Simulation of energy transfers in waves generated by granular slides

International audienceThis paper presents a multi-fluid Navier-Stokes modelling of the waves generated by two granular slides (subaerial and submarine) which were previously studied experimentally and a pure synthetic submarine case used for results interpretation. In the numerical model, air and water are considered as Newtonian fluids. The slide is modelled as a Newtonian fluid whose viscosity is adjusted to fit the experimental results. Once the viscosity is adjusted, the first and the second waves are shown to be accurately reproduced by the model even though the computed slide is slower. For the subaerial case, the viscosity value found is shown to be consistent with the granular µ(I) rheological law. The second part of this work focuses on the energy transfers between a slide and its generated waves. Energy balance is computed in each phase. The wave energy is evaluated in the wave propagation zone. Energy dissipation, kinematic and potential energies are taken into account in the computation of energy transfer ratio allowing for a better understanding of the phenomena. In light of these results, the wave train generation process is discussed as well as the importance of the slide dynamics in the wave generation stage. The amount of energy transferred to wave is not constant with time and the transfer rate depends strongly on the definition of this rate as well as the case considered. For instance, in the subaerial case simulated, the energy transferred to surface waves is 30 % of the energy transferred to water at the time the transfer stops, but this conversion rate is only equal to 4 % of the overall available potential slide energy at the end of the process. For the two submarine cases simulated, the corresponding values, equal in both cases, are 2 % and 1 %, respectively. The simulation results also show that the slide energy is transferred to the water in a short period of time at the beginning whatever the case considered. This observation may be related to the initial nil slide velocity (subaerial case) and the relatively large slope values considered (both cases). Nevertheless, the results illustrate the importance of accurate simulation of the slide dynamics within the wave generation process

IMPACTS DE VAGUE PAR RUPTURE DE BARRAGE SUR FOND MOUILLÉ WAVE IMPACTS IN WET DAM BREAKING

International audienceCet article présente l'analyse de la typologie des impacts de vagues générées par la rupture d'un barrage sur fond mouillé, en mettant l'accent sur l'impact flip-through. Dans lesétudes précédentes, ce type d'impact s'est avéré très difficileà obtenir età répéter expérimentalement, car il nécessite un ajustement très précis des paramètres du problème. Dans la présenteétude, un modèle numérique de rupture de barrage a d'abordété validé sur la base de résultats expérimentaux antérieurs, puis utilisé pourétudier les impacts de vagues sur un mur dont la position peutêtre ajustée. Les caractéristiques de plus de cinq cents cas d'impact ont ainsi puêtreétudiées. Quatre types d'impacts (impacts de ballottement, impacts flip-through, impacts de poches d'air et impacts aérés) ontété identifiés en fonction des paramètres du modèle de rupture de barrage. Les caractéristiques cinématiques et dynamiques de ces différents impacts sontétudiées dans ce travail. Summary This paper focuses on the analysis of the wave impact typology generated in wet dam breaking with a focus on the flip-through impact. In previous studies, this kind of impact has been proven to be very hard to obtain and repeat experimentally as it requires a very precise adjustment of the parameters. In the present study, a numerical model of the wet dam break flow was first validated on the basis of previous experimental results and then used to study the wave impacts on a wall, whose position can be adjusted. The features of more than five hundred impact cases could thereby be studied. Four types of impacts (sloshing impacts, flip-through impacts, air pocket impacts and aerated impacts) were identified and investigated in relation with the parameters of the dam-break model. The kinematic and dynamic characteristics of these different impacts are finally studied in this work

Nanoforce estimation with Kalman filtering applied to a force sensor based on diamagnetic levitation.

International audienceNano force sensors based on passive diamagnetic levitation with a macroscopic seismic mass are a possible alternative to classical Atomic Force Microscopes when the force bandwidth to be measured is limited to a few Hertz. When an external unknown force is applied to the levitating seismic mass, this one acts as a transducer that converts this unknown input into a displacement that is the measured output signal. Because the little damped and long transient response of this kind of macroscopic transducer can not be neglected, it is then necessary to deconvolve the output to correctly estimate the unknown input force. The deconvolution approach proposed in this article is based on a Kalman filter that use an uncertain a priori model to represent the unknown nanoforce to be estimated. The main advantage of this approach is that the end-user can directly control the unavoidable trade-off that exists between the wished resolution on the estimatedforce and the response time of the estimation

Nanoforce estimation based on Kalman filtering and applied to a force sensor using diamagnetic levitation

International audienceNanoforce sensors based on passive diamagnetic levitation with a macroscopic seismic mass are a possible alternative to classical Atomic Force Microscopes when the force bandwidth to be measured is limited to a few Hertz. When an external unknown force is applied to the levitating seismic mass, this one acts as a transducer that converts this unknown input into a displacement that is the measured output signal. Because the under-damped and long transient response of this kind of macroscopic transducer cannot be neglected for time-varying force measurement, it is then necessary to deconvolve the output to correctly estimate the unknown input force. The deconvolution approach proposed in this paper is based on a Kalman filter that use an uncertain a priori model to represent the unknown nanoforce to be estimated. The main advantage of this approach is that the end-user can directly control the unavoidable trade-off that exists between the wished resolution on the estimated force and the response time of the estimation

Analysis of the trade-off between resolution and bandwidth for a nanoforce sensor based on diamagnetic levitation.

