Impact of Aeroacoustics on Aerodynamic Damping In a Transonic 1 1/2-Stage Axial Compressor

New developments in turbomachinery designs require
longer and thinner blades, which are prone to high vibration
amplitudes and flutter. Predicting aerodynamic damping and
understanding its mechanisms, flutter in particular, have been
a research topic for many years. However, most investigations
have been purely based on numerical simulations. In this paper,
the impact of aeroacoustic mode propagation on aerodynamic
damping in a transonic 1 ½-stage axial compressor is investigated.
Harmonic balance calculations of the compressor are
performed, sequentially adding parts of the compressor to the
domain. The acoustic mode propagation is then investigated by
means of a radial mode analysis. The numerically calculated
aerodynamic damping is compared with experimentally obtained
damping at that operating point by means of an acoustic excitation
system. The results show that neighbouring vane rows can
significantly influence the aerodynamic damping, especially, but
not exclusively, when the acoustic modes are cut-on. The reflection
behaviour of the neighbouring vane rows was also found to
be dependent on the cut-on state of the scatter modes; even parts
which are located further away from the rotor - as e.g. struts -
can influence the aerodynamic damping due to acoustic reflections.
However, in this work the extension of the computational
domain mainly decreases non-physical reflections by the boundary
conditions, leading to a more accurate prediction. Acoustic
waves propagating through the blade passage experience phase
jump at the shock, which can presumably lead to a completely
different behaviour as at subsonic operating points.