Experimental study on the mode switching of strong-amplitude tones in slat noise

For the strong-amplitude tonal noise from a slat cove, the mechanisms of mode switching phenomenon are not well understood. In this paper, an experimental study was conducted on the slat noise of a 30P30N three-element airfoil through synchronized measurements of a far-field microphone array, wall-pressure transducers, and a hot-wire anemometry. In-house wall-pressure microphones were developed, based on MEMS microphones and flexible printed circuit board, and attached to the curved surface of the slat to measure the wall-pressure fluctuations. The time-frequency analysis through the continuous wavelet transform demonstrated that the synchronous measurements captured the temporal switching of dominant mode in slat noise and the intermittent vortex structures corresponding to the dominant mode frequency in the flow field. The dominant mode in the time-averaged spectra of far-field noise and wall-pressure fluctuations arises from the competition of strong amplitude over time between the primary modes. The time-frequency analysis based on wall-pressure microphones at different spanwise positions revealed a temporal variation of the dominant mode along the slat span. The spanwise coherence analysis indicated that the dominant mode showing stronger coherence has a longer spanwise correlation length compared to other secondary modes.