Experimental and Numerical Investigation of Aerodynamic Damping

This thesis introduces a new aeromechanical test facility for axial compressor rotors called ACTIVE (Axial Compressor Test rig for the Investigation of blisk Vibrations with an active Excitation system). Thanks to the closed loop design, the test facility can be operated at variable pressure levels as well as at vacuum without changing the test setup. An electromagnetic excitation system is developed, which allows for the excitation of vibration modes independent of the rotational speed and operating point and without disturbing the flow field due to its location below the hub line. The test facility is equipped with a blade tip-timing system (BTT) to measure blade vibration contactless. A unique validation experiment is presented, where the BTT system is validated by means of a scanning laser Doppler vibrometer in combination with an optical derotator. In the thesis the aerodynamic damping of an axial compressor blisk is investigated in detail. In a first step, the natural modes of the blisk are determined by means of experimental modal analyses to validate the finite element (FE) simulations. Campbell diagram measurements reveal a strong influence of the Coriolis force for some modes causing a significant split of the backward and forward traveling wave modes. Subsequently, detailed flow field measurements are applied to validate the steady-state CDF simulations. Hundreds of excitation experiments are conducted to measure aerodynamic damping of multiple modes of the first bending mode family at various operating points. The aerodynamic damping CFD simulations agree well with experimental results. However, limitations of the simplified CFD model are observed for one mode, where an acoustic resonance is detected in the experiment.