Pierre-Yves Lagrée
University of Paris
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Publication
Featured researches published by Pierre-Yves Lagrée.
Journal of the Acoustical Society of America | 2003
Mickael Deverge; Xavier Pelorson; Coriandre Vilain; Pierre-Yves Lagrée; F Chentouf; Jan Willems; A Avraham Hirschberg
Measurements of pressure in oscillating rigid replicas of vocal folds are presented. The pressure upstream of the replica is used as input to various theoretical approximations to predict the pressure within the glottis. As the vocal folds collide the classical quasisteady boundary layer theory fails. It appears however that for physiologically reasonable shapes of the replicas, viscous effects are more important than the influence of the flow unsteadiness due to the wall movement. A simple model based on a quasisteady Bernoulli equation corrected for viscous effect, combined with a simple boundary layer separation model does globally predict the observed pressure behavior.
European Physical Journal E | 2004
Christophe Josserand; Pierre-Yves Lagrée; Daniel Lhuillier
Abstract.We propose a simple continuum model to interpret the shearing motion of dense, dry and cohesion-less granular media. Compressibility, dilatancy and Coulomb-like friction are the three basic ingredients. The granular stress is split into a rate-dependent part representing the rebound-less impacts between grains and a rate-independent part associated with long-lived contacts. Because we consider stationary flows only, the grain compaction and the grain velocity are the two main variables. The predicted velocity and compaction profiles are in apparent qualitative agreement with most of the experimental or numerical results concerning free-surface shear flows as well as confined shear flows.
Physics of Fluids | 2012
Lydie Staron; Pierre-Yves Lagrée; Stéphane Popinet
The granular silo is one of the many interesting illustrations of the thixotropic property of granular matter: a rapid flow develops at the outlet, propagating upwards through a dense shear flow while material at the bottom corners of the container remains static. For large enough outlets, the discharge flow is continuous; however, by contrast with the clepsydra for which the flow velocity depends on the height of fluid left in the container, the discharge rate of granular silos is constant. Implementing a plastic rheology in a 2D Navier-Stokes solver (following the μ(I)-rheology or a constant friction), we simulate the continuum counterpart of the granular silo. Doing so, we obtain a constant flow rate during the discharge and recover the Beverloo scaling independently of the initial filling height of the silo. We show that lowering the value of the coefficient of friction leads to a transition toward a different behavior, similar to that of a viscous fluid, and where the filling height becomes active in the discharge process. The pressure field shows that large enough values of the coefficient of friction (≃0.3) allow for a low-pressure cavity to form above the outlet, and can thus explain the Beverloo scaling. In conclusion, the difference between the discharge of a hourglass and a clepsydra seems to reside in the existence or not of a plastic yield stress.
European Physical Journal E | 2014
L. Staron; Pierre-Yves Lagrée; Stéphane Popinet
Using a continuum Navier-Stokes solver with the μ(I) flow law implemented to model the viscous behavior, and the discrete Contact Dynamics algorithm, the discharge of granular silos is simulated in two dimensions from the early stages of the discharge until complete release of the material. In both cases, the Beverloo scaling is recovered. We first do not attempt a quantitative comparison, but focus on the qualitative behavior of velocity and pressure at different locations in the flow. A good agreement for the velocity is obtained in the regions of rapid flows, while areas of slow creep are not entirely captured by the continuum model. The pressure field shows a general good agreement, while bulk deformations are found to be similar in both approaches. The influence of the parameters of the μ(I) flow law is systematically investigated, showing the importance of the dependence on the inertial number I to achieve quantitative agreement between continuum and discrete discharge. However, potential problems involving the systems size, the configuration and “non-local” effects, are suggested. Yet the general ability of the continuum model to reproduce qualitatively the granular behavior is found to be very encouraging.Graphical abstract
Journal of Biomechanical Engineering-transactions of The Asme | 2011
Masashi Saito; Yuki Ikenaga; Mami Matsukawa; Yoshiaki Watanabe; Takaaki Asada; Pierre-Yves Lagrée
Pulse wave evaluation is an effective method for arteriosclerosis screening. In a previous study, we verified that pulse waveforms change markedly due to arterial stiffness. However, a pulse wave consists of two components, the incident wave and multireflected waves. Clarification of the complicated propagation of these waves is necessary to gain an understanding of the nature of pulse waves in vivo. In this study, we built a one-dimensional theoretical model of a pressure wave propagating in a flexible tube. To evaluate the applicability of the model, we compared theoretical estimations with measured data obtained from basic tube models and a simple arterial model. We constructed different viscoelastic tube set-ups: two straight tubes; one tube connected to two tubes of different elasticity; a single bifurcation tube; and a simple arterial network with four bifurcations. Soft polyurethane tubes were used and the configuration was based on a realistic human arterial network. The tensile modulus of the material was similar to the elasticity of arteries. A pulsatile flow with ejection time 0.3 s was applied using a controlled pump. Inner pressure waves and flow velocity were then measured using a pressure sensor and an ultrasonic diagnostic system. We formulated a 1D model derived from the Navier-Stokes equations and a continuity equation to characterize pressure propagation in flexible tubes. The theoretical model includes nonlinearity and attenuation terms due to the tube wall, and flow viscosity derived from a steady Hagen-Poiseuille profile. Under the same configuration as for experiments, the governing equations were computed using the MacCormack scheme. The theoretical pressure waves for each case showed a good fit to the experimental waves. The square sum of residuals (difference between theoretical and experimental wave-forms) for each case was <10.0%. A possible explanation for the increase in the square sum of residuals is the approximation error for flow viscosity. However, the comparatively small values prove the validity of the approach and indicate the usefulness of the model for understanding pressure propagation in the human arterial network.
