Aeroacoustic Characteristic Analyses Of Biomimetic Blades With A Novel CFD-Informed Noise Visualization Method

Leading-edge(LE)noise is a common source of broadband noise for fans that can be suppressed using appended LE serrations.An integrated study is conducted to explore 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 CFD-informed index is proposed to evaluate the severity of flow separation with adverse pressure gradient and verified to be effective in examining the chordwise separation.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.Acoustic measurements show the robust tradeoff solving capability of the LE 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 over100-3k 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 4k Hz.In addition,this thesis also confirms the noise reduction capability of biogrooveinspired blades,which is even better than the serrated blades.The wavy design is found to be able to suppress noise below 7k Hz but has the problem of high-frequency noise byproduct,whereas the ribleted blade can lower the noise over a wide frequency range though the overall sound reduced is less than wavy one.This thesis thus clarifies the morphological effects of LE serrations on aeroacoustic signatures of rotary devices and confirmed the acoustic effectiveness of biogroove-inspired designs while providing useful methods for acoustic analyses,which can even benefit other fields,like hydroacoustic studies in marine engineering.