Toward Understanding Characteristics of the Stable Boundary Layer that Influence Wind Turbine Loads

Abstract

Stable boundary layer (SBL) flow fields are not routinely considered when evaluating wind turbine loads. They are, known, however, to have distinct characteristics from flow fields associated with the neutral boundary layer (NBL). Especially, for large utility-scale turbines, SBL flow fields might be of interest to evaluate since in the SBL, there can be enhanced wind shear accompanied by wind veer. NBL flow fields, commonly used in turbine loads studies are generated using standard spectral procedures in stochastic simulation; SBL flow fields, on the other hand, require computational fluid dynamics (CFD) procedures that in order to capture the physics and characteristics highlighted as distinct from the NBL. At the present time, large-eddy simulation (LES) appears to be the most efficient CFD procedure that can be employed for SBL flow field generation for win turbine loads studies. Standards, such as from the International Electrotechnical Commission, provide sufficient guidance albeit with several simplifying assumptions (one such deals with constant variance of turbulence over the rotor) and the use of standard target turbulence power spectra and coherence functions to allow NBL flow field simulation. We compare LES-generated SBL flow fields with from stochastic NBL flow fields and loads on a 5-MW turbine in both cases, while systematically seeking to understand the nature of any differences in light of contrasting wind shear, wind veer, and turbulence variation over the rotor plane.