Large-eddy simulation of turbulent flow over a forested hill: Validation and coherent structure identification

Modelling turbulent flow at a very fine scale within and above vegetation canopies over complex terrain is of great interest for many environmental applications. Over the last decade, it has been demonstrated that the large-eddy simulation (LES) technique is able to reproduce in detail many observed features Of turbulent flow over homogeneous and heterogeneous vegetation canopies on flat terrain. For the first time, a nested LES of turbulent flow within and above a forested canopy on an isolated two-dimensional hill is analysed and validated against pressure and velocity data from a wind-tunnel experiment. The model is shown to be able to reproduce accurately the main features observed over a forested hill. The results also confirm recent observations on the intermittency of the recirculation region behind the hill, and on the domination of sweep motions in momentum transfer at the canopy top all along the hill. The main characteristics of turbulent Structures have also been investigated from the analysis of vorticity fields and two-point velocity correlations across the hill. It is shown that the streamwise wind velocity upwind from the Summit 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 the canopy top is only correlated with the flow within the wake region. This result suggests that turbulence within the wake region results from the superposition of various turbulent structures: large turbulent structures induced by the elevated shear layer that may result from the rolling over of Kelvin-Helmotz instabilities, structures induced at the lee-side foot of the hill by an adverse pressure gradient and structures induced by the presence of the canopy. Implications of hilly terrain on canopy-atmosphere exchanges and tree vulnerability to windload are briefly discussed. Copyright (C) 2008 Royal Meteorological Society