Active noise control in axial turbomachines by flow induced secondary sources.

In conventional active noise control experiments, loudspeakers are used to generate the secondary antiphase sound field to be superimposed with the sound waves radiated from the primary source. In the present study, aerodynamic sound sources are used instead for that purpose. This is achieved by disturbing the flow field around the blade tips in such a way that additional periodic forces are set up which in turn form the secondary sound sources. To disturb the flow, either air is blown into the blade tip region through the casing wall, or piezo-electric actuators are installed in the fan casing wall to influence the flow conditions near the blade tips. The resulting aerodynamic sound sources are adjustable in both amplitude and phase. In this way active flow control is used to reduce the tonal noise components of the axial fan. Experimental results are presented for steady and unsteady air jets injected into the main flow at various axial positions relative to the rotor blades. It is shown that the method is successful for plane wave as well as for higher-order mode sound fields. The sound pressure level at the blade passing frequency was reduced by up to 20.5 dB.