Study of the closing mechanism of natural heart valves

At present, there are still some controversial considerations on the closing mechanismof natural heart valves. In this area. a lot of phenomena still remain mysterious.particularlyfor the mechanism of earlier partial closure of the valve white the blood ejection is still in itsacceleration phase. It is the purpose of this study to focus on the problem of earlier closuremechanism of heart valves by using both theoretical analysis and experimentalverifications. A certain two-dimensional mathematical model of closure with allcontributing factors and the whole operating process being considered is investigated with anew analytical method Several new conclusions were found. The main points are asfollows:I. During the opening phase, the main factor of motion of the valve is the acceleratingflow, and the decelerating flow plays a main role during most the of time of the decelerationphase. But during the quasi-steady phase of blood ejection. it is the vortex in the sinus thatcontrols the valve closure.2. For Ih     

PIV EXPERIMENTAL STUDY ON THE HEMODYNAMICS OF AORTIC VALVE UNDER VARIED TILTED ANGLES 1)

瓣叶血栓是主动脉瓣置换术后典型的继发性瓣膜疾病,血流动力学特征异常在其发展过程中至关重要.本文利用粒子图像测速 (particle image velocimetry,PIV) 系统,实验研究了主动脉瓣开口纵向轴线与升主动脉纵向轴线之间倾斜角度 ($\alpha =0^\circ$, $\alpha=5^\circ$,$\alpha =10^\circ$ 和 $\alpha =15^\circ$) 对速度、涡度和黏性剪应力分布等血流动力学特性的影响.研究结果表明：当 $\alpha =0^\circ$ 时,主动脉根部跨瓣血液流动为中心对称流动,而 $\alpha =5^\circ$,$\alpha=10^\circ$ 和 $\alpha =15^\circ$ 时跨瓣血液流动向升主动脉的左冠状动脉一侧倾斜.随着倾斜角度增大,跨瓣血液流动方向倾斜程度增加,血液流动冲击升主动脉壁,损伤内皮细胞导致血栓形成.主动脉瓣倾斜时主动脉窦血液流动速度增大,涡旋也更向主动脉窦底部运动,不利于血液从冠状动脉口流出向心肌供血.同时,主动脉根部的高涡度和高黏性剪应力区域也向升主动脉的左冠状动脉一侧倾斜,主动脉窦的高涡度区域位于主动脉窦底部、高黏性剪应力区域分布于主动脉窦壁面处.主动脉瓣存在倾斜角度时,涡度和黏性剪应力较大,特别是 $\alpha =10^\circ$ 和 $\alpha=15^\circ$,为血栓形成提供了有利环境.研究结果可为临床主动脉瓣置换术参数选择以及继发性瓣膜疾病的避免提供理论依据和技术参考.     

主动脉瓣倾斜角度血流动力学的 PIV 实验研究

瓣叶血栓是主动脉瓣置换术后典型的继发性瓣膜疾病,血流动力学特征异常在其发展过程中至关重要.本文利用粒子图像测速 (particle image velocimetry,PIV) 系统,实验研究了主动脉瓣开口纵向轴线与升主动脉纵向轴线之间倾斜角度 ($\alpha =0^\circ$, $\alpha=5^\circ$,$\alpha =10^\circ$ 和 $\alpha =15^\circ$) 对速度、涡度和黏性剪应力分布等血流动力学特性的影响.研究结果表明:当 $\alpha =0^\circ$ 时,主动脉根部跨瓣血液流动为中心对称流动,而 $\alpha =5^\circ$,$\alpha=10^\circ$ 和 $\alpha =15^\circ$ 时跨瓣血液流动向升主动脉的左冠状动脉一侧倾斜.随着倾斜角度增大,跨瓣血液流动方向倾斜程度增加,血液流动冲击升主动脉壁,损伤内皮细胞导致血栓形成.主动脉瓣倾斜时主动脉窦血液流动速度增大,涡旋也更向主动脉窦底部运动,不利于血液从冠状动脉口流出向心肌供血.同时,主动脉根部的高涡度和高黏性剪应力区域也向升主动脉的左冠状动脉一侧倾斜,主动脉窦的高涡度区域位于主动脉窦底部、高黏性剪应力区域分布于主动脉窦壁面处.主动脉瓣存在倾斜角度时,涡度和黏性剪应力较大,特别是 $\alpha =10^\circ$ 和 $\alpha=15^\circ$,为血栓形成提供了有利环境.研究结果可为临床主动脉瓣置换术参数选择以及继发性瓣膜疾病的避免提供理论依据和技术参考.      

