Significant non-synchronous blade vibrations (NSVs) 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 Reynolds-averaged Navier–Stokes 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, since the appearance of the axially propagating acoustic waves can excite blade vibrations if they coincide with a structural eigenmode, as observed in the presented experiments.

Research of the past decades has shown that the operating range of modern fans and compressors is often limited by aeroelastic phenomena before the onset of pure aerodynamic instability. Prediction of these mechanisms is challenging for state-of-the-art numerical methods, particularly for configurations with flexible wide-chord blades. To provide a benchmark configuration for the community, the composite-material fan stage ECL5, representative of near future ultra-high-bypass ratio architectures has been designed at Ecole Centrale de-Lyon and recently shared as an open-test-case. In research program CATANA, different configurations with variable tuning and intake geometries are investigated experimentally, and here we present a comprehensive aeroelastic study of the tuned reference configuration. The study encompasses the investigation of the whole subsonic and transonic operating range using multiphysical instrumentation. A characterization of structural properties under running conditions is analyzed in comparison to individual blade measurements and FEM-predictions. The stability limit is investigated at different speedlines. At transonic conditions, rotating stall occurred without aeroelastic precursors. Severe nonsynchronous-vibrations were observed at subsonic speeds and limited the operating range before the onset of rotating stall. Through a detailed analysis of the aeroelastic coupling mechanism, a full characterization of interacting modes is presented. The challenging prediction of this coupled phenomenon and the discrepancy to aeroelastic simulations are discussed. The results are a promising benchmark for future method development, particularly involving high-fidelity methods.

With the advancement of modern fan architectures, the lack of experimental benchmarks for the research community became apparent in the recent years. Enormous effort in the method development could not be validated on representative geometries and motivated the development of the open-test-case ECL5/CATANA. A carbon fiber fan stage has been designed by Ecole Centrale de Lyon and shared with the community in 2021. The fan is representative of a modern ultra high bypass ratio (UHBR) architecture with sonic design speed. The reference configuration has been investigated experimentally on a novel test facility with multiphysical instrumentation. In this publication, the results of the first experimental campaign are presented with a detailed analysis of the aerodynamic performance and system symmetry. The presented results comprise full stage mappings across the whole operating range for three main speedlines (55%, 80%, and 100%) using performance rakes, unsteady wall pressure arrays, tip clearance, and stagger angle measurements. Radial profiles of intake boundary layer and rotor exit conditions at selected conditions complete the full validation dataset for steady and unsteady aerodynamic simulations. Results are discussed in comparison to blind-test Reynolds-averaged Navier–Stokes (RANS) simulations of different international institutes. Significant prediction inaccuracies of the tip flow and corner separations of the stator are observed and reveal the requirement of real blade geometry measurements at running conditions. This article is accompanied by a publication with a focus on aeroelastic instabilities observed near the stability limit. The results represent a comprehensive dimensional benchmark dataset and allow method validation on multiple levels of fidelity for the aerodynamic and aeroelastic research community.

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.

The onset of non-synchronous vibrations has been predicted for a composite open-test-case UHBR fan geometry by time-linearized simulations at part-speed conditions. In the current study, time-accurate simulations with harmonic blade oscillations have been conducted to investigate the non-linear behavior. Results for different operating conditions show the evolution of aeroelastic stability. The lock-in of convected aerodynamic disturbances with structural vibration is analyzed for different vibration amplitudes and frequencies. The simulations show the development of multiple wave numbers, mode scattering, and a non-linear dependency on vibration amplitude. The results indicate the possibility of limit-cycle prediction and the necessity of strongly coupled methods for the assessment of non-synchronous vibration.

Non-synchronous vibration (NSV), flutter, or rotating stall can cause severe blade vibrations and limit the operating range of compressors and fans. To enhance the understanding of these phenomena, this study investigated the corresponding mechanisms in modern composite ultra-high-bypass-ratio (UHBR) fans based on the ECL5/CATANA test campaign. Extensive steady and unsteady instrumentation such as stereo-PIV, fast-response pressure probes, and rotor strain gauges were used to derive the aerodynamic and structural characteristics of the rotor at throttled operating conditions. The study focused on the analysis of the transition region from transonic to subsonic speeds where two distinct phenomena were observed. At transonic design speed, rotating stall was encountered, while NSV was observed at 90% speed. At the intermediate 95% speedline, a peculiar behavior involving a single stalled blade was observed. The results emphasize that rotating stall and NSV exhibit different wave characteristics: rotating stall comprises lower wave numbers and higher propagation speeds at around 78% rotor speed, while small-scale disturbances propagate at 57% rotor speed and lock-in with blade eigenmodes, causing NSV. Both phenomena were observed in a narrow range of operation and even simultaneously at specific conditions. The presented results contribute to the understanding of different types of operating range-limiting phenomena in modern UHBR fans and serve as a basis for the validation of numerical simulations.