Active cancellation of rotor/stator interaction tones in turbofan engines

Rotor/stator-generated aerodynamic noise in turbofan engines, HVAC axial fans, and other turbomachinery is often dominated by blade passage harmonics produced by dipole sound sources. Radiation toward the inlet and exhaust depends upon the effective orientation of the dipole sources relative to the propagation angles of the excited modes. Absorption of this tonal noise using passive treatment is often compromised because of a lack of knowledge of the relative phases and amplitudes of the propagating modes. A modal-based active cancellation system has been developed for global suppression of rotor/stator interaction tones. Axially spaced annuli of controlled wall-mounted actuators are located immediately upstream and downstream of the hubs and tips of stator vanes. Wall-mounted error microphone arrays are located in the inlet nacelle and exhaust duct. Actuator drive vectors are determined in an adaptive-quadrature control algorithm that seeks to minimize total radiated noise. The concept was initially developed and tested on a 24-in. axial fan facility and then extended and demonstrated on the 48-in. Active Noise Control Fan facility at NASA Lewis Research Center. Simultaneous suppression of modes (4,0) and (4,1) of 8–17 dB toward the inlet and 3–11 dB toward the exhaust was achieved at the second blade-passage harmonic.