Compressor blade flutter in combined plunge-twist mode shapes

Abstract

Modern aeroengine compressor designs strive to reduce weight and maintain high aerodynamic efficiency. Therefore, the aim of initial compressor designs is to implement novel features that achieve these goals. However, some modern features result in greatly reduced damping, which can lead to vibrational instabilities such as flutter. It is known that the first flap (1F) mode shape is particularly susceptible to flutter in compressors. A feature of the 1F mode shape is that it can be well approximated as a superposition of pure plunge and twist vibration, referred to as a combined mode shape.

This thesis aims to enhance understanding of flutter in combined plunge-twist mode shapes, with a focus on the interaction of flow and mode shape physics. The goal is to develop design guidelines for engine manufacturers early in the design process.

Unsteady CFD computations are conducted to determine the effect of different flow and modal parameters on the flutter stability of compressor cascades. Firstly, the influence of reduced frequency, plunge-twist ratio, and nodal diameter is investigated using experimentally validated CFD. A stability map of these input parameters is built, which shows that increasing the reduced frequency and plunge-twist ratio is beneficial for flutter prevention. Following this, a flutter model was developed to explain the location of the stability limit based on results from the parametric studies of the modal variables. This showed that the flutter limit for a subsonic compressor cascade can be expressed by the `plunge-twist incidence ratio'. Finally, studies were conducted to determine how the inlet Mach number and flow incidence affect the flutter stability of a highly loaded compressor cascade. It was shown that the increased flow incidence and inlet Mach number are destabilising due to the increased complexity of the interaction of the unsteady flow and mode shape at off-design operating conditions.