The Effect of Flow Coefficient on the Self Noise Generated by a Ducted Rotor

Sound generation in low Mach number turbomachines is typically dominated by unsteady fluid forces on rigid surfaces. As a result, the radiated sound is closely related to the unsteady flow field. The present study focused on the self noise that is generated by a ducted rotor separate from the effect of noise due to inflow turbulence. The flow rate through the rotor was independently varied in order change the mean lift on the blades. Measurements of the flow field around a ducted rotor were found to provide insight to the various mechanisms of sound that are present at different mean loading conditions. At lower flow rates the blades were partially stalled, resulting in significantly increased noise levels. The measurements included rotor wake measurements using hot-wire anemometry and far field sound. A simple model to predict the radiated self noise based on the hot-wire measurements is presented.