Non-engine order rotor blade vibration is an aeroelastic phenomenon of major interest for compressor designers resulting from excitation of rotor blade modes through aerodynamic instabilities. Indicators for a comparable type of instability, caused by propagating acoustic modes, have been observed in an experimental multistage high-speed compressor by Safran Helicopter Engines. It is intended to understand the cause of these instabilities by combining experimental data and numerical simulations. Unsteady pressure measurements were carried out by case-mounted and stator-mounted transducers. Rotor tip-timing and magnet-coil sensor systems were installed to measure the blade vibrations. Experimental results show non-engine order signatures in the unsteady pressure signal coherent to the shifted frequency of blade vibrations. In the present paper, the waveform of these oscillations is analyzed in detail, showing a dominant propagating acoustic mode interacting with vibrations of rotor 2. The root cause for the non-synchronous oscillations is identified as an acoustic mode that is cutoff downstream of rotor 3. During the test, the mode changes its frequency and circumferential order, affecting the amplitude of associated blade vibrations.

Non-synchronous blade vibration (NSV) has received enormous attention in turbomachinery research, since it can be safety critical and its occurrence is less predictable than typical aeroelastic instabilities such as flutter. Typical for front stages of core compressors, NSV arises due to the propagation of one or multiple aerodynamic disturbances, traveling around the circumference and locking-in with structural blade vibration modes. Recent studies have shown, that modern low-speed fan architectures are also susceptible for this type of instability. With a specific focus on such aeroelastic instabilities, the open-test-case fan stage ECL5 has been designed by Ecole Centrale de Lyon. It is representative of near-future UHBR fan concepts with carbon-fibre composite blades. To prepare the experiments planned for 2022, unsteady full-annulus simulations have been carried out. Depending on rotation speed, the emergence of aerodynamic disturbances traveling around the circumference at a typical convective speed is observed, such as reported in studies on NSV on different configurations. These aerodynamic disturbances are identified as a possible source for non-synchronous vibration. At design conditions, an unstable aerodynamic behavior is observed, with part-span stall cells developing within the operating range predicted by RANS simulations. A study of intentional aerodynamic mistuning, in form of an alternating leading edge pattern is presented. At design speed, influence on steady performance and the onset of part-span stall is observed. At part-speed, the evolution of small-scale vortical disturbances, which can be responsible for the onset of NSV is affected significantly, providing a possible method to suppress NSV.

Significant non-synchronous blade vibrations (NSV) have been observed in an experimental three-stage high-speed compressor at part-speed conditions. High amplitude acoustic modes, propagating around the circumference and originating in the highly loaded Stage-3 have been observed in coherence with the structural vibration mode. In order to understand the occurring phenomena, a detailed numerical study has been carried out to reproduce the mechanism. Unsteady full annulus RANS simulations of the whole setup have been performed using the solver elsA. The results revealed the development of propagating acoustic modes which are partially trapped in the annulus and are in resonance with an aerodynamic disturbance in Rotor-3. The aerodynamic disturbance is identified as an unsteady separation of the blade boundary layer in Rotor-3. The results indicate that the frequency and phase of the separation adapt to match those of the acoustic wave, and are therefore governed by acoustic propagation conditions. Furthermore, the simulations clearly show the modulation of the propagating wave with the rotor blades, leading to a change of circumferential wave numbers while passing the blade row. To analyze if the effect is self-induced by the blade vibration, a non-coherent structural mode has been imposed in the simulations. Even at high vibration amplitude the formerly observed acoustic mode did not change its circumferential wave number. This phenomenon is highly relevant to modern compressor designs as observed in the presented experiments.