Large-Eddy Simulation of turbulent flow over a forested hill

Modelling turbulent flow at very fine scale within and above vegetation canopies over complex terrain is of great interest for many environmental applications such as forest management, wind energy applications, forest-atmosphere scalar exchange, forest fire propagation, etc. Over the last decade, it has been demonstrated that Large-Eddy Simulation (LES) technique is able to reproduce the main features of turbulent flow observed over homogeneous and heterogeneous vegetation canopies on flat terrain. However, little work has been achieved over complex terrain.

Here, a nested LES of turbulent flow within and above a forest canopy on an isolated two-dimensional hill is analyzed in details and validated against wind-tunnel data. The model is shown to reproduce accurately the main features observed over a forested hill. The simulations also confirm recent observations on the intermittent character of the recirculation region behind the hill and on the dominance of sweep motions in momentum transfer at canopy top all along the hill.

The main characteristics of turbulent structures are also investigated from vorticity fields and two-point velocity correlations simulated across the hill. It is shown that the streamwise wind velocity upwind from the ridge is not correlated with the flow within the wake region but only with the flow above it, while in the wake region, the streamwise wind velocity at canopy top is only correlated with the flow within the wake region. This result may suggest that turbulence within the wake region results from the superimposition of various turbulent structures: i) large structures induced by the elevated shear layer that may result from Kelvin-Helmoltz instabilities, ii) structures induced by the adverse pressure gradient at the lee-side foot of the hill, and iii) structures induced by the presence of the canopy itself. Implications of hilly terrain on canopy-atmosphere exchanges and tree vulnerability to windload are also briefly discussed.