Experimental investigation of steady flow in rigid models of abdominal aortic aneurysms (original) (raw)
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Experimental unsteady flow study in a patient-specific abdominal aortic aneurysm model
2011
The velocity field in a patient specific abdominal aneurysm model including the aorto-iliac bifurcation was measured by 2-D PIV. Phase averaged velocities obtained in 14 planes reveal details of the flow evolution during a cycle. The aneurysm expanding asymmetrically towards the anterior side of the aorta causes the generation of a vortex at its entrance, covering the entire aneurysm bulge progressively before flow peak. The fluid entering the aneurysm impinges on the left side of its distal end, following the axis of the upstream aorta segment, causing an increased flow rate in the left (compared to the right) common iliac artery. High shear stresses appear at the aneurysm inlet and outlet as well as along the posterior wall, varying proportionally to the flow rate. At the same regions elevated flow disturbances are observed, being intensified at flow peak and during the deceleration phase. Low shear stresses are present in the recirculation region, being two orders of magnitude smaller than the previous ones. At flow peak and during the deceleration phase a clockwise swirling motion (viewed from the inlet) is present in the aneurysm due to the out of plane curvature of the aorta.
2009
Pulsatile flow through a sinusoidal bulge in an otherwise straight circular tube is used to model the fluid mechanics within a fusiform abdominal aortic aneurysm. Three-dimensional flow is computed using a high-order spectral-element/Fourier method, driven by an anatomically realistic heartbeat waveform. Model dimensions and parameters are chosen to describe human abdominal aortic aneurysms considered both low and high risk in terms of their likelihood of rupture. A Reynolds number of 330, a Womersley number of 10.7, and aneurysm length and diameter ranges of 2.9-5.2 and 1.3-2.1 times the vessel diameter, respectively, are investigated. Variation in wall shear stress with both time and as a function of aneurysm dimension is computed. From computations on a bulge with maximum diameter approximately twice the undilated tube diameter, the flow is found to be naturally three-dimensional under conditions consistent with the human abdominal aorta. However, the dominant feature of the flow remains an axisymmetric vortex ring, which is generated at the proximal end of the bulge during systole. Both three-dimensional flow and non-uniformity in azimuthal wall-shear-stress distribution are most pronounced in the vicinity of the distal end of the bulge during the resting phase of the heartbeat. The axial distribution of wall shear stress scales approximately with the length of the bulge. The flow is sensitive to changes in the bulge diameter: a bulge with maximum diameter 1.9 times the vessel diameter invokes significantly more complex dynamics than a modest bulge of 1.3 diameters.
In-Vitro Simulation of the Blood Flow in an Axisymmetric Abdominal Aortic Aneurysm
Applied Sciences, 2019
We investigated the blood flow patterns and the hemodynamics associated with an abdominal aortic aneurysm detected in an in vitro measurement campaign performed in a laboratory model of an aneurysm with rigid walls and an axisymmetric shape. Experiments were run in steady flow conditions and by varying the Reynolds number in the range 410 < Re < 2650. High spatial and temporal resolution 2D optical measurements of the velocity field were obtained through a particle tracking technique known as Hybrid Lagrangian Particle Tracking. Conversely to classical Particle Image Velocimetry, both the fluid particle trajectories and the instantaneous and time-averaged velocity fields are provided without constraints on the grid size and very close to the vessel boundary. All the most relevant quantities needed to investigate the flow features were evaluated, and in particular, we focused on the wall shear stress distribution both in the healthy aortic portion and within the aneurysm. Resul...
Dynamics of pulsatile flow through model abdominal aortic aneurysms
Journal of Fluid Mechanics, 2014
To contribute to the understanding of flow phenomena in abdominal aortic aneurysms, numerical computations of pulsatile flows through aneurysm models and a stability analysis of these flows were carried out. The volume flow rate waveforms into the aneurysms were based on measurements of these waveforms, under rest and exercise conditions, of patients suffering abdominal aortic aneurysms. The Reynolds number and Womersley number, the dimensionless quantities that characterize the flow, were varied within the physiologically relevant range, and the two geometric quantities that characterize the model aneurysm were varied to assess the influence of the length and maximal diameter of an aneurysm on the details of the flow. The computed flow phenomena and the induced wall shear stress distributions agree well with what was found in PIV measurements by Salsac et al. (J. Fluid Mech., vol. 560, 2006, pp. 19–51). The results suggest that long aneurysms are less pathological than short ones, ...
CFD and PTV Steady Flow Investigation in an Anatomically Accurate Abdominal Aortic Aneurysm
Journal of Biomechanical Engineering-transactions of The Asme, 2009
There is considerable interest in computational and experimental flow investigations within abdominal aortic aneurysms (AAAs). This task stipulates advanced grid generation techniques and cross-validation because of the anatomical complexity. The purpose of this study is to examine the feasibility of velocity measurements by particle tracking velocimetry (PTV) in realistic AAA models. Computed tomography and rapid prototyping were combined to digitize and construct a silicone replica of a patient-specific AAA. Three-dimensional velocity measurements were acquired using PTV under steady averaged resting boundary conditions. Computational fluid dynamics (CFD) simulations were subsequently carried out with identical boundary conditions. The computational grid was created by splitting the luminal volume into manifold and nonmanifold subsections. They were filled with tetrahedral and hexahedral elements, respectively. Grid independency was tested on three successively refined meshes. Velocity differences of about 1% in all three directions existed mainly within the AAA sack. Pressure revealed similar variations, with the sparser mesh predicting larger values. PTV velocity measurements were taken along the abdominal aorta and showed good agreement with the numerical data. The results within the aneurysm neck and sack showed average velocity variations of about 5% of the mean inlet velocity. The corresponding average differences increased for all velocity components downstream the iliac bifurcation to as much as 15%. The two domains differed slightly due to flow-induced forces acting on the silicone model. Velocity quantification through narrow branches was problematic due to decreased signal to noise ratio at the larger local velocities. Computational wall pressure and shear fields are also presented. The agreement between CFD simulations and the PTV experimental data was confirmed by three-dimensional velocity comparisons at several locations within the investigated AAA anatomy indicating the feasibility of this approach. Fig. 2 Overview of the PTV setup. Panel "a…: Photograph of the image acquisition system. Its main components are labeled as "1… camera, "2… four way splitter prism, "3… mirrors and "4… optical rail. Panel "b…: Photographic output showing the four different perspectives.
