On the effect of aortic root geometry on the coronary entry-flow after a bileaflet mechanical heart valve implant: a numerical study

The simultaneous replacement of a diseased aortic valve, aortic root and ascending aorta with a prosthesis is known as Bentall procedure (Bentall and De Bono in Thorax 23:338, 1968). This is a nowadays standard surgical approach in which the Valsalva sinuses of the aortic root are sacrificed and the coronary arteries are reconnected directly to the graft. The important function of the natural sinuses in the presence of the natural valve is well established; however, very little information is available about whether or not their presence can affect the functioning of a prosthetic bi-leaflet valve and the coronary flow. In the present work, we study the effect of the aortic root geometry on the blood flow through such devices, focusing the attention on the coronary entry-flow. Three root geometries have been considered, two mimicking the prostheses used in practice by surgeons (a straight tube, and the more recent tube with a circular pseudo-sinus), and a third maintaining the natural shape with three sinuses, obtained by Reul et?al. (J Biomech 23:181?C191, 1990) by averaging numerous angiographies of the aortic root in healthy patients. Direct numerical simulations of the flow inside the three prostheses, assumed as undeformable, under physiological pulsatile inflow conditions are presented. The dynamics of the valvular leaflets is obtained by a fully-coupled fluid?Cstructure-interaction approach and the coronary perfusion is reproduced by modulating in time an equivalent porosity, an thus the resistance, of the coronary channels. The results indicate that the sinuses do not significantly influence the coronary entry flow, in agreement with the in vivo observations of De Paulis et?al. (Eur J Cardio-thorac Surg 26:66?C72, 2004). Nevertheless, the peak pressure at the joints of the coronary arteries is smaller in the natural-like aortic root geometry. The latter also produces a further beneficial effect of a reduction in the leaflets?? angular velocity at the closure onto the valvular ring. These phenomena, if confirmed in more realistic clinical conditions, suggest that the use of a prothesis with physiologic sinuses would potentially reduce the local pressure peak, with the associated risk of post-operative bleeding and pseudo-aneurysm formation. It would also reduce the haemolysis effects caused by the red blood cells squashing between impacting solid artificial surfaces.     

Blood flow in the human ascending aorta: a combined MRI and CFD study

Blood flow through the ascending aorta of two individuals is studied numerically. Realistic flow simulation is enabled by the combination of MRI and CFD. The aim of this study is the validation of the calculated flow field and, on the other hand, a comparison between flow distal to an artificial heart valve and native flow of a healthy volunteer. Three-dimensional, time-dependent computer models of the human ascending aorta were reconstructed from three-directional data sets acquired by MRI in the subjects studied. MRI velocity measurements downstream of the aortic valve provided the inflow conditions for the computational study. The pulsatile flow is described by the ALE-modified Navier-Stokes equations with respect to the time-varying flow domain. The numerical approach applies our own developed finite-element solver. During systolic acceleration the flow patterns distal to the valves do not show major differences between the two configurations. During flow deceleration, however, a significant influence of the disturbed inflow conditions can be found in the whole segment. Using the methods proposed, simulation of blood flow in the ascending aorta of the two subjects could be successfully performed. There was good qualitative agreement of blood velocities predicted by CFD and velocity data measured by MRI. In conclusion, the approach described herein might offer a new way towards an improved assessment of detailed in vivo flow conditions and alterations of blood flow associated with heart valve prostheses in particular. Combining CFD and MRI potentially extends the quantification of hemodynamic variables in vivo at a scale beyond the resolution limit inherent to MRI.     

Slamming in marine applications

Practical slamming problems for ships and ocean structures are briefly described. Theoretical status and future challenges for water entry on an initially calm free surface, wetdeck slamming, green water and sloshing are presented. It is emphasized that slamming should be considered in the framework of structural dynamics response and integrated with the global flow analysis around a ship or ocean structure or with violent fluid motion inside a tank. Two-phase flow can give important loading and needs to be better understood. Slamming on a VLFS with shallow draft is dealt with in detail.     