采用瓣叶复合材料的生物心脏瓣膜的力学分析

采用微观组织结构分析及宏观复合材料分析结合的方法,分析了猪主动脉瓣的非线性复合材料性质,提出了一种适用于猪主动脉瓣的非线性复合材料本构模型,用提出的非线性复合材料本构模型,对闭合承载状态下的等厚度与变厚度几何模型的猪主动脉瓣的应力分布及变形进行了有限元数值模拟,发现：与各向同性瓣叶相比,单向增强复合材料的瓣叶不但具有较强的承载能力,而且具有较大的柔软性。     

Comparative hemodynamics in an aorta with bicuspid and trileaflet valves

Bicuspid aortic valve (BAV) is a congenital heart defect that has been associated with serious aortopathies, such as aortic stenosis, aortic regurgitation, infective endocarditis, aortic dissection, calcific aortic valve and dilatation of ascending aorta. There are two main hypotheses to explain the increase prevalence of aortopathies in patients with BAV: the genetic and the hemodynamic. In this study, we seek to investigate the possible role of hemodynamic factors as causes of BAV-associated aortopathy. We employ the curvilinear immersed boundary method coupled with an efficient thin-shell finite-element formulation for tissues to carry out fluid–structure interaction simulations of a healthy trileaflet aortic valve (TAV) and a BAV placed in the same anatomic aorta. The computed results reveal major differences between the TAV and BAV flow patterns. These include: the dynamics of the aortic valve vortex ring formation and break up; the large-scale flow patterns in the ascending aorta; the shear stress magnitude, directions, and dynamics on the heart valve surfaces. The computed results are in qualitative agreement with in vivo magnetic resonance imaging data and suggest that the linkages between BAV aortopathy and hemodynamics deserve further investigation.     

不同心排出量下主动脉瓣血流动力学的PIV实验研究

主动脉瓣发生病变时导致心排出量(cardiac output, CO)减少,而心排出量减少与主动脉瓣血流动力学耦合作用, 引发瓣膜继发性疾病.本文基于医学影像数据三维重构带有冠状动脉的主动脉根部,制备高度光滑和透明的主动脉根部实验模型, 构建体外脉动循环模拟系统,利用粒子图像测速技术(particle image velocimetry,PIV)研究冠状动脉存在时心排出量对主动脉瓣速度分布、黏性剪应力(viscous shear stress, VSS)和雷诺剪应力(Reynolds shear stress, RSS)等血流动力学的影响.研究结果表明: 冠状动脉的存在改变了主动脉窦中的涡旋运动和涡度,冠状动脉存在时流体经由冠状动脉流出, 主动脉窦中的涡旋运动逐渐消失,涡度较早开始减小. 峰值期, 中心对称流动两侧区域存在正、负高黏性剪切区域,存在冠状动脉一侧的升主动脉下游存在高雷诺剪应力区域.心排出量显著影响主动脉瓣的速度分布、VSS和RSS等血液流动和受力状况.随着心排出量增大, 冠状动脉存在时峰值期的最大速度、VSS和RSS增大, 即$CO=2.1$, 2.8, 3.5和4.2 l/min时, 最大速度分别为0.98, 1.13, 1.21和1.37 m/s, 最大VSS分别为0.87, 0.95, 0.96和1.02 N/m$^{2}$, 最大RSS分别为103.76, 116.25, 138.68和146.55 N/m$^{2}$. 心排出量较低时,主动脉瓣较低的跨瓣流动速度和黏性剪应力易导致血栓形成,研究结果可为主动脉瓣置换术提供理论参考.      

