Efficient Design Method for Non-Synchronous Vibrations Using Enforced Motion

Abstract

This paper presents the results of a new enforced motion method using harmonic balance computational fluid dynamics (CFD) analysis to design for NSV. Currently, most researchers employ a time domain CFD technique to directly find the frequency of the underlying flow instability which can take significant computational time. NSV is said to occur when the frequency of the instability coincides with a blade mode frequency. The enforced motion design method uses blade motion to attempt to force the fluid frequency to lock-on to the blade vibration frequency at a specified amplitude. For a fixed critical amplitude and blade mode frequency, a range of interblade phase angles (IBPAs) is investigated to determine the aerodynamic damping. A negative value of damping at any IBPA deems the design unacceptable. Furthermore, a procedure for blade re-design (frequency changing) is presented. At the least stable IBPA, the damping is determined for a range of blade frequencies and amplitudes to determine the Limit Cycle Oscillation (LCO) amplitude. A better design is then at the blade frequency that minimizes the blade vibration amplitude. Therefore, these preliminary results indicate that it is advantageous to include blade motion in NSV design approaches. Most significantly, it gives designers a quick and efficient method to assess a design for NSV.