Biorheology - Volume 39, issue 3-4

Authors: Laganà, K. | Dubini, G. | Migliavacca, F. | Pietrabissa, R. | Pennati, G. | Veneziani, A. | Quarteroni, A.

Article Type: Research Article

Abstract: This work was motivated by the problems of analysing detailed 3D models of vascular districts with complex anatomy. It suggests an approach to prescribing realistic boundary conditions to use in order to obtain information on local as well as global haemodynamics. A method was developed which simultaneously solves Navier–Stokes equations for local information and a non‐linear system of ordinary differential equations for global information. This is based on the principle that an anatomically detailed 3D model of a cardiovascular district can be achieved by using the finite element method. In turn the finite element method requires a specific boundary condition …set. The approach outlined in this work is to include the system of ordinary differential equations in the boundary condition set. Such a multiscale approach was first applied to two controls: (i) a 3D model of a straight tube in a simple hydraulic network and (ii) a 3D model of a straight coronary vessel in a lumped‐parameter model of the cardiovascular system. The results obtained are very close to the solutions available for the pipe geometry. This paper also presents preliminary results from the application of the methodology to a particular haemodynamic problem: namely the fluid dynamics of a systemic‐to‐pulmonary shunt in paediatric cardiac surgery. Show more

Keywords: Computational fluid dynamics, paediatric cardiac surgery

Citation: Biorheology, vol. 39, no. 3-4, pp. 359-364, 2002

Authors: Sherwin, S.J. | Doorly, D.J. | Franke, P. | Peiró, J.

Article Type: Research Article

Abstract: Building on previous studies of unsteady flow within model distal bypass grafts we analyse the near wall residence times and shear exposure in a 45 degrees anastomosis under symmetrical and symmetry breaking geometric configurations. We define residence time as the minimum time for a particle to exit a spherical region and shear exposure as a temporal integral of the Huber‐Henky‐von‐Mises criterion along a particle path over a fixed time interval. Decomposing the pulsatile cycle into four equal intervals we find that the interval of peak residence time in the host vessel is from mid‐deceleration to peak diastole and peak diastole …to mid‐acceleration. The asymmetric model is shown to have a significantly lower residence time during these intervals. Considering the shear exposure prior to the residence time evaluation we determine that a higher average shear exposure exists in the asymmetric model associated with the upstream geometry modification. Analysis of the regions of high residence time and shear exposure suggests that the “toe” region and the interface between the “heel” and bulk flow are more significant than the bed and heel region. Although the asymmetric model considered in this study reduces residence times in the host artery, the product of the measure of shear exposure and residence time is not found to be preferable. If shear exposure were to be considered as an important factor in particle activation, the findings imply that for junction optimisation, greater consideration needs to be given both to the local junction asymmetry and upstream influence on the shear history. Show more

Citation: Biorheology, vol. 39, no. 3-4, pp. 365-371, 2002

Authors: Smith, F.T.

Article Type: Research Article

Abstract: Theoretical modelling of bending and branching tube flows at medium‐to‐high flow rates is described for current industrial and biomedical projects. This mostly uses slender‐flow modelling. Much pressure loss occurs in bends, with increased swirl, large variations in velocity components and wall shear stress, skewing of the downstream motion and reduced flow rate, but the flow regime which is established shows sensitive dependence on the imposed pressure drop and entrance conditions. A small side‐branch off a mother tube produces most rapid variation in pressure and velocity near the daughter entrance, this variation now being quantifiable. A multiple branching yields large flow …rates and nonunique flow patterns, depending on the form of the imposed pressure differences. Show more

Keywords: Bends, branchings, modelling, wall shear stress, pressure, multiple branches

