Edge flutter of long beams under follower loads

The linear instability of a beam tensioned by its own weight is considered. It is shown that for long beams, in the sense of an adequate dimensionless parameter, the characteristics of the instability caused by a follower force do not depend on the length. The asymptotic regime significantly differs from that of short beams: flutter prevails for all types of follower loads, and flutter is localized at the edge of the beam. An approximate solution using matched assymptotic expansion is proposed for the case of a semi-infinite beam. Using a local criterion based on the stability of waves, the characteristics of this regime as well as its range of application can be well predicted. These results are finally discussed in relation with cases of flow-induced instabilities of slender structures.Comment: to appear in Journal of Mechanics of Materials and Structure

Flow-induced pruning of branched systems and brittle reconfiguration

Whereas most plants are flexible structures that undergo large deformations under flow, another process can occur when the plant is broken by heavy fluid-loading. We investigate here the mechanism of such possible breakage, focusing on the flow-induced pruning that can be observed in plants or aquatic vegetation when parts of the structure break under flow. By computation on an actual tree geometry, a 20-yr-old walnut tree (Juglans Regia L.) and comparison with simple models, we analyze the influence of geometrical and physical parameters on the occurrence of branch breakage and on the successive breaking events occurring in a tree-like structure when the flow velocity is increased. We show that both the branching pattern and the slenderness exponent, defining the branch taper, play a major role in the breakage scenario. We identify a criterion for branch breakage to occur before breakage of the trunk. In that case, we show that the successive breakage of peripheral branches allows the plant to sustain higher flow forces. This mechanism is therefore similar to elastic reconfiguration, and can be seen as a second strategy to overcome critical events, possibly a widespread solution in plants and benthic organisms.Comment: 9 pages, 9 figure

A space-averaged model of branched structures

Many biological systems and artificial structures are ramified, and present a high geometric complexity. In this work, we propose a space-averaged model of branched systems for conservation laws. From a one-dimensional description of the system, we show that the space-averaged problem is also one-dimensional, represented by characteristic curves, defined as streamlines of the space-averaged branch directions. The geometric complexity is then captured firstly by the characteristic curves, and secondly by an additional forcing term in the equations. This model is then applied to mass balance in a pipe network and momentum balance in a tree under wind loading.Comment: 10 pages, 11 figure

Drag Reduction, from Bending to Pruning

Most plants and benthic organisms have evolved efficient reconfiguration mechanisms to resist flow-induced loads. These mechanisms can be divided into bending, in which plants reduce their sail area through elastic deformation, and pruning, in which the loads are decreased through partial breakage of the structure. In this work, we show by using idealized models that these two mechanisms or, in fact, any combination of the two, are equally efficient to reduce the drag experienced by terrestrial and aquatic vegetation.Comment: 5 pages, 5 figure

Methodological advances in predicting flow-induced dynamics of plants using mechanical-engineering theory

International audienceThe modeling of fluid-structure interactions, such as flow-induced vibrations, is a well-developed field of mechanical engineering. Many methods exist, and it seems natural to apply them to model the behavior of plants, and potentially other cantilever-like biological structures, under flow. Overcoming this disciplinary divide, and the application of such models to biological systems, will significantly advance our understanding of ecological patterns and processes and improve our predictive capabilities. Nonetheless, several methodological issues must first be addressed, which I describe here using two practical examples that have strong similarities: one from agricultural sciences and the other from nuclear engineering. Very similar issues arise in both: individual and collective behavior, small and large space and time scales, porous modeling, standard and extreme events, trade-off between the surface of exchange and individual or collective risk of damage, variability, hostile environments and, in some aspects, evolution. The conclusion is that, although similar issues do exist, which need to be exploited in some detail, there is a significant gap that requires new developments. It is obvious that living plants grow in and adapt to their environment, which certainly makes plant biomechanics fundamentally distinct from classical mechanical engineering. Moreover, the selection processes in biology and in human engineering are truly different, making the issue of safety different as well. A thorough understanding of these similarities and differences is needed to work efficiently in the application of a mechanistic approach to ecology

