液流动形态没有大的改变;血管的弹性因素轻微影响了流量的分布和搭桥管两端的压降;搭桥管上血管的弹性对近心段吻合口处的影响高于对远心端吻合口处的影响。在CS术治疗法洛四联症的数值模拟仿真中血管壁为刚性这一假设是可以接受的,而流固耦合的数值模拟将得到更为可信的仿真结果。     

A multi-dimensional approach for describing internal bleeding in an artery: implications for Doppler ultrasound guiding HIFU hemostasis

Doppler ultrasound has shown promise in detecting and localizing internal bleeding. A mathematical approach was developed to describe the internal bleeding of the injured artery surrounded by tissue. This approach consisted of a two-dimensional (2D) model describing the injured vessel and a one-dimensional model (1D) mimicking the downstream of the vessel system. The validity of this approach was confirmed by both the numerical simulation and in vivo measurement of a normal porcine femoral artery. Furthermore, the artery was injured using a 16-gauge needle to model a penetrating injury. The velocity waveform at the puncture site was modeled and compared with those at the upstream and downstream of the artery. The results demonstrated that there was a significant increase in magnitude and a phase lag for the peak systolic velocity at the injury site. These results were qualitatively in agreement with the in vivo experiment. Flow turbulence indicated by this approach was also observed in a color Doppler image in the form of a checkered color pattern. This approach might be useful for quantitative internal bleeding detection and localization. Also, the phase lag of the peak systolic velocity was indicated to be potential in the application of internal bleeding detection.     

Numerical analysis and experimental observation of guidewire motion in a blood vessel model

We have developed a computer-based system to simulate a guidewire in blood vessels for surgical planning, intra-operative assistance, and to facilitate the design of new guidewires. In this study, we compared simulation results with experimental results for validation of the simulation system. First, we inserted a commercial guidewire into a poly (vinyl alcohol) hydrogel blood vessel model using a two-axis automatic stage and measured the position of the guidewire tip and the contact force between the guidewire and the vessel. The experimental apparatus can be used not only for the validation of numerical analyses, but also as a simulation system. Second, similarly to the experiment, the motion of the guidewire in the blood vessel model was calculated when the proximal part of the guidewire model was pushed and twisted. The model of the guidewire is constructed with viscoelastic springs and segments, and the proximal part of the guidewire model is constrained by the fixed catheter model. Collisions between the guidewire and the vessel are calculated, and the contact forces are determined according to the stiffness of the vessel wall. The same tendency was seen in the trajectories and the contact force of both the experimental and simulated guidewire tips.     

A predictive study of the mechanical behaviour of coronary stents by computer modelling

Intravascular stents are small tube-like structures expanded into stenotic arteries to restore blood flow perfusion to the downstream tissues. The stent expansion is an important factor to define the effectiveness of the surgical procedure: it depends on the stent geometry and includes large displacements and deformations, geometric and material non-linearity. Numerical analyses seem appropriate to study such a complex behaviour after a free stent expansion. In this study the finite element method (FEM) was applied to a new generation coronary stent. Results from computations were compared with those from a laboratory experiment in terms of radial expansion and elastic recoil. By means of a scanning electronic microscopy the area of plastic deformation were also detected and compared with those obtained in the numerical simulation. Matching between the different measurements was quite satisfactory even if some discrepancies were present due to the absence of the balloon in the numerical model.     

Hemodynamic effects of the geometric dimensions of graft vessels in coronary artery bypass graft models.

The objectives of this investigation are to evaluate the rheologic properties in atherosclerotic disease treated with the various coronary artery bypass graft (CABG) models by numerical analysis, we used four different CABG models for the assessment of spatial fluctuation in wall shear stress, pressure variation and mass flow rate with Carreau model and Navier-Stokes equation. Wall shear stress was higher in a naturally tapered model (model 1) and a constant (non-tapered) diameter of the graft vessel the same as the distal LAD (model 4) than in others. Pressure variation along the native coronary artery and graft vessels was higher in a model 4, model 1 than in a reverse tapering model (model 2) and a constant diameter of the graft vessel the same as the proximal LAD (model 3). The mass flow rate of the distal part (kg/sec,.m(o)) was the highest in model 3. This study suggests that in vitro spatial simulation following CABG revealed that small caliber or tapered graft vessels have adverse hemodynamic effects on the native and graft vessels. By this technique it is possible to simulate the optimal distribution of local hemodynamic variables in patients treated with CABG, also to minimize the degeneration of graft vessel.     

Fundamental Study of Ultrasonic-Measurement-Integrated Simulation of Real Blood Flow in the Aorta

Acquisition of detailed information on the velocity and pressure fields of the blood flow is essential to achieve accurate diagnosis or treatment for serious circulatory diseases such as aortic aneurysms. A possible way to obtain such information is integration of numerical simulation and color Doppler ultrasonography in the framework of a flow observer. This methodology, namely, Ultrasonic-Measurement-Integrated (UMI) Simulation, consists of the following processes. At each time step of numerical simulation, the difference between the measurable output signal and the signal indicated by numerical simulation is evaluated. Feedback signals are generated from the difference, and numerical simulation is updated applying the feedback signal to compensate for the difference. This paper deals with a numerical study on the fundamental characteristics of UMI simulation using a simple two-dimensional model problem for the blood flow in an aorta with an aneurysm. The effect of the number of feedback points and the feedback formula are investigated systematically. It is revealed that the result of UMI simulation in the feedback domain rapidly converges to the standard solution, even with usually inevitable incorrect upstream boundary conditions. Finally, an example of UMI simulation with feedback from real color Doppler measurement also shows a good agreement with measurement.