Valérie Deplano

Experimental and numerical studies on the starting effect on the secondary flow in a bend

A numerical and experimental modelling study was carried out in a curved tube to analyse the behaviour of unsteady flows in a bend. Based on a test bench, with no mechanical disturbances, the flow behaviour was observed using fluorescein injection. Velocity measurements were performed using hot-film anemometry. In addition, a finite volume method was used to perform three-dimensional unsteady numerical simulations. Womersley parameter values between 8 and 21 and Dean number values between 110 and 420 were used to assess the parameters affecting the flow behaviour. Secondary motions were observed, experimentally and numerically, showing the complexity of the flow patterns. The initiation and subsequent development are explained quantitatively. Based on our analysis of the starting effect, the secondary patterns were found to be highly dependent on both the initial conditions and the flow waveforms.

Numerical and experimental study of blood flow through a patient-specific arteriovenous fistula used for hemodialysis

Arteriovenous fistula (AVF) pathologies related to blood flow necessitate valid calculation tools for local velocity and wall shear stress determination to overcome the clinical diagnostic limits. To illustrate this issue, a reconstructed patient-specific AVF was investigated, using computational fluid dynamics (CFDs) and particle image velocimetry (PIV). The aim of this study was to validate the methodology from medical images to numerical simulations of an AVF by comparing numerical and experimental data. Two numerical grids were presented with a refinement difference of a factor of four. A mold of the same volume was created and mounted on an experimental bench with similar boundary conditions. The patients acquired echo D006Fppler flow waveform was injected at the arterial inlet. Experimental and numerical velocity vector cartography qualitatively produced similar flow fields. Quantification with a point-to-point approach thoroughly investigated the velocity profiles using the mean difference between both results. The finest mesh generated CFD results with a mean percentage of the difference in velocity magnitude, taking the PIV as reference, did not exceed 10%. At specific zones, the coarse mesh required adaptive meshing to improve fitting with experimental data. Meshing refinement was necessary to improve velocity accuracy at wide diameters and wall shear stress at narrow diameters. Provided that these criteria were properly respected, we show through this difficult example the validity of using CFD to properly describe flow patterns in image-based reconstructed blood vessels.

Validation of a numerical 3-D fluid―structure interaction model for a prosthetic valve based on experimental PIV measurements

A numerical 3-D fluid-structure interaction (FSI) model of a prosthetic aortic valve was developed, based on a commercial computational fluid dynamics (CFD) software program using an Arbitrary Eulerian Lagrangian (ALE) formulation. To make sure of the validity of this numerical model, an equivalent experimental model accounting for both the geometrical features and the hydrodynamic conditions was also developed. The leaflet and the flow behaviours around the bileaflet valve were investigated numerically and experimentally by performing particle image velocimetry (PIV) measurements. Through quantitative and qualitative comparisons, it was shown that the leaflet behaviour and the velocity fields were similar in both models. The present study allows the validation of a fully coupled 3-D FSI numerical model. The promising numerical tool could be therefore used to investigate clinical issues involving the aortic valve.

New insights into the understanding of flow dynamics in an in vitro model for abdominal aortic aneurysms

An in vitro dynamics set-up of the flow in a compliant abdominal aortic aneurysm (AAA) model with an anterior posterior asymmetry, aorto-iliac bifurcation, and physiological inlet flow rate and outlet pressure waveforms was developed. The aims were first to show that the structural mechanical behavior of the used material to mimic the AAA wall was similar to this of patients with AAA and then to study the influence of the aorto-iliac bifurcation presence and to study the influence of the imbalanced flow rate in the iliac branches on the AAA flow field. 3D visualizations, never performed in the literature, have clearly put into evidence the development of a vortex ring generated at the AAA proximal neck during the decelerating phase of flow rate, which detaches and progresses downstream during the cardiac cycle, impinges on the anterior wall in the distal AAA region, breaks up, and separates into two vortices of which one rolls on upstream along the anterior wall. 2D particle image velocimetry measurements, swirling strength and enstrophy calculations allowed quantification of the vorticity, vortex trajectory and energy for the different geometrical and hydrodynamical conditions. The main results show that the instant and the intensity of the vortex ring impingement depend on the presence of the aorto-iliac bifurcation, with higher intensity, by about 90%, for an AAA without bifurcation. The imbalance of the flow rates into the iliac branches induces different propagation velocities of the vortex ring and lowers the intensity of the vortex impact by about 60%. The potential influence of the AAA dynamics is discussed in terms of AAA remodeling and rupture.

Stereoscopically Observed Deformations of a Compliant Abdominal Aortic Aneurysm Model

A new experimental setup has been implemented to precisely measure the deformations of an entire model abdominal aortic aneurysm (AAA). This setup addresses a gap between the computational and experimental models of AAA that have aimed at improving the limited understanding of aneurysm development and rupture. The experimental validation of the deformations from computational approaches has been limited by a lack of consideration of the large and varied deformations that AAAs undergo in response to physiologic flow and pressure. To address the issue of experimentally validating these calculated deformations, a stereoscopic imaging system utilizing two cameras was constructed to measure model aneurysm displacement in response to pressurization. The three model shapes, consisting of a healthy aorta, an AAA with bifurcation, and an AAA without bifurcation, were also evaluated with computational solid mechanical modeling using finite elements to assess the impact of differences between material properties and for comparison against the experimental inflations. The device demonstrated adequate accuracy (surface points were located to within 0.07 mm) for capturing local variation while allowing the full length of the aneurysm sac to be observed at once. The experimental model AAA demonstrated realistic aneurysm behavior by having cyclic strains consistent with reported clinical observations between pressures 80 and 120 mm Hg. These strains are 1-2%, and the local spatial variations in experimental strain were less than predicted by the computational models. The three different models demonstrated that the asymmetric bifurcation creates displacement differences but not cyclic strain differences within the aneurysm sac. The technique and device captured regional variations of strain that are unobservable with diameter measures alone. It also allowed the calculation of local strain and removed rigid body motion effects on the strain calculation. The results of the computations show that an asymmetric aortic bifurcation created displacement differences but not cyclic strain differences within the aneurysm sac.

