Thesis

Impacts of nonsteady load responses on utility scale wind turbine main bearings by the passage of daytime atmospheric turbulence eddies

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Awarding institution
  • University of Strathclyde
Date of award
  • 2025
Thesis identifier
  • T17407
Person Identifier (Local)
  • 202094109
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Department, School or Faculty
Abstract
  • Main bearings (MB) in wind turbines are prematurely failing, sometimes before 6 years in service, yet the exact damage mechanisms are still debated. This research hypothesises premature main bearing failures may be linked to repetitive force changes driven by the passage of energy-dominant atmospheric turbulence eddies through the wind turbine rotor in the atmospheric boundary layer. A high-resolution large-eddy simulation (LES) of a daytime atmospheric boundary layer was developed using the AMRWind finite volume code, and validated against the existing literature. The [Brasseur and Wei, 2010] framework was employed to improve the accuracy in the surface layer, where LES typically struggles. The overshoot in the prediction of the normalised mean velocity gradient was minimised, though not be fully removed, likely due to the contributions from numerical dissipation in the AMR-Wind code. Moderately convective boundary layers (MCBL) were simulated for several eddy-turnover times until quasistationarity was achieved. A novel methodology was developed to quantify the eddy passage time of the MCBL, showing good agreement with the literature. A sensitivity analysis parametrised a state-of-the art actuator line model (ALM). The sensitivity analysis showed low sensitivity with a blade sweep ratio (BSR) less than 1.0, though a possible connection between the maximum BSR and the non dimensional parameter ϵ/∆ is discussed. The classical ALM shows low sensitivity with a smoothing-ratio of 0.85 or greater. However, this is increased to 300 actuator points for ALM with the addition of the filtered-lifting line correction (FLLC). The FLLC significantly improved the accuracy compared to the classical ALM in both fixed and chord-varying ϵ configurations. The latter could achieve similar accuracy, but grid refinement limitations, due to computational cost, were mitigated by using the FLLC. A static force balance model was used to explore mechanisms driving time variations in the MB force vector. The analysis showed the modified out-of-plane bending moment vector drives the time variations in the MB force vector, due to the contribution from the hub forces being 1-3 order of magnitude smaller in comparison. Furthermore, rotor weight contributes only to the average MB force vector and not to the time variations, in a rigid rotor configuration. It is demonstrated that asymmetry in the velocity field over the rotor disk drives variations in the MB force. Comparing the out-of-plane bending moment generated by ABL turbulence against a steady shear inflow, atmospheric turbulence generates high levels of fluctuations at the low and high frequency content and modulates the time variations in the 3P frequency content. Leading to the classification of three distinct frequency ranges (low, 3P, high). Specific periods of high-frequency and 3P activity were identified and found to qualitatively align with low-frequency peaks in MB force. However, limited overlap between peak ”bursting” events across frequencies suggests distinct underlying mechanisms. A novel blade asymmetry vector was introduced, revealing that blade asymmetry is the dominant driver of MB force variability across all frequency bands. But there are subtle differences in the way asymmetry drives the time variations over the three frequency ranges. Using a novel methodology high-frequency fluctuations were shown to cause sub-second force jumps comparable in magnitude to rotor weight, posing a risk for edge loading, flange impact, and reduced lubrication film thickness. These extreme transients may be contributing to surface-initiated failure mechanisms. Further analysis demonstrated that LSS contribute more to rotor asymmetry and large force jumps than HSR, likely due to their higher coherence and internal velocity gradients. Finally, the influence of blade flexibility was assessed by re-running simulations with deformable blades. A less than 10% change between the calculations indicated that the core findings from the rigid rotor analysis remain valid. High-frequency loading events persist, suggesting that ABL turbulence, particularly in the form of coherent eddies like LSS, plays a key role in triggering dynamic load variations that may initiate MB failure.
Advisor / supervisor
  • Hart, Edward
Resource Type
DOI

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