TIME-AVERAGED ACTIVE CONTROLLER FOR TURBOFAN ENGINE FAN NOISE-REDUCTION

An active control system has been developed and used experimentally on an operational turbofan engine to reduce inlet tonal noise at the Pan blade passage frequency. Both single channel and multichannel systems were tested. The control approach used is the feedforward derivative measurement time-averaged adaptive algorithm. This algorithm is capabl e of adapting to a nonstationary system, requiring no prior knowledge of the system, A reference sensor mounted in the casing: of the fan pr ovides the fan blade passing frequency reference signal to the control ler. Microphones with a large sensing area are placed in the acoustic far field of the engine. The control algorithms minimize a cost functi on, the sum of the error signal mean-square-values, by activating loud speakers that are mounted around the inlet of the engine. The control signals are obtained by filtering the reference signal with finite imp ulse response digital filters, The control algorithm finds the optimum Biter weights by using statistical estimates of the cost function to be minimized. A single channel control system was tested on an axisymm etric dominant mode case and produced reductions of up to 17 dB at the error microphone. The controller maintained reductions in the fan ton e even as the engine speed was increased. In a traverse of the radiate d sound held of the engine the fan blade passage tone was reduced over a sector of 30 deg around the error sensor with a considerable spillo ver increase in the fan tone outside of this sector, Two six channel c ontrol systems were developed and tested on both axisymmetric and spin ning mode dominant cases, Reductions of up to 19.7 dB were achieved ov er large sectors around the error microphones with significantly reduc ed spillover.