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La déformation et la relaxation d'une capsule dans un écoulement confiné sont étudiées au moyen d'un solveur de Navier-Stokes, couplé avec les équations de la membrane par une méthode de frontière immergée (IMB). Ce couplage fluide-structure donne accès à l'évolution temporelle de la forme de la capsule au travers d'un élargissement, puis à la relaxation de celle-ci.

Quatre modèles d'ordre réduit sont proposés pour simuler les écoulements sanguins. Ils sont tous issus des équations de Navier-Stokes. Les interactions fluide-structure sont prises en compte via une loi de pression correspondant à divers comportements de paroi. Les applications concernent les effets d'entrée, la solution de Womersley et la sténose artérielle. Le modèle multi-anneau donne les meilleurs résultats.

International audience; This article focuses on the numerical study of the evaporation of sessile dropletsusing a Volume-Of-Fluid (VOF) method in the free-software Basilisk. We con-sider pure liquid droplets forming a spherical-cap onto a smooth or rough andnon-corrosive substrate and we investigate two different modes of evaporation:the unpinned mode where the contact angle is constant and the pinned modewhere the wetting area is constant. The numerical method used to implementthe contact angle and for the reconstruction of the interface is fully described,especially for the pinned mode where we propose a new VOF implementation.In the unpinned mode, we perform parametric studies and we show the influenceof the relative humidity and the contact angle on the evaporation process. Forall the explored parameters, the simulations predict that the volume decreaseslinearly with time, which matches the signature behaviour of evaporating un-pinned droplets, irrespective of the geometrical parameters. In the pinned mode,the contact angle analysis indicates a linear decrease in time which was expectedaccording to the theory and validated with some experiments we performed.

The creation of a communication between an artery and a vein (arteriovenous fistula or AVF), to speed up the blood purification during hemodialysis of patients with renal insufficiency, induces significant rheological and mechanical modifications of the vascular network. In this study, we investigated the impact of the creation of an AVF with a zero-dimensional network model of the vascular system of an upper limb and a one-dimensional model around the anastomosis. We compared the simulated distribution of flow rate in this vascular system with Doppler ultrasound measurements. We studied three configurations: before the creation of the AVF, after the creation of the AVF, and after a focal reduction due to a hyper flow rate. The zero-dimensional model predicted the bounds of the diameter of the superficial vein that respects the flow constraints, assuming a high capillary resistance. We indeed highlighted the importance of knowing the capillary resistance as it is a decisive parameter in the models. We also found that the model reproduced the Doppler measurements of flow rate in every configuration and predicted the distribution of flow in cases where the Doppler was not available. The one-dimensional model allowed studying the impact of a venous constriction on the flow distribution, and the capillary resistance was still a crucial parameter.

Stenosis, defined by a partial or full obstruction of the arteries, is a frequent anomaly in the cardiovascular system. The pressure drop across a stenosis indicates the severity of the pathology. There is currently no non-invasive method to obtain this pressure drop. In this communication, we use four different blood flow models to compute the pressure in an idealized geometry of stenosis: the steady RNSP model, the Multi-Ring model, the 1D model, and algebraic models. We compare these models on a test case under a steady flow. We then developed a gradient-based parameter estimation method to compare the complex models (1D and Multi-Ring) with algebraic formulas. We used the parameter estimation to evaluate the influence of the geometry, wall elasticity and flow parameter on the empirical coefficients of the algebraic formulas.

Blood pulsatility, aneurysms, stenoses and general low shear stress hemodynamics enhance non-Newtonian blood effects which generate local changes in the space-time evolution of the blood pressure, flow rate and cross-sectional area of elastic vessels. Even though these local changes are known to cause global unexpected hemodynamical behaviors, all one-dimensional (1D) blood flow models are built under Newtonian fluid hypothesis. In this work, we present a time-dependent non-Newtonian extension of a 1D blood flow model, able to describe local space-time variations of the viscous behavior of blood. The rheological model is based on a simplified Maxwell viscoelastic equation for the shear stress with structure dependent coefficients. We compare the numerical predictions of the 1D non-Newtonian model to experimental rheological data available in the literature. Specifically, we explore four well documented shear stress protocols and we show that the results predicted by the 1D non-Newtonian model in a single artery accurately compare, both qualitatively and quantitatively, to the steady and unsteady shear stresses measured experimentally. We then use the 1D non-Newtonian model to compute the flow in idealized healthy and pathological symmetric and asymmetric networks of increasing size. We show that aggregation occurs in such networks occurs, leading to non-Newtonian blood behaviors especially in the presence of pathologies. This non-Newtonian extension of a 1D blood flow model will be useful in the future to improve our understanding of the large-scale hemodynamics in micro- and macro-circulation networks.