Citation: Biorheology, vol. 39, no. 3-4, pp. 373-378, 2002

Authors: Tadjfar, Mehran | Himeno, Ryutaro

Article Type: Research Article

Abstract: A parallel, time‐accurate flow solver is devised to study the human cardio‐vascular system. The solver is capable of dealing with moving boundaries and moving grids. It is designed to handle complex, three‐dimensional vascular systems. The computational domain is divided into multiple block subdomains. At each cross section the plane is divided into twelve sub‐zones to allow flexibility for handling complex geometries and, if needed, appropriate parallel data partitioning. The unsteady, three‐dimensional, incompressible Navier–Stokes equations are solved numerically. A second‐order in time and third‐order upwind finite volume method for solving time‐accurate incompressible flows based on pseudo‐compressibility and dual time‐stepping technique is …used. For parallel execution, the flow domain is partitioned. Communication between the subdomains of the flow on Riken's VPP/700E supercomputer is implemented using MPI message‐passing library. A series of numerical simulations of biologically relevant flows is used to validate this code. Show more

Keywords: Computational fluid dynamics, bio‐fluid mechanics, hemodynamics, parallel computations, finite volume method, arterial branches

Citation: Biorheology, vol. 39, no. 3-4, pp. 379-384, 2002

Authors: Butty, V.D. | Gudjonsson, K. | Buchel, P. | Makhijani, V.B. | Ventikos, Y. | Poulikakos, D.

Article Type: Research Article

Abstract: We are presenting computational fluid dynamics simulation results for the flow in an anatomically accurate right internal carotid artery, exhibiting two saccular aneurysms close to each other. Our study focuses on the investigation of passage times for blood cells through the two‐aneurysm malformation. We construct residence time maps that exhibit strong non‐uniformity, linked to the entry of fluid in only the first, only the second, or in both aneurysms. An entrance index is computed, showing qualitatively the regions at an arterial section upstream of the aneurysms, where cells following one of these scenarios emanate. The significance of the residence time …profiles and entry scenarios obtained is discussed with respect to thrombosis and pharmacokinetics. Preliminary evidence that the inflow–outflow patterns of the two aneurysms may be leading to particularly complex flow and to chaotic mixing is discussed. Show more

Keywords: CFD, cerebral aneurysm, residence times, basins of attraction, chaotic advection

Citation: Biorheology, vol. 39, no. 3-4, pp. 387-393, 2002

Authors: Hayashi, Hiroyuki | Yamaguchi, Takami

Article Type: Research Article

Abstract: A computational fluid dynamics study was conducted using a simplified model of the right coronary artery, which deforms with contraction of the heart. The right coronary artery was modeled using an ordinary helix, whose torsion and curvature changed in time with the contraction and dilatation of the heart which was modeled as a cylinder. In the computational result, the flow in the model right coronary artery was thought to be more affected by the change of the curvature compared to that of the torsion.

Keywords: Wall shear stress, asymmetry of contraction, vortex formation, moving boundary, computational fluid dynamics

Citation: Biorheology, vol. 39, no. 3-4, pp. 395-399, 2002

Authors: Ivankovic, A. | Karac, A. | Dendrinos, E. | Parker, K.

Article Type: Research Article

Abstract: Blood flow through arteries represents a very complex, fluid–structure interaction (FSI) problem. Strong coupling between the blood and artery is due to the relatively low stiffness of the artery compared to that of blood. Hence, the pressure exerted by the flowing blood on the artery wall can result in considerable deformations of the artery, and vice‐versa, arterial deformations can in turn affect the blood flow. In the present work, the finite volume method is employed to solve the problem where compressible fluid, representing blood, flows in healthy arteries as well as in unhealthy, i.e., partly stiffened arteries. The stiffening of …the arterial wall is assumed to be the first key stage in the development of atherosclerosis. The comparison between various deformation profiles of healthy and unhealthy arteries demonstrates significant and measurable differences, in particular in the radial direction. This is hoped to help toward establishing procedures for early diagnosis of the disease. Show more

Keywords: Finite volume (FV) method, fluid–structure interaction, blood flow, atherosclerosis

Citation: Biorheology, vol. 39, no. 3-4, pp. 401-407, 2002

Authors: Kilner, Philip J. | Yang, Guang‐Zhong | Firmin, David N.

