Morphological effects of leading-edge serrations on the acoustic signatures of mixed flow fan

Abstract

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 to accurately quantify the spatial distributions of acoustic sources. A CFD-informed index is proposed to evaluate the severity of flow separation with the pressure gradient and verified to be effective in examining the chord-wise separation. Acoustic measurements show the robust trade-off 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 100–3000 Hz. A smaller interval is more desirable while an optimal length exists in association with tonal noise. Moreover, small inclination angles (≤20°) enable the deceleration of oncoming flows with stagnation relieved, and consequently, further suppress the LE noise, by a flow-buffering effect. Heavy inclination angles (≥40°) induce an additional tip vortex, causing high-coherence turbulence impingement noise and resulting in a drastic increase in broadband noise at frequencies exceeding 4000 Hz. Our study, thus, clarifies the morphological effects of LE serrations on aeroacoustic signatures of rotary devices while providing useful methods for acoustic analyses.