In this paper, we propose a two-dimensional nonlinear "multiring" model for blood flow in axisymmetric elastic arteries. It is designed to overcome the numerical difficulties of three-dimensional fluid-structure interaction simulations of blood flow without using the oversimplifications necessary to obtain one-dimensional models of blood flow. This multiring model is derived by integrating over concentric rings of fluid the simplified long-wave Navier-Stokes equations coupled to an elastic model of the arterial wall. The resulting system of balance laws provides a unified framework in which both the motion of the fluid and the displacement of the wall are dealt with simultaneously. The mathematical structure of the multiring model allows us to use a finite volume method that guarantees the conservation of mass and the positivity of the numerical solution and can deal with nonlinear flows and large deformations of the arterial wall. We show that the finite volume numerical solution of the multiring model provides at a reasonable computational cost an asymptotically valid description of blood flow velocity profiles and other averaged quantities (wall shear stress, flow rate, ...) in large elastic and quasi-rigid arteries. In particular, we validate the multiring model against well-known solutions such as the Womersley or the Poiseuille solutions as well as against steady boundary layer solutions in quasi-rigid constricted and expanded tubes.

The purpose of this work is to construct a simple, efficient and accurate well-balanced numerical scheme for one-dimensional (1D) blood flow in large arteries with varying geometrical and mechanical properties. As the steady states at rest are not relevant for blood flow, we construct two well-balanced hydrostatic reconstruction techniques designed to preserve low-Shapiro number steady states that may occur in large network simulations. The Shapiro number S h = u / c is the equivalent of the Froude number for shallow water equations and the Mach number for compressible Euler equations. The first is the low-Shapiro hydrostatic reconstruction (HR-LS), which is a simple and efficient method, inspired from the hydrostatic reconstruction technique (HR). The second is the subsonic hydrostatic reconstruction (HR-S), adapted here to blood flow and designed to exactly preserve all subcritical steady states. We systematically compare HR, HR-LS and HR-S in a series of single artery and arterial network numerical tests designed to evaluate their well-balanced and wave-capturing properties. The results indicate that HR is not adapted to compute blood flow in large arteries as it is unable to capture wave reflections and transmissions when large variations of the arteries' geometrical and mechanical properties are considered. On the contrary, HR-S is exactly well-balanced and is the most accurate hydrostatic reconstruction technique. However, HR-LS is able to compute low-Shapiro number steady states as well as wave reflections and transmissions with satisfying accuracy and is simpler and computationally less expensive than HR-S. We therefore recommend using HR-LS for 1D blood flow simulations in large arterial network simulations. Two new hydrostatic reconstruction techniques are introduced for blood flow.Both preserve low-Shapiro steady states relevant for blood flow.Both accurately capture wave reflections for arbitrary large impedance variations.Capturing wave reflections is necessary for large pathological network simulations.

The retina offers the most accessible imaging of microvascular networks. Actually current developed techniques at the Hopital des Quinze-Vingts using adaptive optic ophthalmoscopy and the holograph...