Methodological inaccuracies in clinical aortic valve severity assessment: insights from computational fluid dynamic modeling of CT-derived aortic valve anatomy

Aortic stenosis is the most common valvular heart disease. Assessing the contribution of the valve as a portion to total ventricular load is essential for the aging population. A CT scan for one patient was used to create one in vivo tricuspid aortic valve geometry and assessed with computational fluid dynamics (CFD). CFD simulated the pressure, velocity, and flow rate, which were used to assess the Gorlin formula and continuity equation, current clinical diagnostic standards. The results demonstrate an underestimation of the anatomic orifice area (AOA) by Gorlin formula and overestimation of AOA by the continuity equation, using peak velocities, as would be measured clinically by Doppler echocardiography. As a result, we suggest that the Gorlin formula is unable to achieve the intended estimation of AOA and largely underestimates AOA at the critical low-flow states present in heart failure. The disparity in the use of echocardiography with the continuity equation is due to the variation in velocity profile between the outflow tract and the valve orifice. Comparison of time-averaged orifice areas by Gorlin and continuity with instantaneous orifice areas by planimetry can mask the errors of these methods, which is a result of the assumption that the blood flow is inviscid.     

Numerical simulation of the non-Newtonian blood flow through a mechanical aortic valve

This work focuses on the comparison between Newtonian and non-Newtonian blood flows through a bileaflet mechanical heart valve in the aortic root. The blood, in fact, is a concentrated suspension of cells, mainly red blood cells, in a Newtonian matrix, the plasma, and consequently its overall behavior is that of a non-Newtonian fluid owing to the action of the cells’ membrane on the fluid part. The common practice, however, assumes the blood in large vessels as a Newtonian fluid since the shear rate is generally high and the effective viscosity becomes independent of the former. In this paper, we show that this is not always the case even in the aorta, the largest artery of the systemic circulation, owing to the pulsatile and transitional nature of the flow. Unexpectedly, for most of the pulsating cycle and in a large part of the fluid volume, the shear rate is smaller than the threshold level for the blood to display a constant effective viscosity and its shear thinning character might affect the system dynamics. A direct inspection of the various flow features has shown that the valve dynamics, the transvalvular pressure drop and the large-scale features of the flow are very similar for the Newtonian and non-Newtonian fluid models. On the other hand, the mechanical damage of the red blood cells (hemolysis), induced by the altered stress values in the flow, is larger for the non-Newtonian fluid model than for the Newtonian one.     

Scale-up of an unsteady flow field for enhanced spatial and temporal resolution of PIV measurements: application to leaflet wake flow in a mechanical heart valve

A scale-up approach is developed to enhance effective spatial and temporal resolution of PIV measurements. An analysis shows that complete similarity can be maintained for certain unsteady flows and that all types of error in PIV are either reduced or unaffected by scale-up. Implementation and results are described for flow through a mechanical heart valve (MHV), in which high resolution is necessary to advance understanding of the effects of small-scale flow structure on blood cells. With a large-scale model geometry and a low-viscosity model fluid, spatial and temporal resolutions are increased by factors of 5.8 and 118, respectively, yielding the finest resolution to date for MHV flow. Measurements near the downstream tip of a valve leaflet detect eddies as small as 400 μm shed in the leaflet wake. Impulsively started flow exhibits vortex shedding frequencies broadly consistent with the literature on flat-plate and aerofoil wakes, while the physiological unsteady flow waveform promotes 40% higher frequency at peak flow.     

A time-dependent numerical analysis of flow in a mechanical heart valve: Comparison with experimental results

There is a great need to fabricate heart valves that have similar haemodynamic properties with the natural ones. Towards this goal, we examine the dynamics of fluid flow in a mechanical heart valve with one leaflet. The fluid is incompressible and Newtonian and the leaflet is a neo-Hookean material. The Arbitrary Lagrangian Eulerian method is used to model the fluid-leaflet interaction, and the system of equations is solved using the Finite Element method. The pseudo solid approach along with a set of algebraic equations are used to deform the mesh, while care is taken to avoid remeshing of the domain, at the moment of valve closure. The computational results are compared against the experimental results, and we find an excellent agreement for the time period of valve closure, the time the valve is fully opened, and the value of the maximum valve opening angle. This study indicates that the present model is capable of describing the valve dynamics in physiological geometries.     

