Parametric dependencies of aeroengine flutter for flutter clearance applications

Abstract

This thesis describes the effects of operational parameters upon aeroengine flutter stability. The study is composed of three parts: theoretical development of relevant parameters, exploration of a computational model, and analysis of fully scaled test data. Results from these studies are used to develop a rational flutter clearance methodology-a test procedure to ensure flutter-free operation. It is shown, under conditions relevant to aeroengines, that four nondimensional parameters are necessary and sufficient for flutter stability assessment of a given rotor geometry. We introduce a new parameter, termed the reduced damping, g/p*, which collapses the combined effects of mechanical damping and mass ratio (blade mass to fluid inertia). Furthermore, the introduction of the compressible reduced frequency, K*, makes it possible to uniquely separate the corrected performance map from the non-dimensional operating environment (including inlet temperature and pressure).

Simultaneous plots of the performance map of corrected mass flow and corrected speed, (mc,nc), with the (K*, g/p*) map provide a dimensionally complete and fully integrated view of flutter stability, as demonstrated in the context of a historic multimission engine. A parametric, .computational study was conducted using a 2D, linearized unsteady, compressible, potential flow model of a vibrating cascade. This study showed the independent effects of Mach number, inlet flow angle, and reduced frequency upon flutter stability in terms of critical reduced damping, which corroborates the 4D view of flutter stability. Test data from a full-scale transonic fan, spanning the full 4D parameter space, were also analyzed. A novel boundary fitting tool was developed for data processing, which can handle the generic case of sparse, multidimensional, binary data.

The results indicate that the inlet pressure does not alone determine the flight condition effects upon flutter, which necessitates the use of the complete 4D parameter set. Such a complete view of the flutter boundary is constructed, and sensitivities with respect to various parameters are estimated. A rational flutter clearance procedure is proposed. Trends in K* and g/p* allow one to rapidly determine the worst-cases for testing a given design. One may also use sensitivities to extend the results of sea level static (SLS) testing, if the worst case is relatively close to the SLS condition.