Aortic cross-clamping is a common strategy during vascular surgery, however, its instantaneous impact on hemodynamics is unknown. We, therefore, developed two numerical models to estimate the immediate impact of aortic clamping on the vascular properties. To assess the validity of the models, we recorded continuous invasive pressure signals during abdominal aneurysm repair surgery, immediately before and after clamping. The first model is a zero-dimensional (0D) three-element Windkessel model, which we coupled to a gradient-based parameter estimation algorithm to identify patient-specific parameters such as vascular resistance and compliance. We found a 10% increase in the total resistance and a 20% decrease in the total compliance after clamping. The second model is a nine-artery network corresponding to an average human body in which we solved the one-dimensional (1D) blood flow equations. With a similar parameter estimation method and using the results from the 0D model, we identified the resistance boundary conditions of the 1D network. Determining the patient-specific total resistance and the distribution of peripheral resistances through the parameter estimation process was sufficient for the 1D model to accurately reproduce the impact of clamping on the pressure waveform. Both models gave an accurate description of the pressure wave and had a high correlation (R2 >.95) with experimental blood pressure data. Fil: Ventre, Jeanne. Centre National de la Recherche Scientifique; Francia Fil: Politi, Teresa. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Fisiología y Biofísica Bernardo Houssay. Universidad de Buenos Aires. Facultad de Medicina. Instituto de Fisiología y Biofísica Bernardo Houssay; Argentina Fil: Fernández, Juan M.. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Fisiología y Biofísica Bernardo Houssay. Universidad de Buenos Aires. Facultad de Medicina. Instituto de Fisiología y Biofísica Bernardo Houssay; Argentina Fil: Ghigo, Arthur R.. Université de Toulouse; Francia Fil: Gaudric, Julien. Centre National de la Recherche Scientifique; Francia. Université de Toulouse; Francia Fil: Wray, Sandra. Universidad Favaloro; Argentina Fil: Lagaert, Jean Baptiste. Université Paris Sud; Francia Fil: Armentano, Ricardo Luis. Universidad de la República; Uruguay Fil: Capurro, Claudia Graciela. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Fisiología y Biofísica Bernardo Houssay. Universidad de Buenos Aires. Facultad de Medicina. Instituto de Fisiología y Biofísica Bernardo Houssay; Argentina Fil: Fullana, José Maria. Centre National de la Recherche Scientifique; Francia Fil: Lagrée, Pierre Yves. Centre National de la Recherche Scientifique; Francia

Stenosis, defined by a partial or full obstruction of arteries is one of the most frequent anomalies in the cardiovascular system. In clinical studies, blood pressure drop across the stenosis gives...

Implicit parallelism computing is an active research domain of computer science. Most implicit parallelism solutions to solve partial differential equations, and scientific simulations, are based on the specificity of numerical methods, where the user has to call specific functions which embed parallelism. This paper presents the implicit parallel library SkelGIS which allows the user to freely write its numerical method in a sequential programming style in C++. This library relies on four concepts which are applied, in this paper, to the specific case of network simulations. SkelGIS is evaluated on a blood flow simulation in arterial networks. Benchmarks are first performed to compare the performance and the coding difficulty of two implementations of the simulation, one using SkelGIS, and one using OpenMP. Finally, the scalability of the SkelGIS implementation, on a cluster, is studied up to 1024 cores.

Background The curvature of the aortic arch is associated with the risk of endoleak formation after thoracic endovascular aortic repair (TEVAR). However, the adequate assessment of the angles of the aorta continues to represent a major difficulty. We developed a new program based on three-dimensional (3D) reconstructions of computed tomography (CT) scans to objectively identify the location of the aortic points of maximum curvature, and to automatically calculate the main aortic arch angles, comparing final values with visual assessment methods. Methods This is a cross-sectional validation study of a convenience sample of subjects with multislice CT angiography scans of the thoracic aorta from an institutional imaging database. The center lumen line (CLL) of the aorta was determined semi-automatically using Endosize software. The points of maximum curvature on the CLL were determined by two methods: visually by two physicians and through a custom program. Results The study enrolled 9 subjects: 4 with thoracic aneurysms and 5 with normal aortas. The inter-observer and inter-method correlation, agreement and reliability for each of the 3D spatial coordinates of the points of maximum curvature were appropriate. However, the aortic angles determined by visual assessment showed a very low to moderate correlation and reliability with those determined by our custom program. Conclusion An automated custom program can reflect clinician's intuitive assessment of the location of points of maximum curvature and translate it into aortic angles with an apparently higher precision, reducing potential error and user time.

