The interaction of horizontal axis wind turbines (HAWTs) and large-scale energetic flows is not fully understood. The unsteady flow approaching turbines could result in highly unsteady blade forces and root bending moments (RBMs) which would have a strong influence on the turbine's operational life span, especially for large-scale wind turbines. To provide a better understanding of this interaction, numerical simulations of a three-bladed HAWT with a cylinder located at its upstream to produce energetic motions of varying scale relative to the turbine size have been carried out to examine the temporary variations of the turbine root bending moments. The predicted spectra reveal that the energetic large-scale motions produced by the cylinder strongly interact with the instantaneous blade forces and, thus, root bending moments. It specifically shows how dominant large-scale motions approaching the turbine significantly influence the spectral characteristics of flapwise and edgewise RBMs in terms of level and trend. The comparison shows that below about half of the blade passage frequency, RBMs spectra correlate well with those of upstream velocity, whereas above this frequency, RBMs spectra are mainly influenced by the turbine rotational frequency. This study also shows that the strong scale-to-scale interaction between the upstream flow and turbine loading reported previously does not appear at high Reynolds numbers in the present study.

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