Meso- to microscale modeling of atmospheric stability effects on wind turbine wake behavior in complex terrain

Adam S Wise,Julie K Lundquist,Fotini K Chow,Robert S Arthur,James M T Neher,Jeffrey D Mirocha

doi:10.5194/wes-7-367-2022

Abstract

Abstract. Terrain-induced flow phenomena modulate wind turbine performance and wake behavior in ways that are not adequately accounted for in typical wind turbine wake and wind plant design models. In this work, we simulate flow over two parallel ridges with a wind turbine on one of the ridges, focusing on conditions observed during the Perdigão field campaign in 2017. Two case studies are selected to be representative of typical flow conditions at the site, including the effects of atmospheric stability: a stable case where a mountain wave occurs (as in ∼ 50 % of the nights observed) and a convective case where a recirculation zone forms in the lee of the ridge with the turbine (as occurred over 50 % of the time with upstream winds normal to the ridgeline). We use the Weather Research and Forecasting Model (WRF), dynamically downscaled from the mesoscale (6.75 km resolution) to microscale large-eddy simulation (LES) at 10 m resolution, where a generalized actuator disk (GAD) wind turbine parameterization is used to simulate turbine wakes. We compare the WRF–LES–GAD model results to data from meteorological towers, lidars, and a tethered lifting system, showing good qualitative and quantitative agreement for both case studies. Significantly, the wind turbine wake shows different amounts of vertical deflection from the terrain and persistence downstream in the two stability regimes. In the stable case, the wake follows the terrain along with the mountain wave and deflects downwards by nearly 100 m below hub height at four rotor diameters downstream. In the convective case, the wake deflects above the recirculation zone over 40 m above hub height at the same downstream distance. Overall, the WRF–LES–GAD model is able to capture the observed behavior of the wind turbine wakes, demonstrating the model's ability to represent wakes over complex terrain for two distinct and representative atmospheric stability classes, and, potentially, to improve wind turbine siting and operation in hilly landscapes.

Highlights

Wind turbines are commonly sited in complex terrain to take advantage of topographic flow enhancement, such as the acceleration of flow over ridge lines and hill tops
The stable case is influenced by a mountain wave event
To validate the accuracy of this event in Weather Research and Forecasting Model (WRF)–large-eddy simulation (LES)–generalized actuator disk (GAD), we compare the model with multi-Doppler lidar scans obtained by the DTU lidars

Summary

Introduction

Wind turbines are commonly sited in complex terrain to take advantage of topographic flow enhancement, such as the acceleration of flow over ridge lines and hill tops. In addition to topographic acceleration, other terrain-induced flow phenomena in the atmospheric boundary layer (ABL) affect wind turbine performance and wake propagation and characteristics (Xia et al, 2021; Draxl et al, 2021). These microscale processes include mountain/lee waves (and associated rotors), hydraulic jumps, valley flows, and flow separation and recirculation (Fernando et al, 2019; Baines, 1998). Wise et al.: Multi-scale modeling of a wind turbine wake over complex terrain flow separation or recirculation often occurring during daytime convective (or unstably stratified) conditions. Increased knowledge of wind flows in complex terrain is critical to improve predictions to support growing wind energy resources (Veers et al, 2019)

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Conclusion