CATANA : Composite AeroelasTics ANd Aeroacoustics

The application of composite fans enables disruptive design possibilities but increases sensitivity to multi-physical resonance between aerodynamic, structure dynamic and acoustic phenomena. As a result, aeroelastic problems increasingly set the stability limit. Test cases of representative geometries without industrial restrictions are a key element of an open scientific culture but are currently non-existent in the turbomachinery community. In order to provide a multi-physical validation benchmark representative of near-future UHBR fan concepts, the open-test-case fan stage ECL5 was developed at Ecole Centrale de Lyon. The design intention was to develop a geometry with high efficiency and a wide stability range that can be realized using carbon fibre composites. This publication aims to introduce the final test case, which is currently fabricated and will be experimentally tested. The fan blades are composed of a laminate made of unidirectional carbon fibres and epoxy composite plies. Their structural properties and the ply orientations are presented. To characterize the test case, details are given on the aerodynamic design of the whole stage, structure dynamics of the fan and aeroelastic stability of the fan. These are obtained with a state-of-art industrial design process: static and modal FEM, RANS and LRANS simulations. Aerodynamic analysis focuses on performance and shows critical flow structures such as tip leakage flow, radial flow migration and flow separations. Mechanical modes of the fan are described and discussed in the context of aeroelastic interactions. Their frequency distribution is validated in terms of resonance risk with respect to synchronous vibration. The aeroelastic stability of the fan is evaluated at representative operating points with a systematic approach. Potential instabilities are observed far from the operating line and do not compromise experimental campaigns.

Non-synchronous vibrations (NSV) arising near the stall boundary of compressors are a recurring and potentially safety-critical problem in modern axial compressors and fans. Recent research has improved predictive capabilities and physical understanding of NSV but prevention measures are still lacking. This paper addresses this by systematically studying the influence of aerodynamic and structural mistuning on NSV. This is achieved by incorporating mistuning effects in a validated linear model, in which individual blade modes are modelled as single-degree of freedom mass oscillators coupled by a convected aerodynamic disturbance term. The results demonstrate that both structural and aerodynamic mistuning are effective. While structural mistuning improves stability by preventing aero-structure lock-in, aerodynamic mistuning, which locally reduces the tip blockage, attenuates the aerodynamic disturbance causing NSV. In the latter case, the circumferentially-averaged conditions are shown to be most influential, while the pattern plays a minor role. A combination of moderate aerodynamic and structural mistuning (1%) was also found to be effective. These findings are relevant for design decisions, demonstrating that small blade-to-blade variations can suppress NSV.

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.

A specific phenomenon that has been observed in many experimental studies on turbomachinery compressors and fans is discussed under the term ‘rotating instabilities’. It is associated to a local aerodynamic phenomenon, typically occurring in the tip region at highly loaded near stall conditions and often linked to blade vibrations. Even though the effect has been discussed over more than two decades, a very ambiguous interpretation still prevails. A particular problem is that certain signatures in measurement data are often considered to characterize the phenomenon despite possible misinterpretations. The present paper illustrates that a specific image of a pulsating disturbance that has been established in the 1990s needs to be reconsidered. At the example of a recent investigation on a composite fan the difficulties concerning sensor placement and post-processing techniques is discussed with a focus on spectral averaging, isolation of non-synchronous phenomena and multi-sensor cross-correlation methods.

In Part-1, the ECL5 open-test-case has been introduced. Details on design methodology, geometry, and aerodynamics of the whole stage have been presented. Part-2 focuses herein on structure dynamics and aeroelastic stability. This paper aims to provide the mechanical and aeroelastic stability characteristics of the fan stage obtained with a state-of-art industrial design process. The fan blades are composed of a laminate made of unidirectional carbon fibres and epoxy composite plies. Fibre orientations of each ply are parameters which enable to modify the mechanical behaviour with minimal impact on the aerodynamic performance. Details on the structural properties, the manufacturing process and the ply orientations are presented. First mechanical modes of the fan are described and discussed in the context of aeroelastic interactions. Their frequency distribution is validated in terms of synchronous vibration. Aeroelastic stability of the fan is evaluated at representative operating points with a systematic approach. Potential instabilities are observed far from the operating line. Therefore , they do not compromise experimental campaigns.

Application of composite rotors enables disruptive design possibilities but demands for a fundamental understanding of the dynamic behaviour to ensure robust design and safe operation. Sensitivity to multi-physical resonance between aerodynamic, structure-dynamic and acoustic phenomena is amplified in modern low speed fan designs for UHBR application. Very thin blades, which are required to maintain high efficiency at transonic flow conditions, are flexible and prone to vibrations. As a result, aeroelastic and aeroacoustic problems increasingly set the stability limit. Test cases of representative geometries without industrial restrictions are a key element of an open scientific culture but currently non-existent in the turbomachinery community. The most commonly used test cases in computational fluid dynamics (e.g. NASA Rotor37/67; TUD Rotor 1 etc.) were designed over two decades ago, and their aeroelastic characteristics are not representative of modern turbomachinery. Also, available experiments have not been conducted with a focus on coupling-phenomena and hence did not comprise multi-physical instrumentation. In order to provide a multi-physical validation benchmark representative of near-future UHBR fan concepts, the open-test-case fan stage ECL5 has been developed at Ecole Centrale de Lyon. Design intention was to develop a geometry with high efficiency and a wide stability range that can be realized using layered carbon fibre composites. The final design iteration of the fan stage is currently fabricated and will be experimentally tested within the European CleanSky-2 project CATANA (Composite Aeroelastics and Aeroacoustics, catana.ec-lyon.fr). In Part-1 of this publication, the test case is introduced with details on geometry, methodology and aerodynamic design of the whole stage, whereas Part-2 focuses on structure dynamics and aeromechanical stability. An analysis of the calculated aerodynamic performance with a focus on critical flow structures like tip-leakage flow, radial flow migration and flow separations is presented. Furthermore, details on the experimental campaign comprising multi-physical instrumentation anticipated for 2021 are given to highlight the research focus.

A novel test facility for transonic fans has been constructed and commissioned at École Centrale de Lyon (ECL) within the Project PHARE-2. The facility is instrumented with multi-physical measurement systems to deeply investigate aeroelastic and aeroacoustic phenomena. To enable long term fundamental research the composite material Open-Test-Case Rotor ECL5 is currently under development which shall be established as a new reference case for method development. Within the present publication the project objectives, the current rotor design, ECL5v2, and details on the test facility are presented. With the goal to establish future academic collaborations the authors aim to initiate a discussion within the research community in an early stage of the test case development to receive constructive feedback on the design and research approach.