Computer methods in …, 2008
Abdominal aortic aneurysm (AAA) rupture is the clinical manifestation of an induced force exceeding the resistance provided by the strength of the arterial wall. This force is most frequently assumed to be the product of a uniform luminal pressure acting along the diseased wall. However fluid dynamics is a known contributor to the pathogenesis of AAAs, and the dynamic interaction of blood flow and the arterial wall represents the in vivo environment at the macro-scale. The primary objective of this investigation is to assess the significance of assuming an arbitrary estimated peak fluid pressure inside the aneurysm sac for the evaluation of AAA wall mechanics, as compared with the non-uniform pressure resulting from a coupled fluid -structure interaction (FSI) analysis. In addition, a finite element approach is utilised to estimate the effects of asymmetry and wall thickness on the wall stress and fluid dynamics of ten idealised AAA models and one non-aneurysmal control. Five degrees of asymmetry with uniform and variable wall thickness are used. Each was modelled under a static pressure-deformation analysis, as well as a transient FSI. The results show that the inclusion of fluid flow yields a maximum AAA wall stress up to 20% higher compared to that obtained with a static wall stress analysis with an assumed peak luminal pressure of 117 mmHg. The variable wall models have a maximum wall stress nearly four times that of a uniform wall thickness, and also increasing with asymmetry in both instances. The inclusion of an axial stretch and external pressure to the computational domain decreases the wall stress by 17%.
ZAMM - Journal of Applied Mathematics and Mechanics / Zeitschrift für Angewandte Mathematik und Mechanik, 2018
In this study, we develop a physiologic internal pressure and wall stress analysis procedure and apply it to a patient-specific abdominal aortic aneurysm model. Timedependent pressure loading of the inner vessel wall was experimentally measured in a 3D printed aneurysm phantom. The results were used as boundary conditions for finite element calculations of von Mises stresses throughout the AAA model over the cardiac cycle. A nonlinear hyperelastic constitutive law with parameters based on biaxial stress-deformation data from aneurysmal tissue samples was used to describe the mechanical behavior of the aneurysm wall. The internal pressure was found to be fairly spatially uniform (within 10%) over most of the cardiac cycle, but average internal pressure varied by more than a factor of two between systole and diastole. The aneurysm wall stress was highly spatially nonuniform. The highest value of von Mises stress was localized in a small area within the aneurysm bulge and remained in the same place throughout the cardiac cycle, suggesting that this area was the most likely point of rupture. Large variations in wall stress over the cardiac cycle suggest calculations that assume steady flow are a poor approximation for physiological stresses.
Journal of Biomechanics, 2014
The aim of this work is to develop a unique in vitro set-up in order to analyse the influence of the shear thinning fluid-properties on the flow dynamics within the bulge of an abdominal aortic aneurysm (AAA). From an experimental point of view, the goals are to elaborate an analogue shear thinning fluid mimicking the macroscopic blood behaviour, to characterise its rheology at low shear rates and to propose an experimental device able to manage such an analogue fluid without altering its feature while reproducing physiological flow rate and pressure, through compliant AAA. Once these experimental prerequisites achieved, the results obtained in the present work show that the flow dynamics is highly dependent on the fluid rheology. The main results point out that the propagation of the vortex ring, generated in the AAA bulge, is slower for shear thinning fluids inducing a smaller travelled distance by the vortex ring so that it never impacts the anterior wall in the distal region, in opposition to Newtonian fluids. Moreover, scalar shear rate values are globally lower for shear thinning fluids inducing higher maximum stress values than those for the Newtonian fluids. Consequently, this work highlights that a Newtonian fluid model is finally inadequate to obtain a reliable prediction of the flow dynamics within AAA.
Fluid-structure interaction in abdominal aortic aneurysms: structural and geometrical considerations
International Journal of Modern Physics C, 2014
Rupture of the abdominal aortic aneurysm (AAA) is the result of the relatively complex interaction of blood hemodynamics and material behavior of arterial walls. In the present study, the cumulative e®ects of physiological parameters such as the directional growth, arterial wall properties (isotropy and anisotropy), iliac bifurcation and arterial wall thickness on prediction of wall stress in fully coupled°uid-structure interaction (FSI) analysis of¯ve idealized AAA models have been investigated. In particular, the numerical model considers the heterogeneity of arterial wall and the iliac bifurcation, which allows the study of the geometric asymmetry due to the growth of the aneurysm into di®erent directions. Results demonstrate that the blood pulsatile nature is responsible for emerging a time-dependent recirculation zone inside the aneurysm, which directly a®ects the stress distribution in aneurismal wall. Therefore, aneurysm deviation from the arterial axis, especially, in the lateral direction increases the wall stress in a relatively nonlinear fashion. Among the models analyzed in this investigation, the anisotropic material model that considers the wall thickness variations, greatly a®ects the wall stress values, while the stress distributions are less a®ected as compared to the uniform wall thickness models. In this regard, it is con¯rmed that wall stress predictions are more in°uenced by the appropriate structural model than the geometrical considerations such as the level of asymmetry and its curvature, growth direction and its extent.