International audienceNanoforce sensors based on passive diamagnetic levitation with a macroscopic seismic mass are a possible alternative to classical Atomic Force Microscopes when theforce bandwidth to be measured is limited to a few Hertz.When an external unknown force is applied to the levitating seismic mass, this one acts as a transducer that converts this unknown input into a displacement that is the measured output signal. Because the inertia effect due to the mass of such macroscopic transducers cannot be neglected for timevarying force measurement, it is necessary to deconvolve the displacement to correctly estimate the unknown input force.A deconvolution approach based on a Kalman filter and controlled by a scalar parameter has been recently proposed.The adjustement of this parameter leads to a trade-off that is analysed in this paper in term of resolution and bandwidth of the estimated force. Associated tools to help the end-user to set this parameter are also described

Modeling and experimentation of a passive low frequency nanoforce sensor based on diamagnetic levitation.

International audienceThis paper is focused on the study of a new low frequency micro and nanoforce sensor based on diamagnetic levitation. The force sensitive part is a tencentimeter long macroscopic capillary tube used as a levitating seismic mass. This tube presents a naturally stable equilibrium state with six degrees of freedom thanks to the combination of diamagnetic repulsive and magnetic attractive forces. It is only used as a one-direction force sensing device along its longitudinal axis. This force sensor is passive. The force measurement is based on the displacement of the capillary tube and in steady-state this displacement is proportional to the force. This sensor is characterized by an under-damped second-order linear force-displacement dynamic which remains linear on several hundred micrometers and can thus measure a wide range of microforces. Because of the magnetic springs con guration used, the capillary tube presents a horizontal mechanical sti ness that can be adjusted between 0.01 and 0.03 N/m (similar to the sti ness of a thin AFM cantilever). The measurement range typically varies between 50 N. Bandwidth is 4 Hz. The resolution depends on the sensor used to measure the capillary tube displacement and on noises induced by environmental conditions (ground and air vibrations). The resolution typically reached with a STIL confocal chromatic sensor is 5 nN inside a test chamber located on a anti-vibration table. This study is illustrated by a pull-off force measurement

Modélisation numérique Navier-Stokes/VOF de tsunamis générés par un glissement de terrain aérien

La génération de vagues par des glissements de terrain est étudiée à l'aide d'un modèle basé sur les équations de Navier-Stokes et un suivi d'interface PLIC-VOF ou TVD Lax Wendroff. L'originalité de l'étude repose sur la prise en compte implicite du couplage glissement/fluide. Le modèle est validé dans un premier temps, dans le cas de génération d'un soliton par la chute d'un bloc rigide à l'extrémité d'un canal. Une étude numérique de l'influence du caractère déformable du glissement est ensuite proposée. Ce travail met en évidence l'importance et la complexité du rôle de cette déformation sur les caractéristiques des vagues générées. Il convient donc de prendre en compte de manière plus fine la rhéologie du glissement dans le processus de prédiction des tsunamis

Fully three-dimensional direct numerical simulation of a plunging breaker

International audienceThe scope of this paper is to show the results obtained for simulating three-dimensional breaking waves by solving the Navier–Stokes equations in air and water. The interface tracking is achieved by a Lax–Wendroff TVD scheme (Total Variation Diminishing), which is able to handle interface reconnections. We first present the equations and the numerical methods used in this work. We then proceed to the study of a three-dimensional plunging breaking wave, using initial conditions corresponding to unstable periodic sinusoidal waves of large amplitudes. We compare the results obtained for two simulations, a longshore depth perturbation has been introduced in the solution of the flow equations in order to see the transition from a two-dimensional velocity field to a fully three-dimensional one after plunging. Breaking processes including overturning, splash-up and breaking induced vortex-like motion beneath the surface are presented and discussed

Analyse statistique du lien entre les variables d'aléa et les dommages par submersion

International audienceUne base de données de niveau de dommages consécutif à la submersion couvrant les 60 dernières années et focalisée sur la Grande Plage de Biarritz a été récemment élaborée. Cette base repose sur l'analyse d'archives et de documents de presse locaux. Parallèlement, une base de données de variables d'aléas couvrant la même période a également été constituée. Elle utilise des simulations numériques pour les vagues et une analyse harmonique pour les niveaux d'eau, validées par les observations disponibles sur site. Dans le présent article, nous détaillons l'analyse du lien statistique existant entre les deux bases de données. L'objectif est de trouver quelles variables d'aléa ou combinaison de variables expliquent le mieux l'occurrence de la submersion de la Grande Plage et la survenue de dommage. Les résultats préliminaires montrent que le couple flux maximum d'énergie des vagues et niveau d'eau maximum durant l'événement expliquent très bien les dommages répertoriés sur cette plage spécifique

Development of a new nanotribometer with multi asperity contact.

International audienceIt is often difficult to make a connection between the tribological properties really assessed for a single asperity and the ones which involve the whole asperities in a microcontact. We propose a new apparatus development based on passive diamagnetic levitation (PDL) that can be used to study friction in microcontact with a lower range of force than with classical nanotribometers. This sensor measures micro and nanoforces. Its sensitive part is a ten centimeters long glass capillary tube used as a horizontal levitating seismic mass (20 to 80 mg). This rigid part is connected to a magnetic spring with stiffness between 0.01 and 0.03 N/m. The measurement range is 100μN with a resolution between one and five nanonewton. In order to validate this sensor in a multi asperity nanotribological context, this part is used to measure tangential friction forces generated by spherical micro-objects sliding on flat substrate. Results are compared with the ones provided by a classical multiasperity nanotribometer