Unsteady aerodynamic forcing due to distortion in a boundary layer ingesting fan

Abstract Boundary Layer Ingestion (BLI) is a concept that has the potential to reduce energy consumption needed for aircraft propulsion. For the fan design this results in a need to work well in an environment of increased continuous distortion. The objective of this study is to increase the understanding of unsteady aerodynamic forces due to total pressure distortion in a BLI. This is done by using CFD to analyze a fan design intended for a BLI installation. The range of results includes low subsonic to transonic operating speeds, as well as a wide range of circumferential wavenumbers. The forcing exhibits a significant dependency on the aeroacoustic cut-on/cut-off condition. This is in particular true at part speed where the normalized unsteady force in the circumferential direction rises sharply before dropping suddenly along an increasing speed or engine order number. At the same point the unsteady pressure levels in the blade passages are reduced and undamped pressure waves propagating out from the blade row appear. The unsteady forces on the first 3 modes exhibit different sensitivities to the distortion wavelength but all are affected by the acoustic condition. By comparing the results with a similar fan with a higher blade count it is found the interaction is dominated by the cascade properties of the fan blade. A result of this is that a higher blade count fan may be less affected by the distortion, and be less prone to propagate noise due to low engine order distortion.