Numerical Nonlinear Flutter Analysis of a 2D Compressor Blade Using Time and Frequency Domain Methods

Kurzfassung

The aim of this thesis, carried out at the German Aerospace Centre (DLR) - Institute of Aeroelasticity in Göttingen, is to perform numerical flutter predictions for a compressor rotor blade. The unsteady analysis is performed on a two-dimensional linear cascade, whose profile was investigated experimentally in a non-rotating annular test facility at EPFL (École polytechnique fédérale de Lausanne), in the context of the FUTURE project. Unsteady CFD simulations are performed with the DLR in-house code TRACE for internal flows, with the purpose to compare numerical flutter results, obtained with the nonlinear harmonic balance and time domain method, with the time domain method set as a reference. For Validation purposes both nonlinear approaches are compared with the TRAF code, developed at the University of Florence. After an introduction focused on the thesis objectives and a literary survey, in the third chapter theoretical aspects of aeroelasticity are presented, with a particular emphasis on the aeroelastic phenomena involved in turbomachinery compressor blades. The numerical methods for flutter are described, with a special emphasis on the two nonlinear methods used in this work, time domain and harmonic balance. In this context the periodic boundary conditions for the flutter analysis, full annulus and phase lag, are presented. In the fifth chapter, after the description of the test case, details about fluid and solid domain are provided and the computational Setup of the unsteady analysis is shown. Finally, the results of flutter analysis are reported for two operating points, one subsonic and one transonic. The comparison of the results in terms of aerodynamic damping and unsteady pressure coefficient highlights the capability of the harmonic balance method to efficiently capture non linear effects, to accurately predict flutter behaviour with a reduced computational time compared to the time domain method.