制冷压缩机阀片的流体振荡可靠性分析

为了预测受流体冲击的制冷压缩机阀片可靠性，通过引入非线性流体振荡条件，建立流体结构耦合分析的边界积分方程及求解格式，采用等效均值和离差的线性化处理进行可靠度指标计算，提出了流体振荡可靠性分析边界元数值方法。针对B67-30G半封闭制冷压缩机进、排气阀片结构动力可靠性分析显示，该方法能快速、有效地预测阀片结构在流体振荡下的总体性能，为新型阀片的优化和可靠性设计提供依据。     

Application of a coupled Eulerian–Lagrangian method to simulate interactions between deformable composite structures and compressible multiphase flow

Interactions between deformable composite structures and compressible multiphase flow are common for many marine/submarine problems. Recently, there has been an increased interest in the application of composite structures in marine industry (e.g. propulsion system, ship hulls, marine platforms, marine turbines, etc) to take advantage their high stiffness to weight and strength to weight ratios, and high impact/shock resistance characteristics. It is therefore important to evaluate the performance of composite structures subject to dynamic loads. In this paper, a coupled Eulerian–Lagrangian numerical method is proposed to model the two‐dimensional (2D) or axisymmetric response of deformable composite structures subject to shock and blast loads. The method couples an Eulerian compressible multiphase fluid solver with a general Lagrangian solid solver using an interface capturing method, and is validated using analytical, numerical, and experimental results. A 2D case study is shown for an underwater explosion beneath a three‐layered composite structure with clamped ends. The importance of 2D fluid–structure interaction effects on the transient response between composite structures and compressible multiphase flow is discussed. Copyright © 2009 John Wiley & Sons, Ltd.     

Parametric analysis of domestic refrigerators using PCM heat exchanger

In the present study, numerical simulation of refrigeration cycle incorporated with a PCM heat exchanger is carried out. To this end, the refrigeration cycle without PCM has been simulated and then, the performance coefficients of the refrigerator in either with and without PCM are evaluated. The PCM heat exchanger is located in the refrigeration cycle, at a location after the condenser and before the expansion valve. The utilised PCM is N-Octadecane with fusion temperature of 27.5 °C. The simulation of heat exchanger is based on computational fluid dynamics (CFD) in which the flow inside the pipe is considered one-dimensional in the axial extension and PCM surrounding it, is considered two dimensional. Numerical simulation is carried out using MATLAB software. Simulation results show that utilizing PCM in refrigeration cycle of a refrigerator causes an improvement in the convection procedure and results a 9.58% increase in performance coefficient of refrigerator.     

BEM modeling of surface water wave motion with laminar boundary layers

This study is concerned with numerical modeling of viscous surface wave motion using boundary element method (BEM). The equations of motion for thin boundary layers at the solid surfaces are coupled with the potential flow in the bulk of the fluid, and a mixed BEM-finite difference technique is used to obtain the viscosity-related quantities such as wave damping rate, shear stress, and velocity distribution inside the boundary layer. The technique is presented for standing surface wave motion. An excellent agreement is obtained between the numerical predictions and the previous results. The extension to other free surface problems is straightforward.     

膜式燃气表内部流动数值模拟研究

采用CFD数值模拟方法对有柔性膜片的膜式燃气表内部流动进行数值模拟研究,解决了皮膜和旋转阀之间的联动关系以及旋转阀转动与皮膜运动的仿真实现等技术问题;运用滑移网格、动网格及用户自定义函数(UDF)技术实现了膜式燃气表内部流动的动态数值模拟,获得了旋转阀与阀座存在1mm间隙的情况下膜式燃气表内部流场和仪表压损数据.结果表明,数值模拟技术用于膜式燃气表这种复杂结构的内部流动研究是可行的.     

