Wake generator control of inlet flow to cancel flow distortion noise

If the inlet flow to a fan is non-uniform, as is often the case for aircraft engines, then undesirable tonal noise can be generated. A number of authors have suggested using active cancellation to reduce the noise. The secondary field can either be generated by loudspeakers or by the fan itself if secondary non-uniformities are deliberately introduced into the flow. In the research reported here rods inserted radially into the duct were used to generate the secondary field. The distance by which each rod protrudes into the duct was adaptively adjusted in response to an array of in-duct microphones so as to minimise the radiated sound power, whereas previously only fixed rods have been considered. The ability of the steepest-descent algorithm to minimise in-duct sound power under suitable conditions, and hence reduce radiated sound power is demonstrated in both simulations and low Mach number experiments. It is shown how the ability of such a system to control noise depends on the number and position of the controller rods, and the number of acoustic duct modes to be controlled. Thus at low fan speed, when only one mode was present just two controllers achieved an in-duct noise reduction of 25 dB at the blade passing frequency, whereas at a higher fan speed with three modes present six controllers only achieved 2 dB. To implement such a scheme in practice, where large numbers of modes are typically present, it would be necessary to develop controller arrays with many actuators, but with low aerodynamic penalty. Such a system might also be useful in HVAC applications, or in wind-tunnel testing.