Effects of Boundary Layer Ingestion on the Aero Acoustics of Transonic Fan Rotors

Abstract

The use of boundarylayeringesting, embedded propulsion systems can result in inlet flow distortions where the interaction of the boundarylayer vorticity and the inlet lip causes horseshoe vortex formation and the ingestion of streamwise vortices into the inlet. A previouslydeveloped bodyforcebased fan modeling approach was used to assess the change in fan rotor shock noise generation and propagation in a boundarylayeringesting, serpentine inlet. This approach is employed here in a parametric study to assess the effects of inlet geometry parameters (offsettodiameter ratio and downstreamtoupstream area ratio) on flow distortion and rotor shock noise. Mechanisms related to the vortical inlet structures were found to govern changes in the rotor shock noise generation and propagation. The vortex whose circulation is in the opposite direction to the fan rotation (counterswirling vortex) increases incidence angles on the fan blades near the tip, enhancing noise generation. The vortex with circulation in the direction of fan rotation (coswirling vortex) creates a region of subsonic relative flow near the blade tip radius that decreases the sound power propagated to the farfield. The parametric study revealed that the overall sound power level at the fan leading edge is set by the ingested streamwise circulation, and that for inlet designs in which the streamwise vortices are displaced away from the duct wall, the sound power at the upstream inlet plane increased by as much as 9 dB. By comparing the farfield noise results obtained to those for a conventional inlet, it is deduced that the changes in rotor shock noise are predominantly due to the ingestion of streamwise vorticity.