Article Type: Research Article

Abstract: Through cardiac looping during embryonic development, human and other vertebrate hearts adopt sinuous curvatures with marked changes in direction of flow at atrial, ventricular and arterial levels. We used magnetic resonance phase velocity mapping to study flow through the hearts of resting volunteers, and Doppler ultrasound to record changes with exercise. We found asymmetric recirculation of blood during filling phases of all four heart cavities, with blood redirected appropriately for onward passage to the next cavity. Doppler traces showed that biphasic ventricular filling became rapid and monophasic on strenuous exercise. We propose that looped curvatures of the heart have fluidic …and dynamic advantages. Intra‐cavity flow appears to be asymmetric in a manner that preserves stability, and allows momentum of inflowing streams to be redirected towards rather than away from the next cavity. Direction‐change at ventricular level is such that recoil away from ejected blood is in a direction that can enhance rather than inhibit ventriculo‐atrial coupling. These factors may combine to allow a reciprocating, sling‐like, ‘morphodynamic’ mode of action become effective when heart rate and output increase with exercise. Dynamic efficiency of the looped heart may have favoured evolution of large, complex, active species characteristic of the vertebrate line. Show more

Keywords: Cardiac function, fluid dynamics, morphology, looping, asymmetry

Citation: Biorheology, vol. 39, no. 3-4, pp. 409-417, 2002

Authors: King, E. | Xu, X.Y. | Hughes, A.D. | Long, Q. | Thom, S.A. | Parker, K.H.

Article Type: Research Article

Abstract: The carotid bifurcation has been a region of particular interest due to its predilection for clinically significant atherosclerosis. It has been shown that the vessel geometry is a major determinant of the local haemodynamic properties which are believed to be associated with the location of atherosclerotic lesions. Current knowledge of the geometry of the carotid bifurcation is insufficient and restricted to basic geometric parameters. To provide some means of quantifying the degree of complexity of the 3D shape of the bifurcation, we made an initial attempt by evaluating the non‐planarity of an arterial bifurcation based upon the singular value decomposition …theorem. In this paper we present our results obtained on the right carotid bifurcations of six normal subjects, each of whom was scanned twice using the 2D time‐of‐flight MR sequence. The acquired 2D cross sectional images were processed by using our in‐house software which comprises 2D segmentation, 3D reconstruction and smoothing. The centroids of each transverse slices were determined and used as input data for the non‐planarity analysis. Our results using the singular value decomposition method have demonstrated discernible differences in non‐planarity among individuals. Comparisons with the planarity definition proposed by other investigators suggest that the singular value decomposition method offers more information about the linearity and planarity of the bifurcation. However, it is also realised that a single measure of non‐planarity can never fully characterise a bifurcation owing to the great variety of geometries. Show more

Keywords: MRI, carotid bifurcation, singular value decomposition, non‐planarity

Citation: Biorheology, vol. 39, no. 3-4, pp. 419-424, 2002

Authors: Leuprecht, Armin | Perktold, Karl | Kozerke, Sebastian | Boesiger, Peter

Article Type: Research Article

Abstract: Physiological correct modelling of blood flow through the human ascending aorta is done by combining computational fluid dynamics (CFD) and magnetic resonance imaging (MRI). This method provides a relatively new approach in the analysis and quantification of the haemodynamic variables. Velocity patterns and wall shear stress distributions occurring in the ascending aorta of an individual subject are examined. Geometrical data and inflow velocity profiles just downstream of the valve were acquired from MRI measurements. Based on the extraction of arterial cross‐sections a computer model of the time‐dependent geometrical vessel wall was generated. After surface creation the arterial lumen was filled …with an appropriate 3D finite element mesh. The mathematical description of the blood flow uses the Navier–Stokes equations applying an Arbitrary Lagrangian–Eulerian modification with respect to the time‐varying geometry with externally imposed boundary motion. The numerical approach uses our recently developed finite element solver. The computational results agree very well with the measured data. Show more

Keywords: Numerical flow study, magnetic resonance imaging, realistic modelling, wall shear stress

Citation: Biorheology, vol. 39, no. 3-4, pp. 425-429, 2002