Damping by branching: a bioinspiration from trees

Man-made slender structures are known to be sensitive to high levels of vibration, due to their flexibility, which often cause irreversible damage. In nature, trees repeatedly endure large amplitudes of motion, mostly caused by strong climatic events, yet with minor or no damage in most cases. A new damping mechanism inspired by the architecture of trees is here identified and characterized 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 that we call damping-by-branching. It originates in the geometrical nonlinearities so that it is specifically efficient to damp out large amplitudes of motion. A more realistic model, using flexible beam approximation, shows that the mechanism is robust. Finally, two bioinspired architectures are analyzed, showing significant levels of damping achieved via branching with typically 30% of the energy being dissipated in one oscillation. This concept of damping-by-branching is of simple practical use in the design of slender flexible structures.Comment: 10 pages, 10 figure

Drag reduction of flexible plates by reconfiguration

International audienceThrough an extensive and systematic experimental investigation of two geometries of flexible plates in air, it is shown that a properly defined scaled Cauchy number allows collapsing all drag measurements of the reconfiguration number. In the asymptotic regime of large deformation, it is shown that the Vogel exponents that scale the drag with the flow velocity for different geometries of plates can be predicted with a simple dimensional analysis reasoning. These predicted Vogel exponents are in agreement with previously published models of reconfiguration. The mechanisms responsible for reconfiguration, namely area reduction and streamlining, are studied with the help of a simple model for flexible plates based on an empirical drag formulation. The model predicts well the reconfiguration observed in the experiments and shows that for a rectangular plate, the effect of streamlining is prominent at the onset of reconfiguration, but area reduction dominates in the regime of large deformation. Additionally, the model demonstrates for both geometries of plates that the reconfiguration cannot be described by a single value of the Vogel exponent. The Vogel exponent asymptotically approaches constant values for small and for very large scaled Cauchy numbers, but in between both extremes it varies significantly over a large range of scaled Cauchy number. Copyright © Cambridge University Press 2010

Self-similar vortex-induced vibrations of a hanging string

International audienceAn experimental analysis of the vortex-induced vibrations of a hanging string with variable tension along its length is presented in this paper. It is shown that standing waves develop along the hanging string. First, the evolution of the Strouhal number S t with the Reynolds number R e follows a trend similar to what is observed for a circular cylinder in a flow for relatively low Reynolds numbers (32 < Re < 700). Second, the extracted mode shapes are self-similar: a rescaling of the spanwise coordinate by a self-similarity coefficient allows all of them to collapse onto a unique function. The self-similar behaviour of the spatial distribution of the vibrations along the hanging string is then explained theoretically by performing a linear stability analysis of an adapted wake-oscillator model. This linear stability analysis finally provides an accurate description of the mode shapes and of the evolution of the self-similarity coefficient with the flow speed

Flutter of long flexible cylinders in axial flow

International audienceWe consider the stability of a thin flexible cylinder considered as a beam, when subjected to axial flow and fixed at the upstream end only. A linear stability analysis of transverse motion aims at determining the risk of flutter as a function of the governing control parameters such as the flow velocity or the length of the cylinder. Stability is analysed applying a finite-difference scheme in space to the equation of motion expressed in the frequency domain. It is found that, contrary to previous predictions based on simplified theories, flutter may exist for very long cylinders, provided that the free downstream end of the cylinder is well-streamlined. More generally, a limit regime is found where the length of the cylinder does not affect the characteristics of the instability, and the deformation is confined to a finite region close to the downstream end. These results are found complementary to solutions derived for shorter cylinders and are confirmed by linear and nonlinear computations using a Galerkin method. A link is established to similar results on long hanging cantilevered systems with internal or external flow. The limit case of vanishing bending stiffness, where the cylinder is modelled as a string, is analysed and related to previous results. Comparison is also made to existing experimental data, and a simple model for the behaviour of long cylinders is proposed