Arterial cross-clamping is a common strategy used in vascular surgery and its duration is an independent predictor of surgical outcomes. The impact of arterial cross-clamping on the viscoelastic properties of the arterial system and its underlying mechanisms still remain unclear. The aim of this study was to evaluate the effect of arterial cross-clamping on arterial stiffness. A cross-sectional, observational, before-after study was designed to enroll adult patients undergoing vascular surgery. The Augmentation Index normalized to 75 beats-per-minute (AIx@75) and Fractal Dimension (FD) –indirect indicators of arterial stiffness– were calculated from radial arterial pressure tracings during surgery. The arterial pressure tracings from 8 patients were analyzed. Overall data included 4 aortic and 11 iliofemoral interventions. In both aortic and iliofemoral interventions, after arterial clamping, median AIx@75 rose and FD dropped significantly; the opposite occurred after arterial unclamping. Spearman’s correlation suggests a strong significant negative correlation between median AIx@75 and FD during each hemodynamic state for aortic interventions. Our results are consistent at many levels: a) opposite events (i.e., clamping and unclamping) produce changes in different directions, b) two different indicators (i.e., AIx@75 and FD) suggest the same underlying phenomenon, and c) similar results are observed at different vascular locations (i.e., aortic and iliofemoral). Overall, our data consistently suggests an increase in arterial stiffness during clamping and a reduction during unclamping. Despite the large distance from the aortic or iliofemoral intervention sites, radial artery pressure monitoring is still able to detect consistently these vascular events. Fil: Politi, Teresa. Universidad de Buenos Aires. Facultad de Medicina. Departamento de Ciencias Fisiológicas. Laboratorio de Biomembranas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Fisiología y Biofísica Bernardo Houssay. Universidad de Buenos Aires. Facultad de Medicina. Instituto de Fisiología y Biofísica Bernardo Houssay; Argentina Fil: Wray, S. A.. Universidad Tecnológica Nacional; Argentina. Universidad Favaloro; Argentina Fil: Fernández, Juan Manuel Francisco. Universidad de Buenos Aires. Facultad de Medicina. Departamento de Ciencias Fisiológicas. Laboratorio de Biomembranas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Fisiología y Biofísica Bernardo Houssay. Universidad de Buenos Aires. Facultad de Medicina. Instituto de Fisiología y Biofísica Bernardo Houssay; Argentina Fil: Gaudric, J.. Université Pierre et Marie Curie; Francia Fil: Ghigo, A.. Université Pierre et Marie Curie; Francia Fil: Lagrée, P. Y.. Université Pierre et Marie Curie; Francia Fil: Capurro, Claudia Graciela. Université Pierre et Marie Curie; Francia. Universidad de Buenos Aires. Facultad de Medicina. Departamento de Ciencias Fisiológicas. Laboratorio de Biomembranas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Fullana, J. M.. Université Pierre et Marie Curie; Francia Fil: Armentano, R.. Ingeniería Biomédica; Argentina

This work deals with the viscoelasticity of the arterial wall and its influence on the pulse waves. We describe the viscoelasticity by a non-linear Kelvin-Voigt model in which the coefficients are fitted using experimental time series of pressure and radius measured on a sheep's arterial network. We obtained a good agreement between the results of the nonlinear Kelvin-Voigt model and the experimental measurements. We found that the viscoelastic relaxation time-defined by the ratio between the viscoelastic coefficient and the Young's modulus-is nearly constant throughout the network. Therefore, as it is well known that smaller arteries are stiffer, the viscoelastic coefficient rises when approaching the peripheral sites to compensate the rise of the Young's modulus, resulting in a higher damping effect. We incorporated the fitted viscoelastic coefficients in a nonlinear 1D fluid model to compute the pulse waves in the network. The damping effect of viscoelasticity on the high frequency waves is clear especially at the peripheral sites., in Journal of Biomechanical Engineering, American Society of Mechanical Engineers, 2016

We study the behavior of the pulse waves of water into a flexible tube for application to blood flow simulations. In pulse waves both fluid friction and wall viscosity are damping factors, and difficult to evaluate separately. In this paper, the coefficients of fluid friction and wall viscosity are estimated by fitting a nonlinear 1D flow model to experimental data. In the experimental setup, a distensible tube is connected to a piston pump at one end and closed at another end. The pressure and wall displacements are measured simultaneously. A good agreement between model predictions and experiments was achieved. For amplitude decrease, the effect of wall viscosity on the pulse wave has been shown as important as that of fluid viscosity., Comment: Submitted to Journal of Biomechanics, 2015