Intraventricular thrombus formation in the LVAD-assisted heart studied in a mock circulatory loop

Left ventricular assist devices (LVADs) are mechanical pumps that are surgically attached to the left ventricle and aorta. Clinical studies show that LVADs improve patient health and quality of life, and dramatically reduce the mortality of cardiac failure. During periods of high LVAD support, blood flow occurs entirely through the LVAD, the aortic valve is continuously closed, and the heart operates in series with the pump. Thus the normal fluid dynamics of intraventricular flow are altered and linked to the development of thrombus in both the native heart and LVAD. Our goal in this study was to simulate a patient with a recurring thrombus and quantify the variations in the flow field in the LV as the thrombus developed. Particle image velocimetry measurements of transparent silicone models were performed for a range of LVAD support conditions. Results show that the presence of a small thrombus in the LVOT creates a favorable condition for further growth, especially in the presence of high LVAD support. As the thrombus enlarges, it begins to affect the normal vortex-flow pattern, further reducing flow rate and pulsatility in the LVOT. Evaluation of vortex dynamics and stasis regions in both patients and experimental models of LVAD support yield quantitative metrics that can be used to assess the risk of thrombus and the development of strategies to reduce this risk in LVAD patients.     

Large Deborah number flows around confined microfluidic cylinders

Viscoelastic flow around a confined cylinder at high Deborah numbers is studied using microfluidic channels. By varying fluid properties and flow rates, a systematic study of the roles of elasticity and inertia is accomplished. Two new elastic flow instabilities that occur at high Deborah numbers are identified. A downstream instability of disordered and temporally varying streamlines is observed at a Deborah number above 10. This instability is a precursor to an unsteady vortex that develops upstream of the cylinder at higher Deborah numbers. Both instabilities occur at moderate Reynolds numbers but are fundamentally elastic. The size and steadiness of the upstream vortex are primarily controlled by the Deborah and the elasticity number.     

Turbulent time-events in channel with rough walls

This brief communication quantifies the time-events that contribute to the dynamics of wall-bounded flows with rough walls. Lumley’s Proper Orthogonal Decomposition (POD) methodology has been used to extract the energetic modes of the flow. We have used the concept of entropy, a representation of lack of organization in the flow, to represent the extent of spread of turbulent kinetic energy to higher modes. The rough-wall dynamics is dominated by fast activity (short time period) propagating modes and slow activity (long time period) roll modes. A single dominant timescale has been captured for all the propagating modes in flows over smooth walls; multiple dominant timescales representing various vortex shedding events are captured for rough walls. Variable-interval time averaging technique has been used to obtain the bursting frequency. The bursting frequency of rough-wall turbulence is higher compared to smooth-wall turbulence, suggesting that roughness enhances turbulence production activity. Another insightful observation for rough walls revealed by our study is that the vortex shedding frequency of roughness elements is much higher compared to the bursting frequency of rough-wall turbulence. POD provides a straightforward method to extract the natural frequency of shed vortices due to roughness, an important dynamical activity in rough-wall turbulent boundary layers.     

Flow physics of normal and abnormal bioprosthetic aortic valves

Flow physics of transvalvular flows in the aorta with bioprosthetic valves are investigated using computational modelling. For the efficient simulations of flow-structure-interaction in transvalvular flows, a simplified, reduced degree of freedom valve model is employed with a sharp interface immersed boundary based incompressible flow solver. Simulations are performed for normal as well as abnormal valves with reduced leaflet motion that models the effect of early leaflet thrombosis. The structure of the aortic jet and the hemodynamic stresses on the aortic wall are analysed to understand the hemodynamic impacts and possible long-term clinical implications of sub-clinical, reduced leaflet motion. The simulation results have shown that the reduced leaflet motion tilts the direction of aortic jet and generates stronger flow separation and re-attachment on the aortic wall downstream from the reduced motion leaflets. The modified flow pattern increases the wall pressure fluctuation and average wall shear stress on the downstream aortic wall, and results in the asymmetric oscillatory shear index distributions, which may have long-term clinical implications such as aortic wall damage and remodelling.     

