Transient growth and rotor–stator interaction noise in mean swirling duct flow

The phenomenon of spatial transient growth in swirling duct flow with application to aeroengine duct acoustics is investigated. In a typical aeroengine a region of swirling flow exists between the rotor and the downstream stator. The possibility for optimal transient growth of disturbance energy, and of a norm related to the acoustic power radiated back upstream from the stator is shown. Significant transient growth of both energy and noise norms can be observed over length scales relevant to aeroengine size, and over a wide range of parameter values such as the degree of mean swirl, azimuthal wavenumber and frequency. The optimal disturbances are found to have significant three dimensionality. The growth in energy results from increases in the azimuthal and radial components of the disturbance velocity field as it propagates downstream, whilst increases in the azimuthal velocity bring about non-modal growth of the noise measure.