Date

2018

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Mechanical Engineering

First Adviser

Jaworski, Justin W.

Abstract

Floating offshore wind turbines exhibit unique near-wake characteristics due to ocean wave-induced rigid body motion of their rotor. The unique near-wake properties of this wind turbine are studied numerically as a coupled fluid-structure interaction (aeroelastic) phenomenon. The near-wake aerodynamics are modeleld in a Lagrangian sense by the lifting-line free-vortex wake method, and the dynamic rotor-blade structural deformations are described by the beam theory of spinning structures. The aerodynamic and structural domains are strongly coupled using a relaxation and subiteration scheme at each time step. This aeroelastic framework is believed to be the first of its kind with regards to its applications in studying floating offshore wind turbines. A linear eigenvalue stability analysis is used to evaluate the near-wake dynamics and the stability of the wake generated by the aeroelastic simulations. The stability analysis serves as a quantification tool of vortex reactions (divergence rates) and dynamics due to arbitrary disturbances, such as gusts, incoming turbulence, and blade deformation.The aeroelastic framework is validated against below-rated, rated, and above-rated onshore aeroelastic operational conditions of the NREL 5MW reference wind turbine and is compared against rigid rotor simulations. The numerical result highlight qualitatively the difference between wakes generated by rigid and flexible rotors, and indicate the flexible rotors tend to generate more unstable wakes, which lead to earlier wake breakdowns. Aeroelastic simulations indicated that rotor performance metrics, such as rotor power, torque, and thrust, are impacted by the dynamic rotor-blade structural deformations due to the effect blade dynamics have on pressure and velocity deficits across the rotor plane. Simulations of the floating offshore wind turbine operational cases highlight the level of impact that the offshore environment has on the rotor blade dynamics, rotor thrust, rotor power, and rotor torque. Specifically, results show that the offshore environment at below-rated cases provides quasi-static wave-induced motions of the rotor that marginally affect the rotor performance metrics. Rated and above-rated offshore conditions, however, show clear and notable impact on rotor performance, such that performance metrics fluctuate at the frequency of the wave-induced rotor motion.The stability analyses conducted on wakes generated by onshore (fixed) rotors show that divergence-rates of near-wakes fluctuate in time at a rate inversely proportional to the rotor rotation frequency. It is found that the wave-induced motion of the offshore environment breaks this stability trend and causes periodic divergence-rate fluctuations that oscillate under the influence of the wave-induced motion of the turbine. Finally, it is found that blade vibrations introduce higher frequency content in the divergence rate fluctuation in time. However, the dynamic blade deformations dampen the divergence rate fluctuation amplitudes.

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