On the reformulation of topology optimization problems as linear or convex quadratic mixed 0–1 programs

We consider equivalent reformulations of nonlinear mixed 0–1 optimization problems arising from a broad range of recent applications of topology optimization for the design of continuum structures and composite materials. We show that the considered problems can equivalently be cast as either linear or convex quadratic mixed 0–1 programs. The reformulations provide new insight into the structure of the problems and may provide a foundation for the development of new methods and heuristics for solving topology optimization problems. The applications considered are maximum stiffness design of structures subjected to static or periodic loads, design of composite materials with prescribed homogenized properties using the inverse homogenization approach, optimization of fluids in Stokes flow, design of band gap structures, and multi-physics problems involving coupled steady-state heat conduction and linear elasticity. Several numerical examples of maximum stiffness design of truss structures are presented. The research is funded by the Danish Natural Science Research Council and the Danish Research Council for Technology and Production Sciences.     

Investigation on failure process and structural optimization of a high pressure letdown valve

High pressure letdown valve in direct coal liquefaction is used to adjust the flow rate of coal–oil slurry that enters into the downstream separator. Severe erosion–cavitation wear is found on the valve spool, seriously affecting the safety and reliability of unit. The majority of this paper investigates the failure process of valve spool and proposes a corresponding structural optimization via computational fluid dynamics (CFD) methodology. Three geometries of different failure states are selected as the computational domains in the numerical simulation. The Schneer–Sauer model, particle rebound-velocity model and erosion model are employed to calculate the cavitation phenomenon and erosion rates distribution. Experiments of flow rates and cavitation on valve model under different pressure drops are conducted to validate the accuracy of numerical approach. Results showed that the damage development of valve spool aggravates the erosion–cavitation wear. The maximum erosion rates are located on the top of spool head in all the three states. The erosion rates on spool arc surface are two orders of magnitude higher than that on parabolic surface. The decrease in radius of spool head reduces the intensities of erosion–cavitation wear. The numerical results are in agreement with actual failure morphologies of valve spool in different states.     

SPLASH program for three dimensional fluid dynamics with free surface boundaries

This paper describes a three dimensional computer program SPLASH that solves Navier-Stokes equations based on the Arbitrary Lagrangian Eulerian (ALE) finite element method. SPLASH has been developed for application to the fluid dynamics problems including the moving boundary of a liquid metal cooled Fast Breeder Reactor (FBR). To apply SPLASH code to the free surface behavior analysis, a capillary model using a cubic Spline function has been developed. Several sample problems, e.g., free surface oscillation, vortex shedding development, and capillary tube phenomena, are solved to verify the computer program. In the analyses, the numerical results are in good agreement with the theoretical value or experimental observance. Also SPLASH code has been applied to an analysis of a free surface sloshing experiment coupled with forced circulation flow in a rectangular tank. This is a simplified situation of the flow field in a reactor vessel of the FBR. The computational simulation well predicts the general behavior of the fluid flow inside and the free surface behavior. Analytical capability of the SPLASH code has been verified in this study and the application to more practical problems such as FBR design and safety analysis is under way.     

Experimental and computational failure analysis of a high pressure regulating valve in a chemical plant

A high pressure regulating valve for producing ethylene vinyl acetate copolymer in a chemical plant failed after three years in service. The failure analysis of valve was performed by means of visual inspection, stereo microscopy, optical microscopy, scanning electron microscopy and computational fluid dynamics (CFD).The morphology analyses showed that the craters on the valve plug and cage, especially the end of the valve plug, were mainly caused by cavitation erosion, and the ditch damage resulted from the erosion due to impingement and scouring of the high velocity flow. The corrosivity of vinyl acetate was proved to have few effects on the materials damage. The failure reasons were further demonstrated by the simulation results, which presented the distribution of the sites suffering the high velocity flow erosion and possible cavitation erosion. The computed numerical results showed a good agreement with morphology analyses.     

煤化工专用控制阀多相流流场特性研究

针对煤化工专用控制阀易磨损失效的问题,提出基于数值模拟的阀内流场分析方法.今基于可实现k-ε(Realizable k-ε)双方程湍流模型,通过有限元分析法(finite element analysis,FEA),对控制阀的多相流流场中的湍流特征进行数值模拟研究,得到湍流流场内的压力、速度的分布,并结合Preston磨削经验公式,得到控制阀内最易磨损的区域,从而为后续控制阀的结构优化设计及表面强化打下了基础.     

