Switching dominant tones from trailing edge tonal noise

The tonal noise generated at the trailing edge of an airfoil under low-to-moderate Reynolds number conditions can exhibit multiple tones. While time-averaged spectra typically show a single dominant tone, specific cases within a narrow velocity range reveal the existence of two dominant tones. This phenomenon raises an interesting question: are the unsteady occurrences of these two tones concurrent or alternating? To investigate this, an experiment was conducted in the Beihang D7 aeroacoustics wind tunnel using a NACA0012 airfoil. In the frequency domain, far-field noise measurements confirmed the presence of two dominant tones at approximately 744 Hz and 1116 Hz around a freestream velocity of 20 m/s and an angle of attack of 4 degrees. In the time domain, wavelet analysis of the sound pressure signals revealed an intermittent pattern. Concurrently, velocity signals measured at two symmetric spanwise positions in the wake region displayed distinct alternating patterns. Statistical analyses based on permutation entropy and transient tone coding, complemented by cross-correlation analyses between the microphone and each hot-wire, demonstrate that the two dominant tones alternate locally with mutually inconsistent behavior along the spanwise direction, which suggests that the globally observed complex switching behavior is a result of spatially distributed alternating noise sources.