Effect of upwind turbulence length scales on wind turbine wake meandering

Abstract

Wake meandering, the large-scale periodic oscillating motion of the far wake of a wind turbine, affects wake recovery, turbulence intensity, and the overall variability in power production. Two distinct mechanisms have hypothesized its formation. The first is that the wake is convected by large, upwind scales much greater than the wake width. The other is that the turbine rotor induces a motion similar to a bluff body that scales with the rotor diameter. In this work, we focus on elucidating and separating the effects of upwind scales, their interaction with the turbine, and turbine-related scales that impact wake meandering. Precursor simulations are developed to systematically vary only the upper bound of the length scale distribution. We carry out a series of large-eddy simulations under a range of inflow scenarios to investigate influences on wake meandering dynamics. Turbulence wake statistics reveal that the location of peak turbulence kinetic energy in the far wake and the onset of wake meandering moves downwind with lower upwind turbulence levels and short distribution of length scales; however, turbine-added kinetic energy scales well with the thrust velocity across all cases. Spectral analysis indicates that upwind length scales influence the broadening of low-frequency energy modes relevant to wake meandering in the far wake. Profiles tracking the wake center are constructed, and their distribution is shown to scale by the amplitude of the profile, collapsing all cases into a near normal distribution in far-wake locations. Using the wake center profiles, wake meandering is decomposed by a low-pass filter, which indicates two separate scales of wake meandering with different wake center distributions: (1) turbine-influenced scale with regular Strouhal number, St≈0.3, and corresponding harmonics, with the same scale wake center distribution across all upwind conditions and (2) upwind-produced scale if low upwind scale are present, St<0.1, with irregular wake center distributions. Spectral proper orthogonal decomposition confirms the relationship with separate dominant coherent structure modes at corresponding frequencies dependent on upwind conditions.

Publication Title

Physical Review Fluids

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