Preliminary assessment of wake management strategies for reduction of turbomachinery fan noise

Preliminary studies of a new approach for reducing rotor-stator interaction noise are presented. The approach centers on removal and/or addition of fluid from the rotor blades to minimize the shed wakes, thereby making the flow into the stator more uniform, and reducing unsteady loading and radiated noise, High bypass ratio fans typical of next-generation gas turbine engine technology were the focus of the work, The results presented include 1) two-dimensional, unsteady numerical simulations of the impact of rotor blade boundary-layer suction on unsteady stator loading and 2) cascade experiments to investigate the effects of various fan blade boundary-layer suction and trailing-edge blowing strategies on both steady and unsteady aspects of the blade wakes. A two-dimensional linearized panel method was used to estimate the reduction in radiated tone noise achievable with these techniques, Results of an analysis of fan blade design requirements and performance impacts for applications of wake management on full-scale engines are also discussed, The principal conclusion of this study is that methods to control the initiation of the fan wake are feasible for application in high-bypass ratio turbomachines. The data presented suggest that greater than 40% reduction of the steady wake velocity deficit and greater than 35% reduction of the unsteady velocity fluctuations in the wake can be achieved by removal of less than 3% of the fan mass flow through blade boundary-layer suction, or through the addition of Less than 1% of the fan mass flow through trailing-edge blowing, Reductions in the amplitude of propagating circumferential acoustic modes were estimated for the experimentally obtained wake profiles. Mode amplitudes were reduced approximately 4 dB for boundary-layer suction and from 4 to 11 dB for trailing-edge blowing.