A Cyclic Symmetry Analysis for Turbomachine Blade Flutter

Abstract

A frequent cause of turbomachinery blade failure is excessive vibration due to flutter or forced response. One method for dealing with this problem is to increase blade structural damping using either tip or mid-span shroud designs. Unfortunately, most existing aeroelastic analyses deal with a blade alone model which can not be used for system mode analysis. Therefore, judgments based on past experience are used to determine the acceptability of a shrouded blade design. A new cyclic symmetry analysis has been developed to predict shrouded blade flutter. The method provides a system approach, over and above the standard blade alone approach, for predicting potential aeroelastic problems. Using the blade natural frequencies and mode shapes from a cyclic symmetry finite element model, the unsteady aerodynamic forces of the system mode are calculated. A cyclic symmetry flutter analysis is then performed. This analysis has been applied to a typical shrouded fan blade to investigate blade flutter. The predicted system mode flutter demonstrated that the blade alone analysis can be non-conservative.