Modeling and simulating overland flow fed by rainfall is a common issue in watershed surface hydrology. Modelers have to choose among various friction models when defining their simulation framework. The purpose of this work is to compare the simulation quality for the Manning, Darcy-Weisbach, and Poiseuille friction models on the simple case of a constant rain on a thin experimental flume. Results show that the usual friction law of Manning is not suitable for this type of flow. The Poiseuille friction model gave the best results both on the flux at the outlet and the velocity and depth profile along the flume. The Darcy-Weisbach model shows good results for laminar flow. Additional testing should be carried out for turbulent cases., in Journal of Hydrology 2016

Divergent concepts on the origin of the dicrotic notch are widespread in medical literature and education. Since most medical textbooks explain the origin of the dicrotic notch as caused by the aortic valve closure itself, this is commonly transmitted in medical physiology courses. We present clinical data and numerical simulations to demonstrate that reflected pressure waves could participate as one of the causes of the dicrotic notch. Our experimental data from continuous arterial pressure measurements from adult patients undergoing vascular surgery suggest that isolated changes in peripheral vascular resistance using an intravenous bolus of phenylephrine (a selective alpha 1-receptor agonist and thus a potent vasoconstrictor) modify the dicrotic notch. We then explore the mechanisms behind this phenomenon by using a numerical model based on integrated axisymmetric Navier-Stokes equations to compute the hemodynamic flow. Our model illustrates clearly how modifications in peripheral artery resistance may result in changes in the amplitude of the dicrotic notch by modifying reflected pressure waves. We believe that this could be a useful tool in teaching medical physiology courses. Fil: Politi, Teresa. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Fisiología y Biofísica Bernardo Houssay. Universidad de Buenos Aires. Facultad de Medicina. Instituto de Fisiología y Biofísica Bernardo Houssay; Argentina Fil: Ghigo, Arthur. Universite de Paris VI; Francia Fil: Fernández, Juan Manuel Francisco. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Fisiología y Biofísica Bernardo Houssay. Universidad de Buenos Aires. Facultad de Medicina. Instituto de Fisiología y Biofísica Bernardo Houssay; Argentina Fil: Khelifa, Ismaïl. Hôpitaux Universitaires La Pitié-Salpêtrière; Francia Fil: Gaudric, Julien. Hôpitaux Universitaires La Pitié-Salpêtrière; Francia. Universite de Paris VI; Francia Fil: Fullana, José María. Universite de Paris VI; Francia Fil: Lagrée, Pierre Yves. Universite de Paris VI; Francia

Introduccion y objetivos La paraoxonasa 1 (PON1) y la paraoxonasa 2 (PON2) son enzimas antioxidantes cuyos polimorfismos PON1-192 y PON2-311 se han relacionado con el riesgo de infarto agudo de miocardio, con resultados discordantes. El objetivo de este estudio es determinar la asociacion con el riesgo de infarto agudo de miocardio (IAM) de los polimorfismos PON1-192 y PON2-311 y su interaccion. Metodos Se realizo un estudio de casos y controles en el que se recluto a 746 pacientes consecutivos con IAM y 1.796 controles libres de enfermedad cardiovascular seleccionados al azar de la misma poblacion de la que provenian los casos, en 4 comunidades autonomas espanolas entre 1999 y 2000. Se determinaron los polimorfismos PON1-192 y PON2-311, ademas de los factores clasicos de riesgo cardiovascular. Se estimaron modelos de regresion logistica para los analisis multivaribles. Resultados Las odds ratio (OR) del genotipo QQ del polimorfismo PON1-192 y el SS del PON2-311 (presen-tes en el 50 y el 66% de la poblacion, respectivamente) de presentar un IAM fueron 1,26 (intervalo de confianza [IC] del 95%, 1,02-1,55) y 1,25 (IC del 95%, 1,04-1,50), respectivamente, en comparacion con los portadores de los alelos R y C. Ademas, para los sujetos que presentan ambos genotipos QQ y SS, la OR ajustada de tener un IAM se incremento hasta 1,41 (IC del 95%, 1,13-1,76). Conclusiones Nuestros resultados indican que los polimorfismos PON1-192 y PON2-311 son factores de riesgo de IAM independientes en nuestra poblacion.

