The Influence of Stable Boundary Layer Flows on Wind Turbine Fatigue Loads

Abstract

Near-neutral atmospheric stability conditions form the basis for wind turbine design. This is surprising since such near-neutral conditions occur in so-called transition periods only twice each day (around sunrise and sunset). Unstable conditions occur during the day and stable conditions occur generally at night. During nighttime stable conditions, turbulence is typically generated by shear and destroyed by negative buoyancy. Wind shear (both magnitude and direction) under stable conditions is much larger in comparison to that during neutral conditions. Moreover, stable boundary layer (SBL) flows are often accompanied by low-level jets (LLJs); these LLJs can be low enough to impact today’s large utility-scale turbines and thus influence loads. This study compares turbulence, turbine loads, and accumulated fatigue damage for a utility-scale wind turbine in stable versus neutral atmospheric conditions. Our focus is on the varying simulated atmospheric flows that result from (i) different surface cooling rates (which control buoyancy destruction); and (ii) different geostrophic winds (which control shear generation). Inflow turbulence time series are generated and applied over the rotor plane of a 90-meter hub-height 5MW wind turbine based on Large-Eddy Simulation (LES) with refined dynamic sub-grid scale modeling; similarly, neutral boundary layer flows are generated using conventional Fourier techniques for comparison. These simulated wind velocity fields are then fed into an aeroelastic model of the selected wind turbine and turbine fatigue loads are analyzed. Some differences are seen between fatigue loads resulting from neutral versus stable conditions but these are not significant especially when missing high-frequency content in the inflow turbulence from LES-generated flows is augmented by fractal interpolation.