A New Class of Asynchronous Rotordynamic Response in High-Speed Rotors

Virtually all of the nonlinear rotordynamic phenomena studied in the past result in excitation of rotor response at a dominant frequency at or near the critical frequency at rotational speeds other than the critical speed. In experimental work on a “macro-rig” of a micro-rotor in development some years ago at the Gas Turbine Laboratory of MIT, an unforeseen array of asynchronous response frequencies at other than the critical were noted when the rotor was operated at both subcritical and supercritical speeds. However, those responses were not explored in detail at the time. The patterned responses were apparently related to subharmonic and ultra-subharmonic response, but at frequencies lower than the critical frequency. More recently, the author noted a similar pattern of rotordynamic response in the course of operation at subcritical of an experimental turbomachinery component which was experiencing a local rub between the rotor and stator of an interstage seal. Data from that incident revealed a pattern of asynchronous response that had a very similar appearance to the earlier observation. Using a simple numerical model of a rotor employing a single mass mounted on a massless shaft and a piecewise linear (that is, a bilinear) bearing support stiffness to represent the system, it was possible to replicate the response at individual representative points over a range of sub-, trans-, and supercritical high-speed rotor operation. A generalized expression was derived inductively to represent the individual data points. The resultant pattern of data replicated the patterns of data from the two test vehicles which originally inspired the investigation and suggested the means of their suppression where their presence might be undesirable.Copyright © 2007 by ASME