Modelling wind-tree interaction

Windstorms over forests often cause serious damages to the trees. Because of the complexity of the various processes responsible for windthrow in heterogeneous landscapes, modelling both tree and flow dynamics should provide tools for quantifying tree vulnerability to windload at any position in the landscape.

It has been shown that the Large-Eddy Simulation (LES) technique can be applied at the vegetation scale and reproduce the main features of turbulent flow observed over forests. LES allows one to have access to instantaneous dynamic fields and is consequently able to simulate the wind gusts responsible for tree motions.

In order to investigate the possibility of modelling tree motion at the landscape scale, we introduced an equation for tree motion, forced by the instantaneous wind flow, in a LES model. Here, the canopy is simply represented as an infinite row of identical mechanical flexible oscillators. Hence, the two-way coupling between the tree motion and the wind flow occurs through the drag force term.

In a previous step this full wind-plant interaction model was successfully validated on an alfalfa crop canopy. An extension of the model to a forest canopy is presented here. To this purpose, (1) multiple vibration modes are considered instead of a single one as for the crop canopy, and (2) a more realistic tree dynamic model is developed using the Elastica theory instead of the small displacement hypothesis that was considered for crop plants. The tree motion model is first validated against a more detailed finite-element tree model without coupling with the LES airflow model. Simulations of the complete interaction between maritime pine tree motions and turbulent wind flow are then presented at the scale of a continuous homogeneous forest.