Xavier Amandolese

Foliage motion under wind, from leaf flutter to branch buffeting

The wind-induced motion of the foliage in a tree is an important phenomenon both for biological issues (photosynthesis, pathogens development or herbivory) and for more subtle effects such as on wi-fi transmission or animal communication. Such foliage motion results from a combination of the motion of the branches that support the leaves, and of the motion of the leaves relative to the branches. Individual leaf dynamics relative to the branch, and branch dynamics have usually been studied separately. Here, in an experimental study on a whole tree in a large-scale wind tunnel, we present the first empirical evidence that foliage motion is actually dominated by individual leaf flutter at low wind velocities, and by branch turbulence buffeting responses at higher velocities. The transition between the two regimes is related to a weak dependence of leaf flutter on wind velocity, while branch turbulent buffeting is strongly dependent on it. Quantitative comparisons with existing engineering-based models of leaf and branch motion confirm the prevalence of these two mechanisms. Simultaneous measurements of the wind-induced drag on the tree and of the light interception by the foliage show the role of an additional mechanism, reconfiguration, whereby leaves bend and overlap, limiting individual leaf flutter. We then discuss the consequences of these findings on the role of wind-mediated phenomena.

Chaotic Dynamics of Flags from Recurring Values of Flapping Moment

The performance of recently proposed flag-based energy harvesters is strongly limited by the chaotic response of flags to strong winds. From an experimental point of view, the detection of flag chaotic dynamics were scarce, based on the flapping amplitude and the maximal Lyapunov exponent. In practice, tracking the flapping amplitude is difficult and flawed in the large oscillation limit. Also, computing the maximal Lyapunov exponent from time series of limited size requires strong assumptions on the attractor geometry, without getting insurance of their reliability. For bypassing these issues, (1) we use a time series which takes into account the whole dynamics of the flag, by using the flapping moment which integrates its displacements, and (2) we apply an algorithm of detection of chaos based on recurring values in time series.

The transient temporal response of a flexible bridge deck subjected to a single gust

Temporal simulations are increasingly performed in wind effects analysis of flexible structures. By comparison with classical techniques such as spectral methods, temporal simulations provide the advantage of easily combining different kinds of loads, can take nonlinearities into account and also provide the only way to reproduce transient behaviors. In that context this study deals with the transient response of a two-degrees-of-freedom streamlined bridge deck section subjected to a single gust. Experimental evidence of the potentially high level of transient energy amplification due to that kind of extraneous excitation have been recently demonstrated for an airfoil section and for a streamlined bridge deck section, below the critical coupled-mode flutter wind speed. The present study then focuses on the validation of a time-dependent model, based on a simple formulation of both the motion-dependent and the buffeting forces, for catching that kind of transient behavior. A parametric study is also made in order to highlight the impact of the pitch-plunge frequency ratio on the energy amplification below the critical condition.