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There are two major causes of vibrations in machinery structures: (i) excitation by time-varied external forces applied to a structure, and (ii) self-excitation, which can occur even in absence of the excitation forces. In this talk, limit-cycles of the self-excited vibrations are analysed. Examples of such vibrations are (i) flutter-induced vibrations in gas-turbine engines: when the aerodynamic damping is negative and the vibration energy is provided by gas flow; and (ii) self-excited vibrations in car or airplane braking systems resulting in brake disk squeal: when the friction forces between rubbing brake disk and disk pads produce the mechanism inducing vibrations.

Assembled structures comprise friction, gap and other types of nonlinear contact interfaces. The nonlinear contact interfaces can suppress the exponential growth of amplitudes of self-excited vibrations to form a periodic motion: limit cycle oscillations. The frequency, mode shape and the level of amplitudes for the limit-cycle vibrations are determined by contact interface parameters and the intensity of the self-excitation. Appropriate choice of parameter values can reduce the limit-cycle vibrations to levels when they cannot cause immediate failure and, even then, do not contribute to high-cycle fatigue damage accumulation.

In this presentation efficient frequency-domain methods developed recently by the speaker are discussed. The method allows calculation of the sensitivity of amplitudes of the limit-cycle oscillations and of their primary frequency to variation of the nonlinear contact interface parameters and aerodynamic characteristics. Examples of analysis of flutter-excited vibrations in bladed discs will be provided.

By: Luke Scott
Last updated: Monday, 1 December 2014