Experiments on Structurally Mistuned UHBR Open-Test-Case Fan ECL5/CATANA

Abstract

The operational capabilities of turbofan engines encounter limitations due to instabilities arising from tightly coupled interactions among aerodynamics, acoustics, and structural mechanics. Modern fans and compressors exhibit non-synchronous vibration (NSV), leading to safety-critical blade oscillations. In contrast to self-excited phenomena such as flutter, NSV stems from the convection of aerodynamic disturbances that synchronize with blade eigenmodes. Understanding this phenomenon is challenging, as its intricate interaction patterns and the occurrence of flow separations constrain the predictive capabilities of current state-of-the-art methods. To establish a comprehensive benchmark dataset on the aeroelastic behavior of modern ultra-high bypass ratio (UHBR) architectures, the European CleanSky-2 project CATANA aimed to examine a carbon composite fan stage, ECL5, utilizing multiphysical instrumentation. Recently, experiments on a structurally tuned reference configuration were conducted, revealing high-amplitude NSV at multiple subsonic speedlines. The observed interaction modes and instability onset differed significantly from numerical predictions using both linearized Reynolds-averaged Navier–Stokes and unsteady Reynolds-averaged Navier–Stokes with prescribed harmonic blade motion. In an effort to enhance the dataset, two additional fan configurations with identical blade profile geometries were investigated: one featured a structurally mistuned rotor with approximately doubled frequency variation of all eigenmodes compared to the reference, and the other involved a case with locally increased tip clearance on individual blades. This paper presents the experimental results of a sensitivity study and explores the influence of structural mistuning and tip clearance variation associated with manufacturing tolerances. Contrary to the intended outcome, it will be demonstrated that the mistuned case exhibited higher blade vibration amplitudes than the reference case during NSV. Detailed instrumentation reveals that the mistuning pattern was effectively transferred to the rotating system, but aerodynamic mistuning, particularly concerning tip clearance, emerged as a dominant factor. The non-synchronous forced-response nature of NSV during highly throttled operation ultimately dictates the observed response levels under different conditions, necessitating a thorough analysis to evaluate the robustness of a specific configuration. These results contribute valuable insights to the open dataset for the ECL5 configuration, benefiting the research community. Particularly noteworthy is the detailed capture of blade-to-blade variations in this research, which will prove instrumental in validating numerical methods.