Experimental Modal Analysis of 9-meter Research-sized Wind Turbine Blades

Abstract

The dominant and persistent trend with wind turbine technology, particularly in the past three decades, has been growth in the length of the blades. In order to investigate design choices which reduce blade weight, Sandia Labs initiated a study, which is near completion, to evaluate innovative concepts for large blades. The innovations include strategic use of carbon fiber in the spar caps, bend-twist coupling in the composite layup, and thick, flatback airfoils. Several large blades were designed and then built at a down-scaled 9-meter length. Each blade design has undergone a full series of structural tests including modal tests, static tests, and fatigue tests. The modal tests performed for evaluation of these blades is the focus of this paper. Major findings from these tests are summarized, and they include: (1) techniques for experimental quantification of uncertainty in the modal parameters, (2) insight into model calibration using both static load-deflection data and the modal parameters, (3) novel test techniques for reducing the uncertainty in the root boundary condition, and (4) the development of validated structural models. This paper will provide a summary of blade modal testing and structural model validation, and will emphasize recent validation tests using a seismic-mass-on-airbags boundary condition.