# Recent Advances in Acoustics of Transitional Airfoils with Feedback-Loop Interactions: A Review
## 3 Laminar Separation Bubble and Acoustic Feedback Loop Process  
As mentioned above, the controversy surrounding the mechanisms of airfoil tonal self-noise may be attributed to the highly sensitive nature of AFL process. Such sensitivity clearly reveals itself in the impact of experimental facilities on the measured tonal response that led some experimentalists like [PERSON] et al. [11] to believe that AFL's dual ladder-type spectral feature is not an airfoil's intrinsic but rather a facility-induced phenomenon. However, most subsequent experimental works disagreed with such a conclusion by providing evidence of the tonal structure recorded and further investigated using the most advanced measurement techniques. Furthermore, with the advancement of modern computational capabilities, previously prohibitively expensive high-fidelity studies have ment and radiation of instability waves in the transitional flow. The only possible remaining installation effect is the jet-flow deflection due to lateral momentum injection associated with the mean lift as the airfoil is set at a non-zero AoA. However, this was not believed to significantly modify the underlying physics of the observed phenomena. Far-field acoustic measurements performed in the mid-span plane using a single microphone on a rotating arm provide an overview of frequency-angle characterization and spectral directivity of the sound. To correlate acoustics with wall-pressure fluctuations, an elaborate technique of measurements using remote-microphone probes (RMPs) flush mounted along the airfoil surface was employed (see Figure 6). Classical hot-wire anemometry (HWA) and oil-visualization of the flow were used to identify LSB zones involved in the tonal noise mechanism and their dynamics with changing flow velocity and AOA. In addition, to explore the effect of internal and external BL excitations on LSB, AFL, and the resulting trailing-edge noise, both a surface tripping device and an upstream turbulence grid were employed. Several configurations with tripping devices added on one side or the other, as well as on both sides, have been tested, with the tripping device represented by a strip of medical adhesive plaster located around the expected area of maximum local velocity (Figure 6). The tripping forces transition of  
Figure 6: Sketch of the experimental setup used by [PERSON] et al. [15] illustrating pin-hole locations and probe labels of the wall-pressure Remote Microphone Probes (RMP) on the NACA-0012 (bottom left) and SD7003 (bottom right) mock-up. the laminar boundary layer to turbulence and deactivates the generation of tonal noise. The influence of a fine-scale homogeneous upstream turbulence was studied by inserting a turbulence grid before the wind tunnel nozzle contraction (top of Figure 6). Figure 7 illustrates results of the far-field sound measurements obtained in [15] and presented in the form of a color-level map, with the frequency and flow speed as coordinates in logarithmic scales. The results are plotted for NACA-0012 airfoil at AOA = 0\({}^{\circ}\), and for SD7003 airfoil at AOA = 2\({}^{\circ}\) (which geometrical AOA corresponds to the zero effective AOA). The dominant tones at each measured flow speed are marked by red symbols. Clearly, these results feature the ladder-type structure discussed by previous investigators [1; 10; 13; 25; 26] with the dual dependence attributed to shedding and AFL mechanisms to be further addressed below. Sound spectra for SD7003 airfoil extracted from the map in Figure 7b at some indicative flow speeds are shown in Figure 8 revealing zones of multiple-tone generation. A detailed spectrogram analysis in [15] showed non-stationary properties of the acoustic signals observed for both airfoils. In particular, regimes with a single dominant tone, two tones, switching tones, and a regime with intermittent tonal radiation were identified. Figure 7: Frequency-flow speed maps of noise radiation for clean airfoils: NACA-0012 at zero AOA (a), SD7003 airfoil at 2\({}^{\circ}\) geometrical AOA (b) [15].