Influence of casing contouring on flutter boundaries of a jet engine fan

This paper describes a detailed flutter analysis of fan casing contour modifications on a scaled high-speed fan to investigate the influence and effect on flutter boundaries. The flutter analysis focusses on discrete selected members from a previous multidisciplinary study of an automated aero-acoustic optimization with respect to the overall engine performance. The aerodynamic baseline performance of the high-bypass ratio fan is validated with measured rig data using Reynolds-averaged Navier–Stokes (RANS) CFD simulations. Flutter stability predictions are based on the energy method in traveling wave and influence the coefficient formulation using a multi-passage fan assembly in a wide engine operating range. Transonic stall flutter occurs for the first bending mode of blade vibration at part speed, where a few design members show an increased stabilizing aeroelastic behavior especially at approach flight condition. In contrast to that, the results indicate a destabilizing flutter stability effect for certain casing designs at cruise speed related to higher mass flows near choke, which is identified as a transonic unstalled flutter type. Aerodynamic key parameters for the flutter onset mechanism have proved to be the shock/boundary layer interactions in tip region of the fan suction side, which leads to flow separation. A second mechanism is driven by the additional blade vibration in combination with the interaction of shock and tip clearance flow as well as the incoming flow. © 2018, Deutsches Zentrum für Luft- und Raumfahrt e.V.