Spatio-temporal behavior of the far-wake of a wind Turbine model subjected to harmonic motions: Phase averaging applied to Stereo-PIV measurements

Abstract. The complex dynamics introduced by floating platforms present new challenges in the study of wind turbine wakes, and numerous questions remain unresolved due to the early stage of this technology and limited operational experience. Previous studies showed that harmonic motions with realistic amplitude and frequency and under a modelled atmospheric boundary layer have no significant impact on time-averaged values, but that frequency signatures are still visible in spectra of wake parameters. The purpose of this work is to shed light on the spatio-temporal behaviour of the wake during imposed surge, heave and pitch harmonic motions. Wind tunnel experiments on the wake of a porous disc immersed in a modelled marine atmospheric boundary layer were performed and a phase-averaging method with kernel smoothing was applied to the data to extract the harmonic response of the wake. A quasi-steady-state analysis was carried out, showing that the phase-averaged observations appear to be larger than simple steady wake model predictions and revealing the dynamic nature of the wake responses to the motions. Thus, distinct wake dynamic hypotheses are formulated depending on the nature of the motion: (i) for heave, the wake is translated vertically while maintaining its integrity and containing the same power; (ii) for surge, the wake contracts and expands without any displacement of its centre localisation, accompanied with in-phase power modulation; (iii) and for pitch, the wake dynamics include both heave and surge impacts, with a vertical translation of the wake synchronised with crosswise wake surface and power modulations.