Structural dynamics of flexible rotor blades to study aeroelastic phenomena

Abstract

Nowadays, experimental analyses of fluid-structure interactions on flexible rotating blades are few widespread. For this purpose, an experimental rotor test bench for blades with variable flexibility is under development. Different sets of blades have been fabricated with different composite materials and variable thickness, depending on the number of layers considered. The main objective of the present work is to estimate which sets of blades are more sensitive to flutter occurrence based on their Campbell diagrams. For this purpose, a finite element approach is developed to model the dynamic behavior of the blades assumed as a uniform, slender, isotropic and homogeneous beam with out-of-plane bending and torsion deformations, submitted to inertial and centrifugal forces due the rotation effects. Two numerical models were developed by considering Timoshenko and Euler-Bernoulli beam theories; both are validated by means of the software COMSOLr. Later, the mechanical properties of each set of blades have been identified from the frequency response functions (FRFs) obtained through experimental vibration tests. The frequencies and eigenmodes of blades determined experimentally are compared with those obtained with FEM model. Then, rotation effects on the blade frequencies are studied by using Campbell diagrams for all set of blades studied. It is evidenced for some of the sets the presence of intersections between bending and torsional frequencies in the interval of the operational rotating speed of the test bench. This fact might be considered as an indicator of potential flutter occurrence. Aerodynamic load will be consider in the future work.