Quasi-steady prediction of coupled bending–torsion flutter under rotating stall

•In the presence of subsonic stalled flow bending flutter can occur in the range 0.9≤γω<1.0 and torsional flutter occurs for γω≥0.9.•Some values of inter-blade phase angle exhibit stable behavior in bending mode for γω=0.9.•To prevent the early onset of torsional flutter designing subsonic compressor blades at frequency ratio close to one should be avoided.•A bifurcation occurs from bending flutter to torsional flutter in the range 0.9≤γω<1.0 ensuring the existence of only one flutter mode at any time.•A roughly pitchfork pattern of energy distribution occurs between bending mode and torsional mode.

A method is presented in this paper to predict cascade flutter under subsonic stalled flow condition in a quasi-steady manner. The ability to predict the occurrence of aeroelastic flutter is highly important from the compressor design point of view. In the present work, the well known Moore–Greitzer compression system model is used to evaluate the flow under rotating stall and the linearized aerodynamic theory of Whitehead is used to estimate the blade loading. The cascade stability is then predicted by solving the structural model, which is posed as a complex eigenvalue problem. The possibility of occurrence of flutter in both bending and torsional modes is considered and the latter is found to be the dominant one, under subsonic stalled flow, for a large range of frequency ratios examined. It is also shown that the design of compressor blades at frequency ratios close to unity may result in rapid initiation of torsional flutter in the presence of stalled flow. A frequency ratio of 0.9 is primarily emphasized for most part of the study as many interesting features are revealed and the results are physically interpreted. Roughly a pitchfork pattern of energy distribution appears to occur between bending mode and torsional mode which ensures that only one flutter mode is possible at any instant in time. A bifurcation from bending flutter to torsional flutter is shown to occur during which the frequency of the two vibrating modes appear to coalesce for a very short period of time.