Introduction and objectives Two particular polymorphisms, namely PON1-192 and PON2-311, in the genes encoding the antioxidant enzymes paraoxonase-1 (PON1) and paraoxonase-2 (PON2) have been associated with an increased risk of acute myocardial infarction (AMI). However, previous findings have been contradictory. The aim of this study was to investigate the association between the PON1-192 and PON2-311 polymorphisms and their interaction on AMI risk. Methods This case-control study involved 746 consecutive AMI patients and 1796 control subjects without cardiovascular disease, who were randomly selected from the same population from which the patients came. All participants were recruited between 1999 and 2000 from four Spanish autonomous regions. All were assessed for the presence of PON1-192 and PON2-311 and for classical cardiovascular risk factors. Multivariate analysis was carried out using logistic regression modeling. Results The odds ratios (OR) of AMI for patients with the PON1-192 QQ and PON2-311 SS genotypes (who comprised 50% and 66% of the population, respectively) were 1.26 (95% confidence interval [Cl], 1.02-1.55) and 1.25 (95% Cl, 1.04-1.50), respectively, compared with R and C allele carriers. Moreover, in patients with both QQ and SS genotypes, the adjusted OR of AMI increased to 1.41 (95% Cl, 1.13-1.76). Conclusiones Our results indicate that the PON1-192 and PON2-311 polymorphisms were independent risk factors of AMI in our population.

Resume Nous presentons un montage experimental qui modelise le processus de drainage des veines musculaires pendant l'activation des muscles du mollet. Un tube collabable est place verticalement dans une enceinte hermetique connectee a un reservoir capable de fournir des pressions exterieures controlees. Dans le cas d'un drainage correspondant a une marche rapide, nous montrons qu'un phenomene de limitation de debit se produit et que le drainage se fait lentement. Nous presentons des simulations numeriques qui montrent qu'un passage critique, similaire aux ondes de choc dans les fluides compressibles, est a l'origine du phenomene. Pour citer cet article : P. Guesdon et al., C. R. Mecanique 335 (2007).

International audience; We performed numerical simulations of blood flow in arteries with a variable stiffness and cross-section at rest using a finite volume method coupled with a hydrostatic reconstruction of the variables at the interface of each mesh cell. The method was then validated on examples taken from the literature. Asymptotic solutions were computed to highlight the effect of the viscous and viscoelastic source terms. Finally, the blood flow was computed in an artery where the cross-section at rest and the stiffness were varying. In each test case, the hydrostatic reconstruction showed good results where other simpler schemes did not, generating spurious oscillations andnonphysical velocities.

International audience

We investigate experimentally and numerically the filling of a collapsible tube, motivated by venous hemodynamics in the lower limbs. The experiments are performed by filling an initially collapsed flexible tube, applying pressure through a hydraulic circuit. The tube law and the tube tension have been previously measured. The tube shape, the flow rate and the pressure at the two ends of the tube are measured continuously. The filling occurs in three stages: a rapid equilibration of the pressure near the tube entry with atmospheric pressure, a quasi-steady filling of the tube with a linearly rising pressure, and a final stage of tube inflation. Our numerical model is the classical one-dimensional collapsible tube equations. Excellent quantitative agreement is found between computations and experimental data. We show experimentally observed shapes near the tube end that indicate possible three-dimensional effects; however these effects do not impair significantly the ability of the one-dimensional model to describe the experiment. Travelling waves of large amplitude are observed in the simulations and the experiments.

Resume Une methode simple de lissage basee sur la theorie de la regularisation est presentee. On considere le probleme de lissage de donnees comme un probleme d'optimisation dont la fonctionnelle a minimiser reflete les caracteristiques du systeme. On fait appel a la theorie de la regularisation pour definir la courbe optimale qui donne les donnees filtrees. Des simulations numeriques permettent de comparer la methode a une methode spectrale classique. Pour citer cet article : J.-M. Fullana, C. R. Mecanique 330 (2002) 647–652.

