Contemporary design standards for wind turbines (e.g., the International Electrotechnical Commission) recommend stochastic models (e.g., the Kaimal spectral model) for turbulence inflow generation. These simple stochastic models are not very representative of atmospheric boundary layer turbulence, as they do not account for the omnipresent atmospheric stability effects. In this presentation, we will describe and systematically evaluate two competing multiscale, coupled modeling approaches for the generation of high-resolution, four-dimensional, realistic, inflow turbulence fields. Our primary focus will be on stably stratified flows and associated low-level jets.
The workhorse behind both modeling approaches is a state-of-the-art, open-source atmospheric model, called the Weather Research and Forecasting (WRF) model. In the first approach, we perform seamless coupled simulations from synoptic-scale (order of ~100 km) down to turbine-scale (order of a few m) flows. In the second approach, we couple the WRF model with a new-generation, tuning-free (dynamic) large-eddy simulation (LES) code in an offline mode. In this approach, the WRF model only simulates atmospheric flows down to the mesoscale (order of ~1 km) and the turbine-scale simulation burden is carried out by the LES code. We will document the strengths and weaknesses of the both the modeling approaches with the help of observational data (from a sodar and a tall tower) and semi-empirical similarity theories (e.g., spectra, coherence). We will also touch upon the sensitivity of our modeling results with respect to varying subgrid-scale models and spatial resolutions.
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