Measurements of fan inflow distortion noise generation in a low speed fan-part I: aerodynamic analyses

Abstract

In order to shed some light in the understanding of fan noise generation due to non-homogeneous inflow, as is the case for aeroengine with boundary layer ingestion (BLI), a seriesof measurements was performed in a low-speed fan rig. BLI profile generation for fan acousticlaboratory tests has to consider both the average velocity profile, and the turbulence charac-teristics inside the distorted flow region. A question is how representative are experimentsperformed with simple screens in terms of turbulence and velocity profile generation for theinvestigation of fan BLI noise. To answer this question, screens with different shapes and heightcombined with a honeycomb flow straightener were mounted in the inlet of a fan rig. Acousticand aerodynamic measurements were conducted. The average distorted velocity profile wassuccessfully generated by each screen. The turbulence parameters such as Reynold stresses andintegral length scale were assessed upstream the fan. Results indicate a peak in the Reynoldstresses in the region associated to the shear layer, or roughly at a radial position associatedto the screen height. The turbulence levels inside the distorted flow are lower than what isexpected for a BLI configuration.