Physical Understanding and Sensitivities of Low Pressure Turbine 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. Computational fluid dynamics (CFD) analysis will be conducted in a two-dimensional (2D) sense using a frequency domain Reynolds-averaged Navier—Stokes (RANS) solver on a publicly available LPT airfoil geometry: École Polytechnique Fédérale De Lausanne (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 2D acoustic resonance interblade phase angles (IBPAs) are analytically predicted as a function of reduced frequency. Made evident via damping versus 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.