Spectral content and spatial scales in unsteady rotationally augmented flow fields

Frequency spectra were computed from Cntime series acquired during the NASA Ames wind tunnel experiment, for rotationally augmented flow field conditions at zero yaw. Prominent peaks were present in the frequency spectra, corresponding to dominant aerodynamic shedding modes. These shedding modes were well correlated with flow field mean structure, and Strouhal number analyses furnished information regarding time varying flow field kinematics. Specifically, Strouhal number analyses implied that one of two distinct shedding modes was present, depending upon test section speed and local inflow angle. Information regarding these shedding kinematics will help provide understanding of the aerodynamics responsible for unsteady load production during rotationally augmented conditions. This, in turn, will aid in establishing a reliable physical basis for accurately predicting the time varying aerodynamic forces responsible for wind turbine fatigue loads and damage