The phenomena prior to rotating stall were investigated in a high-speed compressor test rig using optical and pneumatic measurement techniques. A number of throttling procedures were performed at transonic and subsonic speedlines with the aim to detect the unsteady effects initiating rotating stall or large amplitude blade vibrations. At transonic speed, radial vortices traveling around the circumference were detected in the upstream part of the rotor using phase-locked particle-image-velocimetry (PIV) measurements above 92% span and unsteady wall pressure measurements. When these radial vortices impinge on a blade leading edge (LE), they cause a forward spill of fluid around the LE. The effects are accompanied by a large-scale vortex breakdown in the blade passage leading to immense blockage in the endwall region. At subsonic speeds, the observed flow phenomena are similar but differ in intensity and structure. During the throttling procedure, blade vibration amplitudes were monitored using strain gauges (SG) and blade tip timing instrumentation. Nonsynchronous blade vibrations in the first torsional eigenmode were measured as the rotor approached stall. Using the different types of instrumentation, it was possible to align the aerodynamic flow features with blade vibration levels. The results show a clear correlation between the occurrence of radial vortices and blade vibrations.

Non-Synchronous-Vibration (NSV) in high-speed turbomachinery compressors is an aeroelastic phenomenon which can have devastating consequences, including loss of rotor blades. Despite extensive research over the past two decades its underlying mechanisms are not yet understood. This paper aims to explain the physical mechanisms causing NSV in a modern transonic compressor rotor. Referring to previous experimental results and using validated computational fluid dynamics (CFD), a parametric study is performed in order to characterize the aerodynamic disturbance causing NSV, and to understand the lock-in mechanism between the fluid and the structure seen during NSV. The results show that the process is driven by aerodynamics in the tip region. Under highly throttled conditions, the tip leakage flow blocks the passage and causes the disturbance, which is characterised as a vorticity fluctuation, to propagate circumferentially in the leading edge plane. It is found that the propagation speed of the disturbance is determined by the mean flow conditions and only its phase is periodically modulated through interaction with oscillating blades. This is the mechanism facilitating lock-in. Based on these findings a semi-analytic model is developed and calibrated with the numerical results. The model is capable of simulating the lock-in process and correctly predicts unstable vibration modes.

A composite fan stage representative of a modern UHBR architecture has been investigated experimentally on a novel test facility at Ecole Centrale de Lyon. These measurements show indications for strong overloading of the tip region resulting in extensive blockage of the blade passage. The performance of the fan is analyzed with extensive instrumentation including radial profiles upstream and downstream of the rotor. Unsteady pressure measurements help to interpret the flow structure in the tip region. The results are presented across a range of operating points on the design speedline. At the stability limit, the machine suffers from Non-Synchronous Vibrations which result from small scale aerodynamic disturbances propagating between the leading edges. A detailed analysis on the occurring waveforms is presented for two operating speeds. In order to further analyze the observed phenomena, a numerical study has been conducted using the RANS solver elsA. The results of steady calculations are discussed in comparison with the detailed experiments. Unsteady simulations near the stability limit accurately predict the aerodynamic disturbances observed during NSV. The obtained results are unusual for typical state-of-the-art transonic fans, as they show the same behavior as high-pressure compressor front stages, dominated by blockage caused by tip leakage flow. Even though flutter is not observed, the observed Non-Synchronous Vibration mechanism is a critical aeroelastic phenomenon which is of great interest for future designs of low speed fans.

Non-synchronous vibrations arising near the stall boundary of compressors are a recurring and potentially safety-critical problem in modern aero-engines. Recent numerical and experimental investigations have shown that these vibrations are caused by the lock-in of circumferentially convected aerodynamic disturbances and structural vibration modes, and that it is possible to predict unstable vibration modes using coupled linear models. This paper aims to further investigate non-synchronous vibrations by casting a reduced model for NSV in the frequency domain and analysing stability for a range of parameters. It is shown how, and why, under certain conditions linear models are able to capture a phenomenon, which has traditionally been associated with aerodynamic non-linearities. The formulation clearly highlights the differences between convective non-synchronous vibrations and flutter and identifies the modifications necessary to make quantitative predictions.