International audience; A model of the fate of colloidal silica in the dermis was designed based on the diffusion of fluorescent silica nanoparticles through collagen hydrogels. The diffusion process was found to depend on particle size (10–200 nm) and surface charge, as well as on collagen concentration (1.5–5 mg mL−1). The presence of human dermal fibroblasts within the hydrogels also significantly impacted on the behaviour of the particles. In particular, the simultaneous monitoring of particulate and soluble forms of silica showed that both the hydrogel network and the cellular activity have a strong influence on the solubilization process of the silica particles, through a combination of surface sorption, uptake and intracellular dissolution. Interactions between silica and collagen in 3D environments also lower the cytotoxicity of 10 nm particles compared to traditional 2D cultures. The results emphasize the complexity of silica chemistry in living tissues and specifically indicate the need for further investigations of the in vivo behaviour of its soluble forms.

This paper is concerned with the robustness of parameter identification methods with respect to the noise levels typically found in experiments. More precisely, we focus on the case of an extended nonlinear system: a system of coupled local maps akin to a discretized complex Ginzburg-Landau equation, modeling a wake experiment. After a brief description of this hydrodynamic experiment as well as of the associated cost function and synthetic data generation, we introduce two inversion methods: a one-time-step approach, and a more sophisticated n -time-step optimization procedure, solved by a backpropagation method. The one-time-step approach reduces to a small linear system for the unknown parameters, while the n -time-step approach involves a backpropagation equation for a set of Lagrage multipliers. The sensitivity of each method with respect to noise is then discussed: while the n -time-step method is very robust even with large amounts of noise, the one-time-step approach is shown to be affected by small noise levels.

We study the flow behind an array of equally spaced parallel cylinders. A system of Stuart-Landau equations with complex parameters is used to model the oscillating wakes. Our purpose is to identify the 6 scalar parameters which most accurately reproduce the experimental data of Chauve and Le Gal [{Physica D {\bf 58}}, pp 407--413, (1992)]. To do so, we perform a computational search for the minimum of a distance $\calj$. We define $\calj$ as the sum-square difference of the data and amplitudes reconstructed using coupled equations. The search algorithm is made more efficient through the use of a partially analytical expression for the gradient $\nabla \cal J$. Indeed $\nabla \cal J$ can be obtained by the integration of a dynamical system propagating backwards in time (a backpropagation equation for the Lagrange multipliers). Using the parameters computed via the backpropagation method, the coupled Stuart-Landau equations accurately predicted the experimental data from Chauve and Le Gal over a correlation time of the system. Our method turns out to be quite robust as evidenced by using noisy synthetic data obtained from integrations of the coupled Stuart-Landau equations. However, a difficulty remains with experimental data: in that case the several sets of identified parameters are shown to yield equivalent predictions. This is due to a strong discretization or ``round-off" error arising from the digitalization of the video images in the experiment. This ambiguity in parameter identification has been reproduced with synthetic data subjected to the same kind of discretization., Comment: 25 pages uuencoded compressed PostScript file (58K) with 13 figures (155K in separated file) Submitted to Physica D

Papier soumis : Journal of Biomechanical Engineering (02/2013); Computational hemodynamic models of the cardiovascular system are often limited to finite segments of the system and therefore need well-controlled inlet and outlet boundary conditions. Classical boundary conditions are measured total pressure or flow rate imposed at inlet and impedances of RLR, RLC or LR filters at outlet. We present a new approach based on a unidirectional propagative hypothesis (UPH) to model the inlet/outlet boundary conditions on the axisymmetric Navier-Stokes equations. This condition is equivalent to a non-reflecting boundary condition in a fluid-structure interaction model of an axisymmetric artery. First we compare the UPH to the best impedance filter (RLC). Second, we apply this approach to a physiological situation i.e. the presence of a stented segment into a coronary artery. In that case a reflection index is defined which quantifies the amount of pressure waves reflected upon the singularity.

International audience

A reliable and fast numerical scheme is crucial for the 1D simulation of blood flow in compliant vessels. In this paper, a 1D blood flow model is incorporated with a Kelvin-Voigt viscoelastic arterial wall. This leads to a nonlinear hyperbolic-parabolic system, which is then solved with four numerical schemes, namely: MacCormack, Taylor-Galerkin, MUSCL (monotonic upwind scheme for conservation law) and local discontinuous Galerkin. The numerical schemes are tested on a single vessel, a simple bifurcation and a network with 55 arteries. The numerical solutions are checked favorably against analytical, semi-analytical solutions or clinical observations. Among the numerical schemes, comparisons are made in four important aspects: accuracy, ability to capture shock-like phenomena, computational speed and implementation complexity. The suitable conditions for the application of each scheme are discussed.

