Measurement of the kinematics of two underplatform dampers with different geometry and comparison with numerical simulation

Underplatform dampers are used as friction damping devices in order to reduce the amplitude of vibrations of blades in turbine rotors. In this paper a study was performed on the forced response of a mock up system which simulates the flexural behaviour of two turbine blades with an interposed underplatform damper. Two different geometries are tested for two different deflection shapes and the dynamic response of the whole system is explained with respect to the damper kinematics. The originality of the work consists in the measurement of the damper displacements by means of a laser Doppler vibrometers (LDV) equipment and in the comparison with the displacements obtained by numerical predictions of a numerical code used for damper design. The strong influence of the rotation of the underplatform damper on the dynamic response is analysed in particular for the in-phase vibration of the two dummy blades. Comparison between calculations and experiments shows that the damper model allows simulating the damper kinematics with good accuracy.