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.

Dissipation of Turbulence in the Wake of a Wind Turbine

“…Future measurement campaigns can exploit collaborative measurements of dissipation rate using in situ measurements as used here and estimates from scanning lidar (e.g. Smalikho et al 2013;Aitken and Lundquist 2014c) to assess the variability of dissipation throughout turbine wakes and the validity of Taylor's frozen turbulence hypothesis for wake measurements. Dissipation likely reaches a maximum value at some distance downwind of the turbine and then decays further downwind.…”

“…Measurements of dissipation rate and the resulting erosion of a turbine wake or of a wind-farm wake are required to assess models of wake behaviour that are used for optimizing wind-farm layouts or quantifying annual energy production yields at wind farms under development. Smalikho et al (2013) showed a strong dependence of wake length with ambient background dissipation rate. Further, with the prospect of improved wake characterization on the horizon, theoretical efforts are now addressing the feasibility of optimizing wind-farm power production based on manipulating wakes (Marden et al 2012;Farm et al 2013).…”

“…The wind-speed deficit in a turbine's wake reduces power generated by downstream turbines (Magnusson and Smedman 1994;Barthelmie et al 2010;Rhodes and Lundquist 2013) and may reduce power generated by distant wind farms (Kaffine and Worley 2010;Fitch et al 2012Fitch et al , 2013. Similarly, the enhanced turbulence of the wake increases damaging loads on downstream turbines (Thomsen and Sørensen 1999;Kelley 2004;Kelley et al 2004;Sim et al 2009) and may enhance turbulent exchange with the surface (Rajewski et al 2013). This enhanced turbulence is an important consideration for predicting the evolution of wakes from individual turbines and for understanding the processes by which individual wakes merge into larger wind-farm wakes and eventually erode downwind.…”

Toward Isolation of Salient Features in Stable Boundary Layer Wind Fields that Influence Loads on Wind Turbines

“…At present, the IEC guidelines explicitly address conditions associated only with the neutral boundary layer (NBL)-i.e., they ignore buoyancy effects in the definition of design load cases. In the present work and in ongoing studies [1][2][3][4], the authors are seeking to extend the design paradigm to allow a reliability-based assessment of wind turbines against fatigue and extreme limit states that includes evaluation of turbine loads for suites of inflow turbulence flow fields that have the spatial structure and characteristics that reflect a wide range of atmospheric stability conditions.…”

“…Wind shear, veer, and elevation-dependent turbulence variance particularly in the SBL (as distinct from the same in the NBL) have been recognized as having a significant influence on power generated as well as on loads experienced by utility-scale wind turbines with large rotor-swept areas [6][7][8]. Physics-based large-eddy simulation can be used to describe various realistic flow fields including those for the SBL [1,2,4,9]; on the other hand, stochastic simulation [10] is the accepted method of choice for the generation of IEC-NBL flow fields and its use, with spectral methods, is well documented in design standards and guidelines.…”