Coupled Mode Flutter Analysis of a 2D Compressor Cascade using Time-Marching Simulations

Abstract

Flutter phenomenon in turbomachinery is typically studied using decoupled methods. These methods assume that the vacuum modes are not affected by modal coupling due to aerodynamic forces. Decoupled methods rely on the energy method and are only reliable for high mass ratios and small logarithmic decrements. However, modern turbomachinery design trends significantly reduce the structure-to-air mass ratio, making systems susceptible to coupled mode flutter. In this context, using classical decoupled methods, also known as work-per-cycle approaches, can lead to unreliable predictions of system stability. Therefore, it is necessary to use time-domain simulations, which, although more expensive in terms of computational resources, provide the reference tool for validating and fine-tuning less demanding approaches. This work was conducted at the German Aerospace Centre (DLR) in order to perform numerical flutter predictions for a compressor cascade. The in-house TRACE solver was utilized to conduct fluid-structure coupled time-marching simulations. These simulations allowed generic heave and pitch degrees of freedom to vibrate freely in response to an initial small perturbation. Various initial conditions were tested to investigate a wide range of IBPAs, and the mass ratio was varied to assess the effect on the stability limits of the system and the modal coupling. The aim of this thesis is to generate a reliable data set to fill the information gap, examine the stability conditions and demonstrate the correlation between low mass ratio values and modal coupling. Additionally, the text discusses the limitations of the proposed methods and provides guidelines for future studies.