Pascal Hémon

Modelling waving crops using large-eddy simulation: comparison with experiments and a linear stability analysis

In order to investigate the possibility of modelling plant motion at the landscape scale, an equation for crop plant motion, forced by an instantaneous velocity field, is introduced in a large-eddy simulation (LES) airflow model, previously validated over homogeneous and heterogeneous canopies. The canopy is simply represented as a poroelastic continuous medium, which is similar in its discrete form to an infinite row of identical oscillating stems. Only one linear mode of plant vibration is considered. Two-way coupling between plant motion and the wind flow is insured through the drag force term. The coupled model is validated on the basis of a comparison with measured movements of an alfalfa crop canopy. It is also compared with the outputs of a linear stability analysis. The model is shown to reproduce the well-known phenomenon of honami which is typical of wave-like crop motions on windy days. 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. The main spatial and temporal characteristics of these waving patches exhibit the same variation with mean wind velocity as that observed with the measurements. However they differ from the coherent eddy structures of the wind flow at canopy top, so that coherent waving patches cannot be seen as direct signatures of coherent eddy structures. Finally, it is shown that the impact of crop motion on the wind dynamics is negligible for current wind speed values. No lock-in mechanism of coherent eddy structures on plant motion is observed, in contradiction with the linear stability analysis. This discrepancy may be attributed to the presence of a nonlinear saturation mechanism in LES.

Applications of biorthogonal decompositions in fluid-structure interactions

Abstract This paper is dedicated to the study of the orthogonal decomposition of spatially and temporally distributed signals in fluid–structure interaction problems. First application is concerned with the analysis of wall-pressure distributions over bluff bodies. The need for such a tool is increasing due to the progress in data-acquisition systems and in computational fluid dynamics. The classical proper orthogonal decomposition (POD) method is discussed, and it is shown that heterogeneity of the mean pressure over the structure induces difficulties in the physical interpretation. It is then proposed to use the biorthogonal decomposition (BOD) technique instead; although it appears similar to POD, it is more general and fundamentally different since this tool is deterministic rather than statistical. The BOD method is described and adapted to wall-pressure distribution, with emphasis on aerodynamic load decomposition. The second application is devoted to the generation of a spatially correlated wind velocity field which can be used for the temporal calculation of the aeroelastic behaviour of structures such as bridges. In this application, the space–time symmetry of the BOD method is absolutely necessary. Examples are provided in order to illustrate and show the satisfactory performance and the interest of the method. Extensions to other fluid–structure problems are suggested.

An Experimental Study of the Acoustic Oscillations by Flows Over Cavities

We present an experimental study of acoustic oscillations induced by an internal airflow over a shallow and a deep cavity. The Kelvin-Helmholtz instability is interacting with an acoustic mode of the cavity or of the duct, leading to a resonance which produces a very high sound level. The influence of upstream boundary layer thickness and neck thickness is studied. Some results obtained by modifying the upstream lip shape, by crenel addition, are also given.

VORTEX INDUCED VIBRATIONS OF A SQUARE CYLINDER IN A WIND TUNNEL

An experimental study of the vortex-induced vibrations of a rigid square cylinder, mounted flexibly, in a wind tunnel is presented. Special attention is paid to keep the structural damping as low as possible. Structural supports are assumed to behave linearly through out the amplitude envelope. Experimental data comprises of amplitude curves and transient slopes. The experimental procedure was repeated to study the cylinder behaviour both under the memory effect and without it. The classical mode switch can be pointed out in both the cases. Hysteresis is however found only in the former case. Measurements consist of the time histories of oscillations using laser displacement sensors. Structural parameters are estimated without airflow. Time evolution of energy of the square cylinder is recorded and later manipulated to calculate the growth rate of oscillation amplitude in the transient regime.Copyright

Study of the Acoustic Oscillations by Flows Over Cavities: Part 1 — Internal Flows

We present a study of acoustic oscillations induced by an internal airflow over a shallow and a deep cavity. The Kelvin-Helmholtz instability is interacting with an acoustic mode of the duct, leading to a resonance which produces a very high sound level. The influence of upstream boundary layer thickness and neck thickness is studied. Some results obtained by modifying the upstream lip shape, by crenel addition, are also given. It is also shown that the numerical simulations using a lattice-gas method give relatively good results by comparison with the experiments. Especially the resonance with the duct acoustics was qualitatively reproduced.© 2002 ASME

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.

Study of the Acoustic Oscillations of Flow Over Cavities: Part 2 — Sound Reduction

We present experimental results obtained with a deep cavity, like an Helmholtz resonator, excited by an airflow. The resonance under the action of the vortices generated in the shear layer is well described and quantified. The mounting of actuators, based on a few piezo-electric elements, allows to generate a series of two-dimensional vortices forced at a frequency which is different than the natural resonance frequency. The sound level in the cavity is strongly decreased and the broadband noise of the turbulence only remains.Copyright