Competing Broadband Noise Mechanisms in Low-Speed Axial Fans

This paper compares two broadband noise mechanisms, the trailing-edge noise or self-noise, and the leading-edge noise or turbulence-ingestion noise, in several blade technologies. Two previously developed analytical models for these broadband contributions arefirst validatedwithwell-definedmeasurements on several airfoils embedded in an homogeneous flow at low-Mach number. Each instrumented airfoil is placed at the exit of an open jet anechoic wind tunnelwith orwithout a grid generating turbulence upstreamof it. Sound ismeasured in the farfield at the same time as the wall-pressure fluctuations statistics close to the airfoil trailing edge and the inlet velocity fluctuation statistics impacting the airfoil leading edge.Themodels are then compared in somepractical cases representative of airframes, wind turbines, and automotive engine cooling modules. The airfoil models of the twomechanisms are then extended to a full rotating machine in open space. The model predictions of both mechanisms are compared with in-flight helicopter measurements and automotive engine cooling modules measurements. In both instances, the turbulence-ingestion noise is found to be a dominant source over most of the frequency range. The self-noise only becomes a significant contributor at high angles of attack close to flow separation. Nomenclature B = number of blades b = nondimensional parameter in Corcos ’ model