The Natural Excitation Technique (NEXT) extracts modal parameters from structures undergoing some natural excitation in which no measure of the excitation force is available. Preliminary damping results are presented for the first blade flatwise modes of the Sandia/DOE 34-m Test Bed (a Vertical Axis Wind Turbine) as a function of both wind speed and tip speed ratio. The current data set is to sparse to show a trend between damping and wind speed, however a trend of higher damping for higher tip speed ratio is apparent. An estimate of the uncertainty of the data is provided. Several improvements to NEXT have been added and are discussed in this paper. These improvements include: the use of a single software environment, the addition of the complex mode indicator function, the partial removal of harmonics, time and frequency domain cross-correlation function calculation, windowing, variable ensemble size capability, filtering and data decimation, and visual convergence checking.
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