Leonardo's vision of flow visualization

Leonardo's studies of cardiovascular systems, in more than 50 surviving pages from two phases of his research (around 1508-1509 and 1513), are a clear demonstration of his observational genius and progressive deduction of cardiac mechanics and the vascular system. He carried out a detailed hemodynamic study of the aortic valve motion and the role of the Sinus of Valsalva in the closure dynamics of the aortic valve, and he accurately correlated the formation of vortices with the separation of a retarded (shear) layer from the lips of the leaflets. In-vivo verification of vortex formation in the Sinus of Valsalva during the systolic phase awaited the application of modern phase-averaged magnetic resonance imaging techniques. Did Leonardo actually build the glass model he twice mentioned, thus performing the first scientific flow visualization of impulsive vortex formation or other fluid mechanical phenomena? Evidence in support of this possibility can be found in both the unusually schematic style he employed for this suite of drawings and the recent flow imaging results obtained in our laboratory through laser-based imaging techniques.     

Dynamics of vortex rings moving counter the lift

The motion of a buoyant vortex ring counter the direction of the lift of given value is experimentally investigated on a wide range of the initial velocity of the ring. The dynamics of its parameters are determined. The experimental results are compared with the calculations according to the earlier developed theoretical model. It is established in which cases the theoretical model describes the dynamics of buoyant vortex ring in motion counter the lift.     

Quantitative interpretation of vortices from a cylinder oscillating in quiescent fluid

 The instantaneous, quantitative patterns of vortices arising from sinusoidal oscillation of a cylinder in quiescent fluid are experimentally characterized for the first time using high-image-density particle image velocimetry. The near-wake does not indicate a separated layer of distributed vorticity leading to a single, large-scale vortex. Rather, for sufficiently high Reynolds number, a sequence of small-scale vorticity concentrations is formed. Agglomeration of only a fraction of the adjacent concentrations forms a larger-scale vortex. Simultaneously, vorticity concentrations of opposite sense are formed along the base (rear) of the cylinder. Streamline patterns typically indicate, however, only the larger-scale vortex; it has a circulation smaller than the total circulation of all vorticity concentrations that are not revealed by the streamlines. These observations are interpreted in the context of the effective resolution of the flow images. Received: 27 October 1995 / Accepted: 27 August 1996     

固体塑性实验室时间尺度的分子动力学模拟概述

随着超级计算机软硬件的飞速提升,基于经验势函数的分子动力学模拟在解析固体塑性的微观机制方面发挥着关键作用.但是,由于传统分子动力学基于牛顿运动方程数值积分,积分时间步长通常为飞秒量级,其模拟的时间尺度通常限于纳秒量级,从而为模拟长时间尺度固体塑性机制带来了巨大的挑战.本文从分子动力学模拟的时间尺度限制切入,介绍目前国际...     

The decay of confined vortex rings

Vortex rings are produced during the ejection of fluid through a nozzle or orifice, which occurs in a wide range of biological conditions such as blood flow through the valves of the heart or through arterial constrictions. Confined vortex ring dynamics, such as these, have not been previously studied despite their occurrence within the biological flow conditions mentioned. In this work, we investigate laminar vortex rings using particle image velocimetry and develop a new semi-empirical model for the evolution of vortex ring circulation subject to confinement. Here we introduce a decay parameter ?? which exponentially grows with increasing vortex ring confinement ratio, the ratio of the vortex ring diameter (D _VR) to the confinement diameter (D), with the relationship $\beta=4.38 \exp(9.5D_{\rm VR}/D),$ resulting in a corresponding increase in the rate of vortex ring circulation decay. This work enables the prediction of circulation decay rate based on confinement, which is important to understanding naturally occurring confined vortex ring dynamics.