International audience; Droplet migration in a Hele–Shaw cell is a fundamental multiphase flow problem which is crucial for many microfluidics applications. We focus on the regime at low capillary number and three-dimensional direct numerical simulations are performed to investigate the problem. In order to reduce the computational cost, an adaptive mesh is employed and high mesh resolution is only used near the interface. Paramet-ric studies are performed on the droplet horizontal radius and the capillary number. For droplets with an horizontal radius larger than half the channel height the droplet overfills the channel and exhibits a pancake shape. A lubrication film is formed between the droplet and the wall and particular attention is paid to the effect of the lubrication film on the droplet velocity. The computed velocity of the pancake droplet is shown to be lower than the average inflow velocity, which is in agreement with experimental measurements. The numerical results show that both the strong shear induced by the lubrication film and the three-dimensional flow structure contribute to the low mobility of the droplet. In this low-migration-velocity scenario the interfacial flow in the droplet reference frame moves toward the rear on the top and reverses direction moving to the front from the two side edges. The velocity of the pancake droplet and the thickness of the lubrication film are observed to decrease with capillary number. The droplet velocity and its dependence on capillary number cannot be captured by the classic Hele–Shaw equations, since the depth-averaged approximation neglects the effect of the lubrication film.

Using axisymmetric simulations of a flow in a rectilinear cylindrical tube, our main purpose here is to present a procedure minimising reflected waves caused by numerical boundary conditions. Since...

We study the stability of a viscous liquid layer of uniform thickness subject only to viscous stresses and surface tension. We show that the growing cylindrical end rim does not typically break into droplets for moderate wavelengths. Thus, other mechanisms are needed to cause the instabilities, which, for instance, lead to the famous milk crown. The problem remains open for very large wavelengths.

Two particular polymorphisms, namely PON1-192 and PON2-311, in the genes encoding the antioxidant enzymes paraoxonase-1 (PON1) and paraoxonase-2 (PON2) have been associated with an increased risk of acute myocardial infarction (AMI). However, previous findings have been contradictory. The aim of this study was to investigate the association between the PON1-192 and PON2-311 polymorphisms and their interaction on AMI risk.This case-control study involved 746 consecutive AMI patients and 1796 control subjects without cardiovascular disease, who were randomly selected from the same population from which the patients came. All participants were recruited between 1999 and 2000 from four Spanish autonomous regions. All were assessed for the presence of PON1-192 and PON2-311 and for classical cardiovascular risk factors. Multivariate analysis was carried out using logistic regression modeling.The odds ratios (OR) of AMI for patients with the PON1-192 QQ and PON2-311 SS genotypes (who comprised 50% and 66% of the population, respectively) were 1.26 (95% confidence interval [CI], 1.02-1.55) and 1.25 (95% CI, 1.04-1.50), respectively, compared with R and C allele carriers. Moreover, in patients with both QQ and SS genotypes, the adjusted OR of AMI increased to 1.41 (95% CI, 1.13-1.76).Our results indicate that the PON1-192 and PON2-311 polymorphisms were independent risk factors of AMI in our population.

Model identifications based on orbit tracking methods are here extended to stochastic differential equations. In the present approach, deterministic and statistical features are introduced via the time evolution of ensemble averages and variances. The aforementioned quantities are shown to follow deterministic equations, which are explicitly written within a linear as well as a weakly nonlinear approximation. Based on such equations and the observed time series, a cost function is defined. Its minimization by simulated annealing or backpropagation algorithms then yields a set of best-fit parameters. This procedure is successfully applied for various sampling time intervals, on a stochastic Lorenz system.

The aim of this work is twofold: to develop a quick and accurate numerical tool for studying the venous return and to set up an ambitious experiment to gain better understanding of treatment and prevention of CVI. [All rights reserved Elsevier].

Simulating the flow in an arterial tree is a challenging problem. Many authors (since for example [7]) have succeeded in simulating an arterial tree with a monodimensional (1D) approach or integral approach, among them [5] and recently [1,6]. The two later [1] and [6] compared successfully an experimental realistic visco elastic model of tubes and such 1D models. Here, we emphasize on the numerical methods and we will simulate results of [6] as test case.