Influence of mechanical mistuning on the forced response of a turbine cascade

An experimental investigation has been conducted in the non-rotating annular test facility of the "Laboratoire de Thermique Appliquée et de Turbomachines" (LTT), "École Polytechnique Fédérale de Lausanne" (EPFL). The influence of frequency mistuning and modeshape mistuning on the forced response of a turbine cascade is thereby determined. The 20 turbine blades of the cascade are excited by gusts generated using a strutted rotor with 13 elliptical bars providing a 13th engine order excitation pattern. The blades vibrate in their first eigenmode, corresponding to either a bending vibration or a torsion vibration. The frequency mistuning study has been carried out for the cascade with a first vibration mode in torsion. The frequency mistuning is introduced by adding or removing mass. A case with a weak (case T1) and a strong random frequency mistuning (case T2) are studied. For the modeshape mistuning study the blades used exhibit a first eigenmode in bending. The modeshape mistuning is introduced by changing the bending direction, using three different directions in the experimental setup. A datum case and two mistuned case are studied: The blades all vibrate in a direction normal to the chord for the datum case, called case B0. For the first mistuned case B1, the bending direction is varied alternately from blade to blade by plus or minus 10°. The second mistuned case B2 corresponds to a random variation of the bending direction. The motions of all blades as well as the unsteady pressure distributions on selected blade surfaces are recorded during the forced response measurements. In order to quantify the forcing function, the unsteady pressure distribution is measured in a second test setup with clamped blades, allowing the isolation of the effects of motion-induced unsteady pressures. Specific objective of this work is to study the influence of mistuning of the structural properties on the forced response of a turbine cascade. Both frequency mistuning and modeshape mistuning are investigated and an attempt is made to separate their impact on the maximum vibration amplitude of the individual blades. The measurement results show that the aerodynamic force that excites the vibration comprises five significant harmonics, the first harmonic at the strut passing frequency being dominant. The dominant first harmonic leads to an aerodynamic excitation of the 13th engine order. The measured forced response amplitude plots for the frequency mistuning study show typical effects of frequency mistuning and coupling (secondary peaks). The measured forced response phase of the blade movement exhibits inter-blade phase angles that are typical for a 13th engine order excitation. The effects of frequency mistuning and coupling can be detected on the phase plots. Besides the frequency mistuning, a significant damping mistuning is present in the cascade. The damping mistuning leads to a wide scatter of maximum response amplitude. A damping factor correction is appl