Airframe noise from commercial aircraft rivals engine noise in the landing phase of flight. Unsteady flow around the leading edge slat of the high-lift system contributes significantly to airframe noise. This research experimentally investigated tonal noise sources arising from the unsteady flow around the leading edge slat from a flow physics standpoint. Using this approach, efficient acoustic control strategies naturally follow. This study resolved two types of acoustic tones, trailing edge vortex shedding noise and acoustic resonance, originating from the slat component of a two-dimensional, three-element airfoil model. The latter noise mechanism had not been documented until the current study. Sharpening the slat trailing edge reduced the overall sound pressure level from the shedding noise by 12dB. The reduction in radiated acoustic pressure was directly proportional to the reduction in the strength of the vortex shedding. The acoustic power was conceptually related to the strength of the global instability in the wake. The onset of the slat aeroacoustic resonance corresponded to the formation of a separation bubble on the upper surface of the slat. The peak acoustic frequency matched the frequency of large-scale structures ejected from the separation bubble. A feedback loop between the near-wake region and the separation point sustained the resonance. A model based on a phase matching condition between the acoustic waves and the generation of the separated shear layer instability waves successfully predicted the acoustic frequencies. The acoustic tone followed the modal variation closest to the subharmonic of the natural instability frequency, which caused the discontinuous jumps in frequency with increasing velocity.
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