Numerical Investigation of the Effect of Flutter Instability of the Blade on the Unsteady Flow in a Modern Low-Pressure Turbine

Unsteady flows through various blade rows of a gas turbine engine often influence the dynamic structure behaviour of the blades leading to either an instantaneous or a high cycle fatigue (HCF) failure of the structure. Most of the modern civil aero engines consist of the Low Pressure Turbine (LPT) which can account for approximately 20-30 percent of the total engine weight. The flow inside an LPT in aero engines is prone to separation due to the low air density resulting in a low Reynolds number. Several studies have been performed to improve the efficiency of the LPTs as well as to reduce the weight and the associated manufacturing costs of the LPTs. However, these designs not only decrease the highly-correlated LPT flutter parameter known as reduced frequency but also introduce the higher per-stage loading. As a result, the LPTs of the modern gas turbines may also be prone to the aeroelastic instability problems similar to those encountered in the compressors and fans due to the high loading conditions in a combination with an increase of the blade aspect ratio and reducing the blade thickness.

As an accurate prediction of flutter and forced response in turbomachines, especially in LPTs, is one of the greatest unsolved challenges faced by the industry, a lot of efforts have been made over the last decades to seek the efficient numerical methods. The existing high fidelity aeroelastic solvers are based on the Unsteady Reynolds Averaged Navier–Stokes (URANS) models. The URANS models are not capable of predicting the unsteady flow behaviour, especially in the flow separation zone due to the interaction between the transient flow and the blade structure, which is usually seen in LPTs. Therefore, the required confidence and accuracy cannot be obtained with the URANS models because of the inadequacy of the turbulence models. Furthermore, the existing aeroelastic models and solvers used in the industry mainly focus on the aeroelasticity parameters such as the value of aerodynamic damping, and disregards the complex physics occurring during the fluid-structure interaction process which gives rise to a black-box effect. The understanding of the interaction between the various sources of unsteadiness and the blade structure is still limited, and it requires further investigations. Therefore, a high fidelity numerical model should be developed to provide understanding of the physics behind the fluid-structure interactions in a modern LPT.

The overarching aim of this paper is to explore the forced response and flutter instability in a modern LPT using a high-fidelity Direct Numerical Simulation (DNS) method in which the various sources of unsteadiness associated with the fluid-structure interaction are included. The present work will provide fundamental understandings of the mechanism behind the interaction between the flow field unsteadiness and the blade structure in a modern LPT. This work will bridge a key gap in the knowledge of aeroelasticity modelling and prediction, and results will be relevant to other turbo-machines prone to aeroelastic instabilities such as steam turbines and wind turbine blades.