Trapped acoustic modes in aeroengine intakes with swirling flow

A theoretical model of an aeroengine intake-fan system is developed in orde r to show the existence of acoustic resonance in the intake. In general thi s phenomenon can be linked to instabilities in aircraft engine inlets. The model incorporates a slowly varying duct intake and accounts for the sw irling flow downstream of the fan. The slow axial variation in cross-sectio n gives rise to turning points where upstream-propagating acoustic modes ar e totally reflected into downstream-propagating modes. The effect of the sw irling flow downstream can be to cut off a mode which is cut on upstream of the fan. It is shown that these two aspects of the flow, coupled with the effects of the fan (represented by an actuator disc), can lead to acoustic modes becoming trapped in the intake, thus giving rise to pure acoustic res onance. A whole range of system parameters, such as axial, fan and swirl Mach numbe rs, which satisfy the conditions for resonance are identified. The effects of a stationary blade row behind the fan are also considered leading to a s econd family of resonant states. In addition we find resonance due to refle ction of acoustic modes at the open (inlet) end of the duct.