Investigations of Flutter and Aero Damping of a Turbine Blade: Part 1 — Experimental Characterization

Flutter is a self-excited and self-sustained aero-elastic instability, caused by the positive feedback between structural vibration and aerodynamic forces. A two-passage linear turbine cascade was designed, built and tested to better understand the phenomena and collect data to validate numerical models. The cascade featured a center airfoil that had its pitch axis as a degree of freedom to enable coupling between the air flow and mechanical response in a controlled manner. The airfoil was designed to be excited about its pitch axis using an electromagnetic actuation system over a range of frequencies and amplitudes. The excitation force was measured with load cells and the airfoil motion was measured with accelerometers. Extraordinary effort was taken to minimize the mechanical damping so that the damping effects of the airflow over the airfoil, that were of primary interest, would be observable. Assembling the cascade required specialized alignment procedures due to the tight clearances and large motion. The aerodynamic damping effects were determined by observing changes in the mechanical frequency response of the system. Detail aero and mechanical measurements were conducted within a wide range of flow conditions. Experimental results indicate interesting changes in aerodynamic damping over a range of Mach numbers from 0.4 to 1.2. The aero damping was also found to be independent of displacement amplitude within the tested range, giving credence to linear numerical approaches.