Contribution to Experimental Validation of Linear and Non-Linear Dynamic Models for Representing Rotor-Blade Parametric Coupled Vibrations

This work gives a theoretical and experimental contribution to the problem of rotor-blades dynamic interaction. A validation procedure of mathematical models is carried out with help of a simple test rig, built by a mass-spring system attached to four flexible rotating blades. With this test rig, it is possible to highlight some dynamic effects and experimentally simulate the structural behavior of a windmill in two dimensions (2D-model).

Only lateral displacement of the rotor in the horizontal direction is taken into account. Gyroscopic effect due to rotor angular vibrations is eliminated in the test rig by attaching the rigid rotor to a flexible foundation. The blades are modeled as Euler-Bernoulli beams. Using three different approaches to describe the beam deformation one achieves: (a) a linear model; (b) a linear beam model with second order terms; (c) a fully non-linear model.

Tip masses at the end of the blades emphasize the coupling between the dynamic and elastic terms. The shape functions are chosen in order to reduce the mathematical model, so that only the first bending mode of beam is taken into account. The resulting equations of motion have five degrees of freedom and illustrate linear, non-linear and time-depending terms in a very transparent way.

Although neither gyroscopic effect due to rotor angular vibrations nor higher blade mode shapes are considered in the analysis, the equations of motion of the rotor-blades system are still general enough for the purpose of the work: validation of different linear and non-linear models with time depending (periodic) coefficients.

Experiments are carried out in the time and frequency domains while the rotor operates with different constant angular velocities.