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. Recent experiments, conducted as part of the European CATANA project on a structurally tuned reference configuration, revealed high-amplitude NSV across multiple subsonic operating conditions. 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 due to manufacturing tolerances. Contrary to the expectations, the mistuned rotor exhibited higher blade vibration amplitudes during NSV compared to the reference case. 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 under conditions of highly throttled operation ultimately dictates the observed response levels, necessitating a thorough analysis to evaluate the robustness of a specific configuration.