Modelling waving crops using Large-Eddy Simulation

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. Finally, the impact of crop motion on the wind flow is discussed.