Experimental study of the multiple tones in trailing edge noise of an aerofoil at low-to-moderate Reynolds number

In this work, aeroacoustic characteristics of the multiple tones of a NACA0012 aerofoil at moderate incidence 5° are studied using anechoic wind tunnel tests. Based on the sound signals measured by the far-field microphones, it is observed that the existence of multiple tones is dependent on Reynolds number ($Re$), and a dynamic acoustic feedback loop model is proposed to explain associated mechanisms in the different flow regimes. In regime 1 ($Re<1.6\times 10^5$), the existing tonal noise acts as an external force, triggering the upstream vortices that will interact with those shed from the trailing edge. The interaction process can modulate both the amplitude and frequency of tonal noise, leading to the observed multiple tones. The fluctuations in the sound wave, in turn, can affect the upstream boundary-layer flow, introducing intermittency into vortex dynamics. The observed modulation of transient acoustic frequency establishes the necessary condition for generating multiple tones in a forced oscillation system. In regime 2 ($1.6\times 10^5<Re<2.3\times 10^5$), the tonal noise is weakened as the feedback loop is suppressed due to the boundary-layer transition on the suction side. As a result, the vortex-shedding occurs at a single fixed frequency so that multiple tones disappear. In regime 3 ($Re>2.3\times 10^5$), the boundary-layer flow on the suction side is fully turbulent. However, flow instability on the pressure side acts as the driving force in the dynamic acoustic feedback model, triggering the modulation process and, therefore, the multiple tones.