Search

Plant vibrations have been studied for many purposes, both at the whole plant scale or at the local organ scale, see for instance in wind induced physical response of trees or crops [1], insect communication through vibration [2] or thigmomorphogenesis [3] . The experimental measurement of vibrational characteristics of plants, such as the frequencies of free motion, has considerably improved in the past ten years thanks to video motion capture techniques and signal processing . Recent tests on Poplar and Arabidopsis showed that these frequencies revealed important information combining the architecture, the geometry and the local mechanical properties of the plant [4] . This led to the idea of using vibrations for phenotyping plants, as a non-destructive, non-contact and fast method. We have developed a way to obtain quickly (less than a minute) a large quantity of information on the dynamics of a plant, more precisely on its dominant vibration modes. The method is based on an excitation by air pulses, a high speed video recording of the motion, and quasi real time signal processing of images to derive the dominant frequency of the captured motion. Tests on water stressed tobacco, on several mutants of Arabidopis thaliana and on mechanically stressed poplar, showed the robustness and performance of the method. More recently an advanced signal processing has been developed that allows extracting several frequencies present simultaneously in the motion, for instance those of leaves. This was tested on poplar and oak foliage. The method is currently under implementation in automatic phenotyping facilities.

Communication oraleCommunication orale; absent

International audience

Poster + résuméPoster + résumé; absent

International audience; Strong dynamic loads in man-made slender structures are likely to produce large amplitude vibrations of due to their flexibility, which often cause functionality losses or inflict irreversible damages. The amplitude of motion results, on one hand, from the characteristics of the loading, and on the other hand, from the characteristics of the structure in terms of inertia, stiffness and damping. A high rate of damping, i.e. mechanical energy dissipation whatever the physical mechanism, is a standard way to limit vibrations of large amplitude and their undesirable consequences. However, adding damping in slender structures remains a challenging problem in engineering. As slender and flexible structures, trees repeatedly endure strong climatic events, yet with minor or no damages in most cases. Therefore, trees regularly submitted to natural flow excitations by wind are likely to possess efficient and specific strategies of damping. In fact, several damping mechanism have been identified in trees: viscoelastic behaviour of wood, aeroelasticity from the interactions with air, and the linear tuned-mass-damper effect between the trunk and the branches. However, the latter, as a linear mechanism, is not amplitude-dependent and therefore does not damp specifically large amplitude motions. A new amplitude-dependent damping mechanism, inspired from trees, is identified and characterised here in the simplest tree-like structure, a Y-shape branched structure. Through analytical and numerical analyses of a simple two-degree-of-freedom model, branching is shown to be the key ingredient in this protective mechanism. This damping-by-branching originates in the geometrical nonlinearities and is therefore specifically efficient to damp out large amplitudes of motion. A more realistic model of the Y-shape structure, using beam finite-element approximation, exhibits the same effect, which seems to be robust. Similarly, two arbitrary multi-branched slender architectures show significant levels of damping of the trunk motion while only the relative branch motion is damped. Finally, the same numerical analysis is performed on a digitalised walnut tree geometry showing the significant damping contribution of this mechanism during large amplitude motions.

Colloque avec actes et comité de lecture. Internationale.; International audience; La dynamique d'une plante est caractérisée par stimulation mécanique à l'aide d'un pot vibrant par balayages entre 0 et 40Hz. On analyse les films obtenus pour obtenir le champ de vitesse, que l'on traite par Décomposition Bi Orthogonale. On détermine ainsi l'existence d'un mode dans la plage de fréquence considérée et la déformée modale associée. D'autre part, un calcul éléments finis des modes à partir des caractéristiques réelles de la plante est effectué et les résultats sont comparés aux expériences. On observe des modes localisés et organisés en modes de tige, de feuilles et d'apex.

International audience; A modal energy transfer mechanism caused by geometrical nonlinearities is identified and characterized in a cantilever Y-shaped structure. Dynamical analysis of a simple two-degree-of-freedom model allow to assess the parameters regulating this mechanism. A numerical simulation of the dynamics of a finite-element model exhibits the same mechanism.

