On the shear-layer oscillation and low-frequency tonal noise in a slat cove

During approach and landing of civil aircraft, the slat of high-lift devices becomes one of the significant airframe noise sources. Currently, the mechanisms remain unclear on the low-frequency tonal noise of a slat cove, i.e., the determination of dominant mode and the first-mode spectral hump. This paper presents an experimental investigation on these issues using a phased microphone array and a hot-wire anemometry. The experimental results revealed such characteristics of low-frequency tones in the slat cove. The wavelet time-frequency analysis of velocity fluctuations demonstrates a switching between the primary resonance modes, while the temporal characteristics of the dominant mode indicate a higher proportion in time than other modes. The shear-layer development path shifts due to the bulk oscillation of shear layer, giving rise to the switching between the primary modes and within the first-mode hump. The key parameter that determines the transition of dominant acoustic mode is closely related to the shear-layer oscillation. A correction to the well-known Terracol's acoustic feedback model is proposed based on shear-layer oscillation effects, which agrees well with the experimental results.