Pressure drop and flashing mechanisms in refrigerant expansion devices

This paper examines a novel pressure drop mechanism as well as flow choking conditions that determine mass flow rate in refrigerant expansion devices. For this study, an ideal situation is considered where an expansion device such as a short tube orifice or a thermostatic expansion valve is modeled as an ideal isentropic nozzle. In addition, a liquid with a certain initial degree of superheat is first expanded in the converging nozzle down to the exit section without any phase transition. At the exit section where the metastable liquid jet flashes to produce a complex axisymmetric two-phase flow, a shock wave may terminate the overall expansion process. The model presented here is based on experimental observations in short nozzles, where the metastable liquid in the central core undergoes a sudden phase transition in the interfacial region, giving rise to a high-speed two-phase flow. A simple 1-D analysis of the radial evaporation wave based on the theory of discontinuities from gas dynamics leads to the Chapman–Jouguet (C-J) solution. Flow choking issues are examined and numerical examples are presented for three common refrigerants: R134a, R-22, and R-600a. Results suggest that the evaporation wave may be the flow controlling mechanism in these devices.     

基于蜂群算法的等压灌装阀阀口流道结构优化

目的为了改善等压灌装阀的灌装性能,分析阀口流道结构参数对液料流场的影响,求解流道内最大压力损失、最大液料流速和最大湍动能均最小化的约束多目标优化问题。方法基于正交试验设计和Fluent流场仿真软件对灌装阀阀口流道流场进行数值模拟,并通过回归分析建立以阀口流道结构参数为自变量的最大压力损失、最大液料流速和最大湍动能的经验方程,进而建立阀口流道结构参数约束多目标优化模型,采用约束多目标人工蜂群算法对优化模型进行求解。结果流道内最大压力损失最小化、最大液料流速最小化和最大湍动能最小化这3个目标之间存在冲突,无法同时达到最优,基于多目标人工蜂群算法获得了阀口流道结构参数的最优Pareto解集。结论约束多目标人工蜂群算法能有效用于等压灌装阀阀口流道结构参数的优化。     

Bridging the length-scale gap—short fibre composite material as an example

A sequential modelling approach consisting of passing information across length scales is presented to simulate macroscopic behavior of composite materials. The modeling procedure utilizes a proper flow of information from molecular scale to macroscopic scale including material characteristics at different length scales. Both molecular dynamics and analytical/numerical methods were used in the multiscale analysis together with some experimental observations obtained from Raman microspectroscopy and X-ray microtomography. The multiscale procedure is systematically applied to short glass fibre polypropylene composite material.     

电子膨胀阀内啸叫噪声特性及发声规律的实验研究

空调系统中电子膨胀阀节流时出现的尖锐啸叫噪声严重影响用户使用舒适性,了解制冷剂流经电子膨胀阀时的啸叫噪声产生机理及其发声规律是解决上述噪声问题的关键。本文设计了可以对阀前后制冷剂状态进行调节控制并对产生的啸叫噪声进行测量分析的实验系统,得到不同工况及阀开度下的啸叫噪声发声规律。研究表明：啸叫噪声来源于阀内流体高频压力脉动引起的流体周期性振荡,其发声特性是阀内环锥形节流通道与阀腔构成的亥姆霍兹共振腔结构对共振频率附近的噪声源信号选择性放大的结果。啸叫噪声声压级主要与阀内制冷剂流速及阀开度有关,阀开度为700 pls下制冷剂速度由2.5 m/s增至3 m/s时,噪声声压级提高了21%;阀内制冷剂流速决定了流体振荡频率,阀开度决定了阀内声腔共振频率;通过改变阀腔结构以增大共振频率,使常见空调工况下阀内制冷剂冲击引起的振荡频率低于共振频率,可以有效避免啸叫噪声产生。     

