Abstract
This thesis describes the development and application of a methodology for modelling fluid-structure interaction (FSI). The method uses a mesh deformation algorithm integrated within an industrial computational fluid dynamics (CFD) code solving Reynolds averaged Navier-Stokes (RANS) equations in the arbitrary LagrangianEulerian (ALE) formulation. Three classes of problems have been investigated: dynamic stall, stall flutter and leaf seal dynamics for turbomachinery applications. In the first one, the flow over an aerofoil pitching from 5◦ to 25◦ at high Reynolds numbers has been studied for different reduced frequencies. CFD results have been found to be in reasonably good agreement with both the experimental and computational results available from the literature. In the subsequent part of the thesis, the CFD code has been coupled with a dynamic structural model of the aerofoil, and stall flutter simulations have been conducted to study the effects of the initial condition on the amplitude of limit cycle oscillation (LCO) in the stable region. The third application study focuses on the aeroelastic instability of a leaf seal in the secondary air system of gas turbine. Leaf seals can be deployed in gas and steam turbine secondary air systems to reduce parasitic leakages close to rotating shafts. They consist of multiple flexible elements which have to withstand substantial pressure loads during operation. Leaf seal issues are anecdotally associated with the risk of vibration due to the high energy contained within the fluid. This may lead to transient forcing imposed on the flexible elements. Modal analysis of a single leaf has shown that the natural frequency is significantly affected by leaf-rotor contact, while pressure loading is less important. A blow-down neutral clearance seal design was identified for further analysis, using one-way FSI. Transient leaf deflections for the first-flap and third twist modes were imposed on the leaf during unsteady CFD simulations. Unsteady flow structures in the vicinity of the leaf pack, including trailing edge vortex shedding, were identified. Fourier spectral analysis and worksum calculations showed that first mode is stable, whereas a possible flutter instability was observed for the third mode due to interaction with vortex shedding.
Influence coefficient method was developed which evaluates the modal forces on the vibrating leaves including the effects of neighbouring leaves. Further, this was implemented in the modal model to study the circumferential travelling waves.
Prediction of travelling waves with first mode are found encouraging and this methodology can be used to assess higher modes.