Influence of Wall Compliance on Hemodynamics in Models of Abdominal Aortic Aneurysm

Purpose: To determine complementary criteria to existing morphological criteria, which are not reliable but are used to justify surgical intervention to treat abdominal aortic aneurysm (AAA). Methods: An experimental study was conducted in which 2 models of AAA, 1 rigid and 1 soft, were used to study the influence of compliance on aneurysm dynamics. The heart rate was 70 beats per minute, and the mean flow rate was 1.02 L/min. Velocity measurements were made with particle image velocimetry in 2 planes parallel to flow (1 vertical and 1 horizontal). Results: The general flow patterns generated in the rigid AAA model were in agreement with the literature. In both models, a vortex occurred at the beginning of systolic deceleration in the proximal part of the AAA, near the anterior wall. The vortex remained confined to the proximal part during the entire cycle in the rigid model, whereas in the soft model, the vortex migrated to the distal segment during the cycle and impacted the AAA walls. This impact generated a local pressure increase on the wall. In the soft model, another vortex was created near the posterior wall. These vortices eroded and weakened the walls of the distal segment, which can cause rupture. Conclusion: Compliance of the aneurysm wall might become another criterion to justify surgical intervention.

Flow dynamics characterisation of a novel perfusion-type bioreactor for bone tissue engineering

The successful in vitro engineering of living tissues relies on numerous parameters that can be grouped in four main families: cells, scaffolds, medium and bioreactor. In the case of regenerative medicine, one principal target is to be able to produce implantable grafts obtained by combining biocompatible, biomimetic 3D resorbable scaffolds, preferably with autologous stem cells, primarily cultured in vitro and properly induced in the right lineage in an ergonomic dynamic perfusion like bioreactor, preferably using a serum-free culture media (Bodle et al. 2011; Rauh et al. 2011). Thepresentwork focuses on the design and experimental PIV characterisation of a versatile dynamic bioreactor that seeks to reach such a goal, particularly in the case of bone tissue engineering (BTE).While able to host various types of cells and media, the bioreactor relies on the use of macroporous hydrogel types of scaffolds in which adiposederived stem cells (ASC) are seeded, cultured and mechanically induced into an osteogenic lineage without any inductive biochemical additives.

RFID implantable pressure sensor for the follow-up of abdominal aortic aneurysm stented

An abdominal aortic aneurysm (AAA) is a dilatation of the aorta at the abdominal level, the rupture of which is a life threatening complication with an 80% mortality rate. Even though those devices keep improving, the failure rate of the endovascular treatment is due to persisting pressure into the excluded aneurysmal sac. Since 2005, several integrated sensors have been designed for the follow-up of the AAA treated by a stent. Solutions are based on the use of a single sensor. Thrombus in the excluded AAA can modify the field of pressure when leaks appeared and a network of sensors should be used. We present in this paper the ENDOCOM project that aims to design an implantable pressure sensor that can be used in a network configuration. To validate the new materials, we developed a framework composed of in vitro experiments and in vivo tests on large animal model. Numerical modeling has been investigated from the experimental data to determine the optimal position of sensor. Some results of those different parts are shown in this paper.

Wall shear stress and endothelial cells dysfunction in the context of abdominal aortic aneurysms

The formation of aneurysm of abdominal aorta (AAA) is a multifactorial and predominantly degenerative process that results from a complex interplay between biological processes in the arterial wall and the haemodynamic stimuli on the wall, i.e. wall shear stresses (WSSs) and elongations of the wall directly applied on vascular endothelial cells (ECs) partly regulate the arterial wall remodelling (Golledge et al. 2006; Humphrey and Holzapfel 2012). In the case of AAA, these mechanical stimuli strongly vary in space and time, leading to strong spatiotemporal gradients (Figure 1). The effect of these abnormal mechanical stimuli leads to EC dysfunction. This phenomenon plays a significant role in AAA pathology, in which damage of ECs, associated with changes in endothelial permeability and ...

Influence of blood shear-thinning behaviour on flow dynamics in abdominal aortic aneurysm in vitro compliant model

Numerous studies have been done on models of abdominal aortic aneurysm (AAA) in order to correlate haemodynamics and AAA growth and/or rupture, as well as haemodynamics and intra-luminal thrombus development. It is now well accepted that whatever the model, experimental or numerical, the interaction between the fluid and the AAA wall must be taken into account. Although in large arteries the hypothesis of blood Newtonian behaviour is usually made (Berger et al. 2000), the AAA geometry involves low values of shear rate, which invalidate this assumption. Indeed, the blood presents a nonlinear behaviour, and its main characteristic in the macrocirculation is its shear-thinning behaviour. This nonlinear behaviour has never been considered in the experimental study of AAA, and the most recent numerical simulations (Biasetti et al. 2011) have been realised in AAA model with rigid wall. Therefore, this work deals with the analysis of the blood shear-thinning behaviour influence on the intra-AAA fluid dynamics in a compliant asymmetric model of AAA-included iliac bifurcation and physiological flow rate and pressure waveforms (Deplano et al. 2012).