AERODYNAMIC DETUNING ANALYSIS OF AN UNSTALLED SUPERSONIC TURBOFAN CASCADE (FLUTTER, FORCED RESPONSE, INFLUENCE COEFFICIENT)

DANIEL HOYNIAK, Purdue University

Abstract

A model for aerodynamic detuning to achieve enhanced supersonic unstalled aeroelastic stability and forced response behavior has been developed. This model analyzes the stability and resonant characteristics of an aerodynamically detuned rotor operating in a supersonic inlet flow field with a subsonic leading edge locus, with the aerodynamic detuning accomplished by means of nonuniform circumferential spacing of adjacent rotor blades. The unsteady aerodynamic forces and moments acting on the blading are defined in terms of influence coefficients in a manner that permits the unsteady loading for both a conventional uniformly spaced rotor configuration as well as that of the detuned nonuniform circumferentially spaced rotor to be determined. The unsteady aerodynamic loading is calculated for two conditions: (1) a transverse gust interacting with a stationary cascade; (2) the harmonic translational and torsional motions of the airfoils. These unsteady aerodynamic loads are incorporated into an aeroelastic model to predict the flutter and forced response characteristics of both the tuned and detuned cascade configurations. Nonuniform circumferential airfoil spacing is demonstrated to have a beneficial effect on the unstalled supersonic flutter behavior of a twelve bladed rotor with Verdon's uniformly spaced Cascade B geometry as a baseline. The improvement in cascade stability is related to the change in the chordwise unsteady pressure difference distribution over the airfoils which results from the movement of the Mach wave airfoil surface intersection locations. The combination of this type of aerodynamic detuning and alternate blade structural detuning is shown to have a very beneficial effect on the flutter behavior with the placement of the high and low frequency airfoils critical. The forced response characteristics of the detuned cascade are shown to be dependent on the nature of the incoming gust. The application of aerodynamic detuning to the coupled bending-torsion flutter of a cascade also had a beneficial effect on stability when the elastic axis and center of gravity were coincident at or foward of the midchord.

Degree

Ph.D.

Subject Area

Mechanical engineering

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