Computer Methods in Biomechanics and Biomedical Engineering | 2015
Xiaofei Wang; Jose-Maria Fullana; Pierre-Yves Lagrée
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, 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.
Journal of Biomechanical Engineering-transactions of The Asme | 2000
Sylvie Lorthois; Pierre-Yves Lagrée; Jean-Pierre Marc-Vergnes; Francis Cassot
Maximal wall shear stress (MWSS) in the convergent part of a stenosis is calculated by the interactive boundary-layer theory. A dimensional analysis of the problem shows that MWSS depends only on a few measurable parameters. A simple relationship between MWSS and these parameters is obtained, validated, and used to calculate the magnitude of MWSS in a carotid stenosis, as a function of the patency of the circle of Willis and the stenotic pattern. This demonstrates the huge effect of collateral pathways. Elevated MWSS are observed even in moderate stenoses, provided they are associated with a contralateral occlusion, a large anterior, and narrow posterior communicating arteries, suggesting a potential risk of embolus release in this configuration.
Journal of Fluid Mechanics | 2010
Olivier Devauchelle; Luce Malverti; Eric Lajeunesse; Pierre-Yves Lagrée; Christophe Josserand; K.-D. Nguyen Thu-Lam
The present paper is devoted to the formation of sand patterns by laminar flows. It focuses on the rhomboid beach pattern, formed during the backswash. A recent bedload transport model, based on a moving-grains balance, is generalized in three dimensions for viscous flows. The water flow is modelled by the full incompressible Navier–Stokes equations with a free surface. A linear stability analysis then shows the simultaneous existence of two distinct instabilities, namely ripples and bars. The comparison of the bar instability characteristics with laboratory rhomboid patterns indicates that the latter could result from the nonlinear evolution of unstable bars. This result, together with the sensibility of the stability analysis with respect to the parameters of the transport law, suggests that the rhomboid pattern could help improving sediment transport models, so critical to geomorphologists.
Physics of Fluids | 2003
Pierre-Yves Lagrée
The two-dimensional laminar quasisteady asymptotically simplified flow with mass transport of sediments is solved over an erodible bed in various laminar hydraulic regimes (infinite depth, finite depth subcritical or supercritical, nondisturbed boundary layer). Compared to the boundary layer thickness, the bump is supposed longer and thinner and the triple deck theory is used. Furthermore, the flow is linearized. Next, a simplified mass transport equation is obtained which includes the two following phenomena: there is a flux of erosion when the skin friction goes over a threshold value, and concentration of sediment in suspension is convected but falls at a constant settling velocity. It is shown that two ingredients (convection of the longitudinal flux or particles and advanced response of the skin friction to the bump changes) are necessary to produce (except in the supercritical regime which, in this flux convected model, is always stable) a band of amplified spatial frequencies. Furthermore, putting the effect of slope limitation makes long wave stable (in the infinite depth case). Examples of evolution in various regimes are presented, wave trains of ripples are created and merge in a unique bump. A very long time is required for this process. This coarsening appends except in the infinite depth case when the effect of slope limitation is turned on: in this case a train of several bumps fills the computation domain.
Physics of Fluids | 2010
Lydie Staron; Pierre-Yves Lagrée; Christophe Josserand; Daniel Lhuillier
In order to test the rheology of granular flows, we performed series of numerical simulations of nearly monodisperse stationary chute flows from rapid to slow and very slow flow regime, namely, close to the jamming transition. We check how existing rheological models (i.e., Bagnold’s model and the I-model) capture the behavior of the numerical flows, and perform an acute characterization of the structure of the flow in terms of grains velocity fluctuations close to the jamming transition. The simulations show that both Bagnold’s and the I-model fail to describe the data points in the slow regime, namely, when I≤2×10−2. Turning to the analysis of grains velocity fluctuations, we compute the associated correlation length λ and show its dependence on the inertial number: λ/d∝I−0.32. The amplitude of the grains velocity fluctuations, namely, the granular temperature, exhibits a power-law dependence on the shear rate and allows for an efficient prediction of the shape of the velocity profiles. The main result consists of a scaling merging all data points for all flow regimes onto the same master curve, and relating granular temperature, shear rate, and the variation of stress between the considered depth and the bottom wall. This scaling can be written as a relation between local stress, local shear rate, and local temperature, provided the introduction of a characteristic length scale ξ=d(H−z)/z where both the distance to the surface and the distance to the bottom wall are involved. This scaling strongly suggests a nonlocal behavior, valid in the flow regime and extending close to the jamming transition, and hints at granular temperature as the variable at the origin of the nonlocality.In order to test the rheology of granular flows, we performed series of numerical simulations of nearly monodisperse stationary chute flows from rapid to slow and very slow flow regime, namely, close to the jamming transition. We check how existing rheological models (i.e., Bagnold’s model and the I-model) capture the behavior of the numerical flows, and perform an acute characterization of the structure of the flow in terms of grains velocity fluctuations close to the jamming transition. The simulations show that both Bagnold’s and the I-model fail to describe the data points in the slow regime, namely, when I≤2×10−2. Turning to the analysis of grains velocity fluctuations, we compute the associated correlation length λ and show its dependence on the inertial number: λ/d∝I−0.32. The amplitude of the grains velocity fluctuations, namely, the granular temperature, exhibits a power-law dependence on the shear rate and allows for an efficient prediction of the shape of the velocity profiles. The main result ...