The continued expansion of wind energy within nations' energy portfolios requires continued reductions in
the levelized cost of energy (LCOE), the ratio of financial cost to purchase and operate wind farms to financial
gain from the electrical power produced by the wind farm. Major contributors to the numerator include
replacement costs for premature component failure on the drivetrain, including, in particular, the main bearing
(Hart et al. 2020a). Whereas main bearing failure is likely not subsurface fatigue-related (Hart, et al. 2019) and
a number of potential mechanisms likely contribute, the dominant processes underlying premature main
bearing failures are not currently known (Hart, et al. 2019,2020a). Work has been undertaken to develop a
systematic approach to the study of main bearing loading and failure mechanisms (Hart 2020b); wherein, it is
demonstrated that a detailed understanding of the loads experienced by the main bearing, and their causal
mechanisms, is a necessary prerequisite to progress in this field. The proposed research programme centers on
the hypotheses that (1) the mechanisms underlying premature main bearing failures result from specific
repetitive time changes in the bearing load-zone, and (2) that these deleterious load-zone forcings are in
response to specific temporal characteristics in the moments and forces on the main shaft that result from the
passage of the energy-dominant atmospheric turbulence eddies through the wind turbine rotor plane.
Utility-scale wind turbines and wind farms respond to turbulence within the "atmospheric boundary layer"
(ABL), the 1-2 km region of the troposphere adjacent to the earth's surface. During the day, the structure of the
turbulence eddies transported within the ABL is driven by strong convection and strong shear, a coherent eddy
structure that varies systematically with the global stability state of the ABL (Khanna & Brasseur 1998, Jayaraman
& Brasseur 2014). As the eddies interact with rotating wind turbine blades, high temporal variabilities in
aerodynamic loads pass from the rotor hub to the main shaft in the form of torque, bending moment and axial
force fluctuations (Vijayakumar, et al. 2016) with three characteristic time scales (Nandi, et al. 2017) in main
shaft moments (Lavely 2017). The shortest of these is below 1 sec. with relative variability on order 50%
(Vijayakumar, et al. 2016). Lavely (2017) showed that, whereas main shaft torque fluctuations respond to rotoraveraged
horizontal winds, shaft bending moment fluctuations respond to time changes in spatial asymmetry in
horizontal velocity over the rotor plane, generated as turbulence eddies pass through the rotor.
If the above hypotheses are valid, it follows that the turbulence-generated deleterious load fluctuations on
the main bearing are likely driven by different classes of turbulence structure and loading response at different
time scales as atmospheric eddies pass through the rotor plane. Since ABL turbulence structure varies with
atmospheric stability, deleterious load characteristics change during the day and among seasons, as well as with
topography. Furthermore, within a wind farm the hypotheses can be extended to include potential deleterious
main bearing responses to combinations of atmospheric and rotor wake turbulence eddying motions due
especially to the generation of spatial asymmetries over wind turbine rotor planes.