Physical Understanding and Sensitivities of LPT Flutter

Abstract

Successful, efficient turbine design requires a thorough understanding of the underlying physical phenomena. This paper investigates the flutter phenomenon of low pressure turbine (LPT) blades seen in aircraft engines and power turbines. CFD analysis will be conducted in a two-dimensional sense using a frequency domain RANS solver on a publicly available LPT airfoil geometry: EPFL’s Standard Configuration 4. An emphasis is placed on revealing the underlying physics behind the threatening LPT flutter mechanism. To this end, flutter sensitivity analysis is conducted on three key parameters: reduced frequency, mode shape, and Mach number. Additionally, exact two-dimensional acoustic resonance inter-blade phase angles (IBPAs) are analytically predicted as a function of reduced frequency. Made evident via damping vs. IBPA plots, the CFD model successfully captures the theoretical acoustic resonance predictions. Studies of the decay of unsteady aerodynamic influence coefficients away from a reference blade are also presented. The influence coefficients provide key insights to the harmonic content of the unsteady pressure field. Finally, this work explores methods of normalizing the work per cycle by the exit dynamic pressure.