Dipole active control of wake-blade row interaction noise

In high-bypass turbofan engines, discrete-frequency tones are generated at blade-pass frequency and its harmonics by the unsteady interaction between the rotor blades and stationary strut and vane rows. Specific circumferenti al acoustic modes are generated, determined by the number of rotor blades a nd stator vanes. However, only certain modes propagate to the far field and result in the discrete-frequency noise received by an observer. This paper is directed at active control of this discrete-frequency noise through can cellation of the propagating acoustic waves. A series of experiments is per formed in the Purdue Annular Cascade Research Facility configured with a 16 -bladed rotor and either one or three airfoil stator vane rows, The active discrete noise control is accomplished utilizing a speaker-dipole arrangeme nt around each stator to generate additional control, propagating acoustic waves that interact and cancel those generated by the rotor-stator interact ion, with the control parameter being the phase angle between the rotor wak e and the speaker-dipole. In both one- and three-airfoil stator vane row co nfigurations, the active noise control system simultaneously attenuated all upstream- and downstream-propagating modes. The system was also demonstrat ed in a monopole configuration that also provided significant noise reducti ons in the three-airfoil stator vane row.