Multi-Scale Modeling of a Wind Turbine Wake Over Complex Terrain in Different Atmospheric Stability Regimes

Most previous modeling and simulation of wind turbine wakes has been done over flat terrain. However, wind turbines are commonly sited in complex terrain to take advantage of accelerating flow over ridgelines. How turbine wakes interact with terrain-induced flow features, such as recirculation zones and localized coherent structures, can also be strongly dependent on thermal stratification. The objective of this work is to model stable, neutral, and convective atmospheric conditions to gain insight into wind turbine wake behavior in complex terrain.

The test case for this study is the site of the 2017 Perdigão field campaign in Portugal which consists of two parallel ridges with a wind turbine on top of one of the ridges. The Weather Research and Forecasting (WRF) model is used to conduct large-eddy simulations (LES) with both ideal and real configurations. Both setups use grid resolutions as fine as 10 m in the horizontal with a generalized actuator disk model used to represent flow interaction with the wind turbine. The idealized setup features an outer domain with flat terrain and periodic boundary conditions to force an inner domain with the complex Perdigão terrain where atmopsheric stability is provided using the surface heat flux. The other setup uses a multi-scale grid nested setup to couple mesoscale forcing with the microscale to generate realistic conditions where atmospheric stability is inherent to the cases selected. Outer domains use a planetary boundary layer closure scheme, while the inner domains use an LES turbulence closure.

Simulation results are compared with data from meteorological towers and lidar scans taken during the Perdigão field campaign covering representative atmospheric conditions where the wind direction is roughly perpendicular to the hills. The wake behavior is dependent on atmospheric stability, showing different amounts of vertical deflection from the terrain and persistence downstream for stable, neutral, and convective conditions. This wake behavior is also influenced by any recirculation zones formed in the valley between the two ridges under the different atmospheric stability regimes.