International audience; Experimental data on the frequencies of a walnut tree are presented. These are derived from several kinds of tests using protocols that are commonly used in vibration analysis: pull and release test, hammer impact tests and wind excitation test. The results from these experiments are then used to validate an analytical approach based on the llometry of the tree. This is made possible by a detailed digitization of the tree geometry.

absent

Wind is a major ecological factor for plants and a major economical factor for forestry. Mechanical analyses have revealed that the multimodal dynamic behavior of trees is central to wind and tree interactions. Moreover, the trunk and branches influence dynamic modes, both in frequency and location. Because of the complexity of tree architecture, finite element models (FEMs) have been used to analyze such dynamics. However, these models require detailed geometric and architectural data and are tree-specific. In this work, closed-form scaling laws for modal characteristics were derived from the dimensional analysis of idealized fractal trees that sketched the major architectural and allometrical regularities of real trees [16]. These scaling laws were compared to three-dimensional FEM modal analyses of two completely digitized trees with maximal architectural contrast. Despite their simplifying hypotheses, the models explained most of the spatiotemporal characteristics of modes that involved the trunk and branches, especially for sympodial trees. These scaling laws reduce the tree to (1) a fundamental frequency and (2) one architectural and three biometrical parameters. They also give quantitative insights into the possible biological control of wind excitability of trees through architecture and allometries. It is shown that the mechanical design of tree depends on branching symmetries and allometries, and is likely to be controlled through thigmomorphogenesis.

National audience; Agricultural landscapes often exhibit large spatial variability caused by the presence of different crops, clearings, roads, forest patches of various height, etc. Such heterogeneities may influence the turbulent structure of the lower atmosphere and be involved in damages to crops or forests induced by windstorms. Because of the complexity of the various processes responsible for windthrow in heterogeneous landscapes, modelling both plant and flow dynamics may provide tools that should allow quantification of tree vulnerability to windload following its position in heterogeneous environments. Over the last decade, it has been demonstrated that the Large-Eddy Simulation (LES) technique can be applied at the vegetation scale and reproduce the main features of turbulent flow observed over homogeneous vegetation canopies as well as downwind from forest leading edges. LES allows one to have access to instantaneous dynamical fields and is consequently able to simulate the wind gusts responsible for plant motions. In these LES models, the canopy is usually simply represented by a drag force term in the momentum equation, with no account for plant motions. In order to investigate the possibility of modelling plant motion at the landscape scale, we introduced an equation for plant motion, forced by the instantaneous wind flow, in a LES model that has been previously validated over homogeneous and heterogeneous canopies. Here, the canopy is simply represented as an infinite row of identical mechanical oscillators where only the fundamental mode of plant vibration is considered. Hence, the two-way coupling between the plant motion and the wind flow occurs through the drag force term. This new model is validated on a crop canopy of alfalfa whose motions have been analyzed in great details from video recordings. It is shown that the model is able to reproduce the well-known phenomenon of Honami which is typical of wavelike crop motions on windy days. Moreover, the wavelength of the main coherent waving patches, extracted using a bi-orthogonal decomposition (BOD) of the crop velocity fields, is in agreement with that deduced from video recordings and exhibits the same dependence as measurements with the magnitude of the mean wind velocity. The impact of crop motion on the wind flow and the relation between organized crop motion and wind flow coherent structures are discussed. Finally, it is shown that the model could be extended in future to forest canopy.

In evaluating the service life of marine structures such as risers, one must assess their dynamic response to vortices excitation (vortex-induced vibrations or VIV). At lock-in, i.e. when the frequency of the shedding of vortices synchronizes with the natural frequency of the structure, the amplitude of vibration of the structure is of the order of its diameter. Sustained vibration of this amplitude causes material fatigue which leads with time to failures. Predicting the flow velocity range for which lock-in occurs is thus of importance. It is also important in cases of flexible structures to predict which mode of vibration will lock-in with the wake. The interest here is in predicting lock-in for rigid and flexible structures like tensioned beams using a linear wake oscillator model: here, lock-in is interpreted as a linear instability caused by the merging of the two natural frequencies of a dynamic system that includes two coupled oscillators, namely the wake and the structure. This instability is also referred as couple mode flutter. It is found that the linear wake oscillator model predicts lock-in range for both elastically supported rigid cylinder and for flexible structures subjected to uniform flows. In particular, the linear approach is able to predict transition from one lock-in mode to another for flexible structures (mode-switching). it is also capable of predicting when more than one mode can lock-in with the wake for a fixed velocity (time sharing).

Colloque avec actes et comité de lecture. Internationale.; International audience; L'observation d'une goutte de liquide mouillant transportée à pression constante dans un microcanal en T permet de mettre en évidence trois comportements à la jonction: en dessous d'une pression seuil, la goutte reste bloquée à l'entrée de la bifurcation; au dessus, la goutte éclate ou se divise en deux gouttes filles dépendamment de sa longueur initiale et la pression de forçage. Nous prédisons les différentes transitions au moyen d'arguments géométriques et dynamiques.

