Experimental Modal Analysis of a Full-Scale Rotating Fan

Abstract

Experimental modal analysis of a full-scale aeronautic fan is performed at design rotating speed and under vacuum conditions. The composite woven fan is excited through embedded piezoelectric actuators and the dynamic response of the system is measured with strain gauges. Presented experiments achieve high quality data measurements of forced responses thanks to a strict control of experimental conditions, an extended instrumentation of the fan and a careful testing method. The study focuses on the two first bending mode families with nodal diameters. The modal testing of the system is performed using frequency stepped sine excitations over a close range around investigated resonances and using travelling wave excitations allowing to investigate specific nodal diameter modes in each family. The forced response measurements are used to estimate the frequency response functions of the rotating fan under different experimental parameters. In particular, the first mode family exhibits a higher modal density which is challenging for modal parameters estimations. Used frequency domain modal identification techniques are presented and adapted to the fitting of frequency response functions. At the end, these techniques are applied to investigate sensitivity of extracted natural frequencies and modal damping ratios with respect to different parameters such as rotational speed, excitation level and nodal diameter content of the excitation pattern.