Full-field optical deformation measurement and operational modal analysis of a flexible rotor blade

Abstract

Abstract This study describes the deformation measurement and operational modal analysis (OMA) of an extremely flexible rotor blade in hover. Blade deformation of a 0.45 m-diameter two-bladed rotor was measured at two different root pitch angles and two rotational speeds up to 1500 RPM by a time-resolved digital image correlation (DIC) technique. The DIC technique successfully measured the time history of three dimensional displacements over the entire rotor blade at approximately 900 measurement locations for the flap, lead-lag, and torsional degrees of freedom. Measured blade deformation data were then processed with the Complexity Pursuit (CP) algorithm, to determine the dynamic characteristics of the rotor blade without input excitation information. The identified modal frequencies were within 5% of those obtained by another OMA technique called NExT-ERA as well as a numerical model. There was good agreement between the mode shapes estimated by the CP algorithm and the numerical model; however there was some discrepancy with the mode shapes identified by NExT-ERA. Although modal damping values were extracted by the CP algorithm, they were less than 3 % for all test cases, which is too low to account for aerodynamic damping. Overall, the modal identification process with the CP algorithm was found to be easier to implement and more accurate than the conventional NExT-ERA approach.