Inflow distortion noise and turbulence measurements in a low speed fan test rig

Abstract

In the context of emerging aircraft with boundary layer ingestion and high bypass ratios, fan broadband noise is expected to increase. On the one hand, broadband noise is excited when the ingested turbulence interacts with the rotor. On the other hand the distorted mean flow in the inlet affects the turbulence generated by the rotor, i.e. the rotor wakes and the tip vortex. In this study turbulence measurements upstream and downstream of the rotor with hot wires are combined with an in-duct sound power measurement technique in a laboratory fan test bench. The sound power is calculated based on a broadband radial mode analysis. Based on the increased turbulence measured downstream of the rotor, the rotor stator interaction is expected to increase with inflow distortion. The sound power measurements do not confirm this expectation. The reason for this contradiction are discussed in the paper. It is assumed that the rotor stator interaction noise is masked by the turbulence rotor interaction noise at high frequencies and possibly by low frequency stall noise that arises when the rotor blade passes through the inflow distortion. These assessments are supported by the hot wire measurements.