Flutter stability is a dominant design constraint of modern gas and  
steam turbines. This makes it difficult to further increase the  
efficiency of turbine stages e.g. by increasing the arodynamic load on the  
last stages or by using new types of composite materials for components.  
Thus, flutter-tolerant designs are currently explored, where the resulting  
vibrations remain within acceptable bounds in order to avoid high cycle fatigue.  
In particular, friction damping has the potential to yield Limit Cycle  
Oscillations (LCOs) in the presence of a flutter instability. To predict  
such LCOs, it is the current practice to model the aerodynamic forces in  
terms of aerodynamic influence coefficients, derived for some normal  
modes of the linearized structural model and fixed oscillation  
frequency. However, this approach neglects that both the nonlinear  
contact interactions and the aerodynamic stiffness cause a change in the  
modal deflection shape and the natural frequency. This, in turn, may have a substantial effect on the aerodynamic damping.  
The goal of this paper is to assess the technical importance of these neglected effects.  
To this end, a state-of-the-art aero-elastic model of a low pressure  
turbine blade row is considered, undergoing nonlinear frictional contact  
interactions in the tip shroud interfaces. On the structural side the finite  
element model is reduced by means of the Craig Bampton method in which the  
relevant Degrees of Freedom (DOFs) at the contact area in the tip shroud are  
retained. The reduced amount of DOFs permits to formulate the motion of the  
blade and thus the motion of the fluid mesh for the flow solver explicitly.  
The symmetry of the investigated traveling wave type motions allow  
to reduce the computational domains for the fluid and the structure to a single reference passage and segment only.  
The LCOs are computed with a fully-coupled Harmonic Balance method, which  
iteratively computes the Fourier coefficients of structural deformation and  
conservative flow variables, as well as the a priori unknown frequency.  
The variable frequency and additional treatment to fix the phase of the solution  
lead to compatibility between the fluid and structural solver and permits the  
solver to compute a physically correct solution.  
The results are compared with the conventional and a refined variant of the energy  
method which is based on a nonlinear modal analysis in combination with a one-way coupled aerodynamic response.  
It is found that there is a noticeable aerodynamic influence on the structural damping which can only be computed by means of two-way coupled methods.


Analysis of Friction-Saturated Flutter Vibrations With a Fully-Coupled Frequency Domain Method

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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