Atmospheric transport of terrain-induced turbulence from high-resolution LES experiments

Knowledge about turbulence intensity at the wind turbine level is important for wind energy industry. Over a complex terrain environment, an important question is how various topography features may generate or alter turbulence and how far the influence can be extended downstream. Direct measurements of turbulence at wind turbine level are difficult, especially over complex terrain. The limitation is the available technology that lacks the ability to provide a long-range snapshot of turbulence as the eddies travel over terrain, interact with each other, change their productive and dissipative properties, and are then observed some 10 km downstream of their source. On the other hand, large eddy simulation (LES) has proven to be an effective computational tool to characterize turbulence in complex terrain environment and associate it with its dominant source and mode of production.

In this study, the Atmospheric Regional Prediction System model (ARPS) is employed for LES modelling of atmospheric transport of terrain-generated turbulence in a neutral atmosphere. The simulations are two-dimensional with an isotropic spatial resolution of 15 m and run to a quasi-steady state. The simulations are designed in such a way to allow an examination of the effects of a bell-shaped mountain with varying height and aspect ratio on turbulence properties generated by another mountain 20 km upstream. The background winds were chosen to be 15 ms-1, so as to fall within the wind energy production power range and the atmosphere is assumed to be neutral. The very high spatial resolution of the simulations enable an detailed examination of turbulent kinetic energy (TKE) budget through tendency partitioning of the energy budget equation while explicitly resolving the turbulent information of eddy scales larger than ~ 15 m.

Preliminary results showed very detailed flow structures that resemble a multitude of eddy scales dynamically interacting while shearing oppositely paired vortices. Changes in the TKE tendency were observed at distances up to 15 km upwind of the hill, which signify the effect of the terrain feature on measurements upwind of it. Also, in the wake region of the experiment with higher aspect ratio, the TKE production from both advection and shear tendencies were dominant for heights lower than 500 m when compared to the experiment with lower aspect ratio. Future work will be focussed on developing clear relationships between TKE tendencies and upwind terrain configurations, and also include the effect on vortex shedding by examining the sizes and intensity of advected/produced vortices through an area downstream of the terrain.