Infinite boundary elements for dynamic problems of 3-D half space

In this paper, a new procedure for solving 3-D dynamic problems of unbounded foundations in the frequency domain by using BEM is studied. For simulations of wave propagations due to far field effects, a type of infinite boundary element (IBEM) is presented for modelling a 3-D regular or irregular half space. The wave type considered could be compressional, shear or a combination of the two. Through the analysis of the asymptotic behaviour of 3-D fundamental solutions for elasto dynamics, a rather feasible technique for obtaining singular integral coefficients for dynamic problems has been developed. Through the analysis of the dynamic response for a 3-D square foundation under a uniform load distribution, excellent accuracy has been achieved in agreement with previous numerical solutions. Another example–analysis of the dynamic compliance of a rigid square plate on a half space–has also shown very good results. The development of this infinite boundary element provides a powerful tool for dealing with 3-D structure foundation interaction or wave propagation problems for irregular foundations such as arch dam canyons.     

Immersed smoothed finite element method for fluid–structure interaction simulation of aortic valves

This paper presents a novel numerical method for simulating the fluid?Cstructure interaction (FSI) problems when blood flows over aortic valves. The method uses the immersed boundary/element method and the smoothed finite element method and hence it is termed as IS-FEM. The IS-FEM is a partitioned approach and does not need a body-fitted mesh for FSI simulations. It consists of three main modules: the fluid solver, the solid solver and the FSI force solver. In this work, the blood is modeled as incompressible viscous flow and solved using the characteristic-based-split scheme with FEM for spacial discretization. The leaflets of the aortic valve are modeled as Mooney-Rivlin hyperelastic materials and solved using smoothed finite element method (or S-FEM). The FSI force is calculated on the Lagrangian fictitious fluid mesh that is identical to the moving solid mesh. The octree search and neighbor-to-neighbor schemes are used to detect efficiently the FSI pairs of fluid and solid cells. As an example, a 3D idealized model of aortic valve is modeled, and the opening process of the valve is simulated using the proposed IS-FEM. Numerical results indicate that the IS-FEM can serve as an efficient tool in the study of aortic valve dynamics to reveal the details of stresses in the aortic valves, the flow velocities in the blood, and the shear forces on the interfaces. This tool can also be applied to animal models studying disease processes and may ultimately translate to a new adaptive methods working with magnetic resonance images, leading to improvements on diagnostic and prognostic paradigms, as well as surgical planning, in the care of patients.     

Robin‐Neumann transmission conditions for fluid‐structure coupling: Embedded boundary implementation and parameter analysis

Partitioned procedures are appealing for solving complex fluid‐structure interaction (FSI) problems, as they allow existing computational fluid dynamics (CFD) and computational structural dynamics algorithms and solvers to be combined and reused. However, for problems involving incompressible flow and strong added‐mass effect (eg, heavy fluid and slender structure), partitioned procedures suffer from numerical instability, which typically requires additional subiterations between the fluid and structural solvers, hence significantly increasing the computational cost. This paper investigates the use of Robin‐Neumann transmission conditions to mitigate the above instability issue. Firstly, an embedded Robin boundary method is presented in the context of projection‐based incompressible CFD and finite element–based computational structural dynamics. The method utilizes operator splitting and a modified ghost fluid method to enforce the Robin transmission condition on fluid‐structure interfaces embedded in structured non–body‐conforming CFD grids. The method is demonstrated and verified using the Turek and Hron benchmark problem, which involves a slender beam undergoing large transient deformation in an unsteady vortex‐dominated channel flow. Secondly, this paper investigates the effect of the combination parameter in the Robin transmission condition, ie, α_f, on numerical stability and solution accuracy. This paper presents a numerical study using the Turek and Hron benchmark problem and an analytical study using a simplified FSI model featuring an Euler‐Bernoulli beam interacting with a two‐dimensional incompressible inviscid flow. Both studies reveal a trade‐off between stability and accuracy: smaller values of α_f tend to improve numerical stability, yet deteriorate the accuracy of the partitioned solution. Using the simplified FSI model, the critical value of α_f that optimizes this trade‐off is derived and discussed.