THE NATURAL EXCITATION TECHNIQUE (NEXT) FOR MODAL PARAMETER EXTRACTION FROM OPERATING STRUCTURES

The Natural Excitation Technique (NExT) is a method of modal testing t hat allows structures to be tested in their ambient environments. This paper contains developments and results since 1990, and a new theoret ical derivation of NExT, as well as a verification using analytically generated data. In addition, results from NExT are compared with conve ntional modal testing for a parked, vertical-axis wind turbine. Also, for a rotating turbine, NExT is used to calculate the modal parameters as functions of the rotation speed, since substantial damping is deri ved from the aeroelastic interactions during operation. Finally, exper imental results calculated using NExT are compared with analytical pre dictions of damping using aeroelastic theory.