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 yearsin service, yet the exact damage mechanisms are still debated. This research hypothesises premature main bearing failures may be linked to repetitive force changes drivenby the passage of energy-dominant atmospheric turbulence eddies through the windturbine 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 [Brasseurand 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 meanvelocity gradient was minimised, though not be fully removed, likely due to the contributions from numerical dissipation in the AMR-Wind code. Moderately convectiveboundary layers (MCBL) were simulated for several eddy-turnover times until quasistationarity was achieved. A novel methodology was developed to quantify the eddypassage time of the MCBL, showing good agreement with the literature. A sensitivityanalysis parametrised a state-of-the art actuator line model (ALM). The sensitivityanalysis showed low sensitivity with a blade sweep ratio (BSR) less than 1.0, thougha possible connection between the maximum BSR and the non dimensional parameter?/? is discussed. The classical ALM shows low sensitivity with a smoothing-ratio of0.85 or greater. However, this is increased to 300 actuator points for ALM with theaddition of the filtered-lifting line correction (FLLC). The FLLC significantly improvedthe accuracy compared to the classical ALM in both fixed and chord-varying ? configurations. The latter could achieve similar accuracy, but grid refinement limitations, due to computational cost, were mitigated by using the FLLC. A static force balancemodel 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 timevariations in the MB force vector, due to the contribution from the hub forces being 1-3order of magnitude smaller in comparison. Furthermore, rotor weight contributes onlyto 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 drivesvariations in the MB force. Comparing the out-of-plane bending moment generated byABL turbulence against a steady shear inflow, atmospheric turbulence generates highlevels of fluctuations at the low and high frequency content and modulates the timevariations in the 3P frequency content. Leading to the classification of three distinctfrequency ranges (low, 3P, high). Specific periods of high-frequency and 3P activitywere identified and found to qualitatively align with low-frequency peaks in MB force.However, limited overlap between peak ”bursting” events across frequencies suggestsdistinct underlying mechanisms. A novel blade asymmetry vector was introduced, revealing that blade asymmetry is the dominant driver of MB force variability across allfrequency bands. But there are subtle differences in the way asymmetry drives the timevariations over the three frequency ranges. Using a novel methodology high-frequencyfluctuations were shown to cause sub-second force jumps comparable in magnitude torotor weight, posing a risk for edge loading, flange impact, and reduced lubricationfilm thickness. These extreme transients may be contributing to surface-initiated failure mechanisms. Further analysis demonstrated that LSS contribute more to rotorasymmetry and large force jumps than HSR, likely due to their higher coherence andinternal velocity gradients. Finally, the influence of blade flexibility was assessed byre-running simulations with deformable blades. A less than 10% change between thecalculations indicated that the core findings from the rigid rotor analysis remain valid.High-frequency loading events persist, suggesting that ABL turbulence, particularlyin the form of coherent eddies like LSS, plays a key role in triggering dynamic loadvariations that may initiate MB failure.
Advisor / supervisor
  • Hart, Edward
Resource Type
DOI

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