1. Introduction
With the application of modern lightweight blade geometries, aeroelastic coupling phenomena tend to reduce the operating range of compressors and fans in aircraft propulsion engines. As synchronous excitation mechanisms are well understood today, the most challenging aeroelastic phenomena for future turbomachinery applications are of a non-synchronous (not a multiple of the shaft rotation speed) nature. A list of these phenomena is presented as follows. This paper is an extended version of our paper published in Proceedings of the European Turbomachinery Conference ETC14 2021, Paper No. 625 and 626, Gdansk, Poland, 12–16 April 2021 [
1,
2].
Flutter is defined as self-excited blade vibration and usually involves blade-to-blade coupling. It is considered as initially small-amplitude blade oscillation in a specific eigenmode that exponentially amplifies through a positive feedback loop with the aerodynamic field (negative aerodynamic damping). Linear modelling approaches are capable of predicting the onset of this mechanism and of determining critical modes and developing countermeasures. The disturbance is only dependent on the blade vibration and disappears as soon as the vibration stops. For large vibration amplitudes, limit-cycle oscillation may occur due to aerodynamic or structure dynamic non-linearity. For turbo-engine fans, critical modes are typically dependent on swirling acoustic modes that develop between the inlet and the fan stage [
3].
Buffeting describes the interaction between an aerodynamic instability (typically vortex shedding) that comprises a characteristic frequency and the blade vibration in a specific eigenmode. Typically, no circumferential blade-to-blade coupling is necessary, but often planar acoustic duct modes establish and synchronize the phase of the aerodynamic instabilities leading to a zero-nodal-diameter vibration [
4].
Rotating stall is a purely aerodynamic phenomenon occurring due to overloading of a blade row and subsequent flow separation, leading to the establishment of circumferentially propagating stall cells. It typically excites structural eigenmodes due to unsteady loading, but for small amplitudes the propagation is not coupled with the blade vibration [
5].
Non-synchronous forced response is a non-synchronous coupling mechanism in multistage compressors that has been observed between a trapped acoustic mode and a coincident structural vibration pattern [
6]. The phenomenon is less relevant for fan applications.
Convective non-synchronous vibration (NSV) typically occurs close to the stability limit but before the onset of rotating stall. A complex lock-in mechanism between propagating aerodynamic vorticity disturbances and blade vibration is described under the term NSV [
7,
8]. Blade vibration leads to a change in free vorticity, or even the formation of radial vortices in the passage flow, which are convected from blade to blade if a sufficient blockage is present in the passage. Interaction with trailing blades leads to modal forcing and the shedding of subsequent vorticity. Propagating aerodynamic disturbances can lock in with structural vibration patterns and lead to a coherent fluid–structural interaction. The phenomenon must be differentiated from flutter, as the aerodynamic disturbance appears without blade vibration and has a characteristic convective propagation speed (typically described under the term rotating instability).
For all the described phenomena, the flow structure in the tip region, particularly the influence of the tip leakage flow and the passage shock, is of great importance. Significant blockage enables the circumferential transport of disturbances or provokes separation of the boundary layer, which may be susceptible to acoustic or structure-dynamic feedback.
For high-speed fans, rotating stall, buffeting and flutter are the most common instability mechanisms and are well understood today. The establishment of UHBR configurations with low-speed fans, however, leads to a substantial change in the relevant characteristics.
Low-speed fans predominantly operate on the flat part of the compression characteristic, making them more susceptible to stall-driven instability [
9].
The flutter frequencies (in the stationary frame) are lower compared to high-speed designs. Acoustic liners in the intake, which are designed to attenuate higher-frequency community noise, do not affect the modes relevant for aeroelastic instability.
The intake length is shorter for low-speed fans, leading to stronger inflow asymmetry and altered acoustic interactions [
10]. This gives rise to stronger broadband excitation and shifted resonance frequencies.
The relative Mach number and shock strength are lower, and the tip clearance relative to the blade chord and solidity (solidity = blade chord length/pitch) are smaller than for conventional direct-drive fans and more sensitive to geometric variability [
11].
A strongly non-linear fluid–structure interaction has been observed at low frequencies for fans with low solidity related to the pressure untwist of the blades. Under transonic conditions, slight deviations of the local stagger angle at the blade tip can cause a fundamentally different shock structure between adjacent blades that affects the stability of distinct rotor sections [
12]. This circumstance affects the applicability of promising methods such as intentional blade mistuning [
13] for suppressing the development of circumferentially propagating modes.
All the fan blades shown have significant 3D features, particularly forward sweep, which is known to be beneficial for aerodynamic performance but emphasizes aeroelastic sensitivity for torsional or chord-wise bending modes and particularly NSV. Recent research [
14,
15] has shown that the use of the anisotropic properties of composites in fans could help to improve both the mechanics and aerodynamics. Composites also present the potential to control flutter by modification of the eigenmodes [
16].
In order to enable further technological advancements in this direction, extensive research is necessary to identify and characterize the relevant instability mechanisms for the novel type of low-speed fan. In particular, the complex flow structure at part load and part speed is challenging for state-of-the-art numerical approaches and requires experimental benchmark data on representative geometries.
To address these research objectives, an extensive research program with collaboration between Ecole Centrale de Lyon and the Von Karman Institute for Fluid Dynamics has been initiated. A fan module, ECL5, fabricated from composite material, has been developed as an open test case. It will be investigated, with a focus on non-synchronous coupling mechanisms between aerodynamics, acoustics and structure dynamics, within the European Clean Sky 2 project CATANA (Composite Aeroelastics and Aeroacoustics, catana.ec-lyon.fr). The fan stage was designed at Ecole Centrale de Lyon and was intended to be representative of near-future composite low-speed fans in terms of the following:
- 1.
General aerodynamic design parameters (Mach number, blade loading, solidity, aspect ratio, hub-to-tip ratio, mass flow density, etc.)
- 2.
Aerodynamic flow structure, due to its influence on instability mechanisms (shock patterns, radial flow migration, secondary flow, separations, etc.)
In the following, the final design and the composite structure of the fan are presented. Details are given on the modelling strategies used to simulate the different physics: stationary flow simulations (RANS), mechanical simulations (FEM) and time-linearized simulations (LRANS). Finally, the aerodynamic behaviour and structural dynamics of the fan are deduced, and its aeroelastic stability is assessed.