Blade flutter is an aeroelastic instability caused by the interaction between a blade vibration and the resulting unsteady pressure distributions on the blades. Depending on the phase angle of the unsteady aerodynamic pressure distribution, it can dampen or amplify the triggered vibration. In the latter case, self-excited vibrations (natural oscillations) occur and the blade becomes aerodynamically unstable. In the case of flutter, the total damping – that is the sum of the natural mechanical damping (always positive) and the aerodynamic damping (positive or negative) – is negative and the oscillation decays exponentially.
Modern, highly loaded blades are especially prone to such motion-induced pressure distributions, which can lead to critical aeroelastic vibrations. Being able to predict these unsteady aerodynamic phenomena for flutter assessment is a challenging task for the designer. Although modern numerical tools allow the aeroelastic behaviour of the blades to be predicted with sufficient accuracy, their validation requires high-quality experimental data. Rotating cascades (test models with turbine or compressor blades) are studied in test rigs to provide a simplified yet valuable insight into the aeroelastic behaviour of vibrating blades: the blades are excited with a predetermined vibration and the unsteady pressure distribution on the blade surface is measured to determine the unsteady aerodynamic forces. From this, the exchange of forces between fluid and structure, as well as the aerodynamic damping, can be determined and, as such, it is possible to determine whether the aerodynamics stabilise or destabilise the blade vibration (fluttering).
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