Colloque avec actes et comité de lecture. Internationale.; International audience; Lorsque de l'eau ruisselle autour d'un cylindre dans un écoulement d'air, deux filets d'eau se forment le long du cylindre au voisinage des points de décollement. Dans cet article, nous étudions l'interaction entre ces filets et le sillage. Nous montrons expérimentalement que, pour certaines vitesses de vent, les filets vibrent à une fréquence proche de la fréquence de détachement tourbillonnaire. Nous supposons que ces vibrations résultent d'ondes capillaires stationnaires sur la surface libre des filets. La fréquence propre du mode capillaire fondamental est estimée par une analyse énergétique. Nous élaborons ensuite un modèle d'interaction entre les filets et le sillage qui prévoit un accrochage entre ces deux degrés de liberté pour un intervalle de vitesse de vent et une amplification simultanée de la force transverse fluctuante s'exerçant sur le cylindre. Ces résultats permettent d'expliquer les expériences conduites récemment par Alam & Zhou (2006).

absent

International audience; A simple model for the near wake dynamics of slender bluff bodies in cross-flow is analyzed. It is based on a continuous distribution of van der Pol oscillators arranged along the spanwise extent of the structure and interacting by diffusion. Diffusive interaction is shown to be able to model cellular vortex shedding in shear flow, the cell size being estimated analytically with respect to the model parameters. Moreover, diffusive interaction succeeds in describing qualitatively the global suppression of vortex shedding from a sinuous structure in uniform flow. [Nous analysons un modèle simple de la dynamique du sillage proche derrière une structure élancée. Le modèle est constitué par une distribution continue, le long de la structure, d'oscillateurs de van der Pol interagissant par diffusion. En écoulement cisaillé, la diffusion permet de décrire le détachement tourbillonnaire par cellules, dont la taille est ici calculée analytiquement en fonction des paramètres du modèle. Dans le cas d'une structure sinueuse en écoulement uniforme, le modèle reproduit qualitativement la suppression globale du détachement tourbillonnaire. ] © 2002 Académie des sciences/Éditions scientifiques et médicales Elsevier SAS.

International audience; On propose, par une approche de développement asymptotique, une valeur approchée de la force suiveuse qui rend instable une poutre semi-infinie suspendue. La validation expérimentale est réalisée à l'aide d'un tuyau sous écoulement interne. On observe la transition entre la solution classique pour la poutre encastrée-libre de faible longueur soumise à une force suiveuse et la solution pour une longueur infinie. [Using matched asymptotic expansions, we give here an approximate value of the follower force that results in the instability of a semi-infinite hanging beam. An experimental verification is shown in the context of fluid-conveying pipes. The transition between the classical short length solution and the proposed solution for infinite length is also observed in the experiments.] © 2001 Académie des sciences/Éditions scientifiques et médicales Elsevier SAS.

International audience; On étudie, à partir de l'équation de dispersion, les conditions dans lesquelles peut apparaître une instabilité absolue sur les parois élastiques d'un canal parcouru par un écoulement potentiel. Le seuil d'instabilité absolue est donné de façon exacte dans le cas des films minces pour des parois formées de plaques en flexion ou de membranes. Le cas général est résolu numériquement et rejoint les résultats classiques pour une paroi en présence d'un domaine fluide semi-infini. On montre que la nature de la rigidité des parois influe fortement sur l'apparition de l'instabilité absolue. [Conditions for absolute instability of the elastic walls of a plane channel with potential flow are investigated. The absolute instability threshold is given in closed form in the thin film approximation for walls made of plates or membranes. The more general case is solved numerically and is compared with more classical results for a single wall in the presence of a semi-infinite fluid domain. It is found that absolute instability is strongly influenced by the membrane or plate nature of the wall stiffness.] (C) 2000 Academie des sciences/Editions scientifiques et medicales Elsevier SAS.

The flutter of a cantilever fluid-conveying pipe is one of the most generic problems of flow-induced vibrations. In its simplest model, linear stability is lost as the flow velocity is increased, at a threshold which depends on the mechanical parameters of the system. This is a global instability. An infinite pipe modeled with the same local equations experience local instability as soon as the fluid is put into motion. When an elastic foundation is present, cases of local stability with global instability, or, conversely, local instability with global stability are found. The purpose of this paper is to address the question of compatibility between local and global approaches. More specifically, when an elastic foundation is present, we give a criterion on traveling waves to allow a global instability to develop.