  Leading-edge (LE) noise is a common source of broadband noise for fans that
can be suppressed using appended LE serrations. We conduct an integrated study
of the morphological effects of interval, length, and inclination angle of
owl-inspired LE serrations on the aeroacoustic characteristics of a mixed flow
fan using experiments, computational fluid dynamics (CFD), and the Ffowcs
Williams-Hawkings (FWH) analogy. A novel method for surface noise strength
(SNS) visualization was developed based on the FWH analogy with large-eddy
simulations, and a CFD-informed index SAPG is proposed to evaluate the severity
of flow separation with pressure gradient, which are verified to be effective
in examining the acoustic sources and chordwise separation. Acoustic
measurements show the robust tradeoff solving capability of the serrations
under various morphologies and the SNS visualizations indicate that the
separation-induced LE noise is suppressed considerably. One-third octave
analyses suggest that extending serration length can lower separation noise
more effectively than shrinking the interval over 100-3k Hz. A smaller interval
is more desirable while an optimal length exists in association with tonal
noise. Moreover, small inclination angles (below 20 degrees) enable the
deceleration of oncoming flows with stagnation relieved, and consequently,
further suppress the LE noise, by a flow-buffering effect. Heavy inclination
angles (over 40 degrees) induce an additional tip vortex, causing
high-coherence turbulence impingement noise and resulting in a drastic increase
in broadband noise at frequencies exceeding 4k Hz. Our study thus clarifies the
morphological effects of LE serrations on aeroacoustic signatures of rotary
devices while providing useful methods for acoustic analyses.
