An Approach for Estimating the Effect of Transient Sweep Through a Resonance

Abstract

The difference of a stationary forced response situation of a turbine or compressor blade relative to a transient resonance sweep is well known and documented in the literature. Different approaches have been used to understand the effect on transient amplitude in comparison with forced response. The dependencies on damping levels and resonance passage speed have been noted. Estimates for a critical or/and maximum sweep velocity have been given. The understanding of transient response during resonance sweep is of practical importance for instance when running a certification stress test for an aircraft engine, where it needs to be decided upfront which acceleration rate (increase in rpm per second) to use to ensure that the maximum airfoil response that could be attained under stationary condition is being measured with sufficient precision. A second reason for understanding of transient response is the verification of correct, if relevant lower, component life usage during transient regimes in operation. This paper gives a proposal for a simple 1D method based on one degree of freedom (1-dof) system considerations for estimating the transient response dependency on the sweep velocity, damping levels and resonance frequency. The method is verified with 3D analyses of more complex blade-disc structures, which have been validated with air jet excitation rig and aero engine tests. Using the results of the 1-dof analysis, an estimate of the expected stationary resonance response increase can be formulated even in cases where the measured data are based on a significant deviation from the desired sweep velocity, where transient effects would be significant.