Using Time-Frequency and Wavelet Analysis to Assess Turbulence/Rotor Interactions

Large loading events on wind turbine rotor blades are often associated with transient bursts of coherent turbulent energy in the turbine inflow. These coherent turbulent structures are identified as peaks in the three-dimensional, instantaneous, turbulent shearing stress field. Such organized inflow structures and the accompanying rotor aeroelastic responses typically have time scales of only a few seconds and therefore do not lend themselves for analysis by conventional Fourier spectral techniques. Time-frequency analysis (and wavelet analysis in particular) offers the ability to more closely study the spectral decomposition of short period events such as the interaction of coherent turbulence with a moving rotor blade. In this paper, we discuss our initial progress in the application of time-frequency analysis techniques to the decomposition and interpretation of turbulence/rotor interaction. We discuss the results of applying both the continuous and discrete wavelet transforms for our application. Several examples are given of the techniques applied to both observed turbulence and turbine responses and those generated using numerical simulations. We found that the presence of coherent turbulent structures, as revealed by the inflow Reynolds stress field, is a major contributor to large load excursions. These bursts of coherent turbulent energy induce a broadband * This paper is a declared work of the U.S. Government and is not subject to copyright protection in the United States. † This work has been supported by the U.S. Department of Energy under contract no. DE-AC36-98-GO10337. ‡ Principal Scientist, National Wind Technology Center, Senior AIAA Member § Senior Engineer, National Wind Technology Center ¶ Professor, Department of Electrical Engineering # Graduate Student, Department of Electrical Engineering aeroelastic response in the turbine rotor as it passes through them. INTRODUCTION Identifying the processes responsible for inducing large loading events on wind turbine rotors remains one of the major unanswered questions of the technology. We know that such events can occur during large yaw excursions or during periods of sustained operation at large yaw angles. Previously we identified large loading events resulting from the interaction of the turbine rotor with organized or coherent turbulent structures in the inflow. We found that the number of cycles contained in the high-loading tails of spectral distributions scales with the hub-height or local mean shearing stress u* and the turbine layer vertical stability. The mean shearing stress or friction velocity is defined as