Electronic Theses and Dissertations

Date

2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Mechanical Engineering

Committee Chair

Daniel Foti

Committee Member

Jeffrey G Marchetta

Committee Member

Ranganathan Gopalakrishnan

Committee Member

Thomas Hagen

Abstract

The increasing demand for clean and sustainable energy has led to the rapid growth of wind energy production. The variability of energy production is affected by wake meandering, a large-scale periodic oscillating motion of the far wake of a wind turbine. However, formation and energy dynamics of the wake meandering are not fully understood. This undermines the development of accurate models for predicting wind farm performance. Two distinct formation mechanisms have been separately hypothesized: (1) modulation by large, upwind coherent structures present in the atmospheric boundary layer and (2) turbine-scale bluff body effects related to the size of the rotor. Numerical investigations employing large-eddy simulation of high-fidelity wind turbine flows are undertaken to elucidate the dependency of lengths scale on wake meandering and assess energy transfer mechanisms between scales. A series of precursory large-eddy simulations of an atmospheric boundary layer are carried out where the streamwise length scale distribution ranges from a magnitude larger than the boundary layer height to much smaller than a utility-scale wind turbine rotor. Length scales are manipulated using a high pass filtering approach to produce upwind length scale scenarios, with varying cutoffs of the largest scale while maintaining the same total energy flux. Wind turbine simulations for each scenario employ the actuator surface with the nacelle model and actuator disk model to assess effects of upwind scale and turbine scale separately. Both near and far wake differences are exhibited in the instantaneous and averaged features related to wake meandering. The peak wake meandering frequency is shown to shift higher as the energy in large upwind scales decreases explaining the variation in wake meandering frequency in previous laboratory, field, and computational results. Energy dynamics including mean and turbulent kinetic energy budgets are assessed to elucidate effects on global energy transfer. Furthermore, scale-to-scale energy dynamics in a wind turbine are based on a new methodology where specific scales of coherent structures are identified through dynamic mode decomposition, whereby the total coherent velocity is separated into a set of velocities classified by frequency. The coherent kinetic energy of a specific scale is defined by a frequency triad of scale-specific velocities. Equations for the balance of scale-specific coherent kinetic energy are derived to interpret inter-scale dynamics. Triadic interactions, the mechanism of energy transfers between scales, manifest as triples of wavenumbers or frequencies and can be characterized through bispectral analyses. We compare the new method to bispectral mode decomposition. The bispectrum from both methods identifies prominent upwind and wake meandering interactions that create a broad range of energy scales including the wake meandering scale. The coherent kinetic energy associated with the interactions shows a strong correlation between upwind scales and wake meandering. The coherent kinetic energy in the rotor scales and the hub vortex scale in the wind turbine interact to produce new scales. The analysis reveals that vortices at the blade root interact with the hub vortex formed behind the nacelle, which has implications for the proliferation of scales in the downwind near wake.

Comments

Data is provided by the student.

Library Comment

Dissertation or thesis originally submitted to ProQuest

Notes

Embargoed until 08-01-2025

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Available for download on Friday, August 01, 2025

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