Comparison of Strategies for the Active Control of Buzz-Saw Tones

Abstract

The results are presented of a feasibility study into the use of active control to reduce the low-engine-order components of buzz-saw noise produced in an aeroengine. It is shown that effective control performance can be achieved using a single ring of circumferentially spaced control actuators and a single ring of error sensors. Two control approaches are investigated: the minimization of the sum of the squared pressures at the error sensors, and the minimization of the sum of weighted squared spinning mode amplitudes. A comparison is made between the performance obtained by these control objectives. It is shown that pressure minimization and minimization of spinning mode amplitudes at the duct walls yields identical control performance. It is also shown that the amplitude of the evanescent modes excited by the control actuators is the fundamental factor in limiting control performance when noise and extraneous modes are absent at the error sensors. The variation of sound power reduction vs tip speed is investigated in detail. It is shown that, following control, a new set of resonances arise because of standing waves set up between the secondary sources and the duct exhaust termination and between the sensors and the duct inlet termination. Considerable improvements in control performance can be achieved by weighting the modal control objective such that control is focussed on the buzz-saw mode.