# Recent Advances in Acoustics of Transitional Airfoils with Feedback-Loop Interactions: A Review
## 2 Tone-Producing Flow Regimes  
Parametric experimental studies conducted by [PERSON] [1], [PERSON] and [PERSON] [10], [PERSON] et al. [19], and later by [PERSON] et al. [20] analyzed acoustic data for NACA-0012 airfoil in order to map the regions of discrete tones in the angle-of-attack (AoA) vs. _Re_ plane (Figure 4). In some of these studies, T-S waves were observed at the lower _Re_ than the one corresponding to the first tone appearance. In other words, T-S instabilities alone were not a sufficient condition for the occurrence of the acoustic radiation and/or the feedback process. Instead, such a process was associated with the presence of a separation bubble at the airfoil pressure side, while the sound intensity was correlated with the length of the bubble.  
Most recently, [PERSON] et al. [15] reported on the results of a comprehensive experimental campaign where far-field sound-pressure measurements were recorded for NACA-0012 and SD7003 airfoils (compared in Figure 5) at various flow speeds and geometrical angles of attack. The tonal data obtained for NACA-0012 airfoil showed both similarities and differences with the previous studies, which suggests the high sensitivity of the AFL mechanism to experimental conditions. The cambered SD7003 airfoil was found to have a much narrower emission area than the NACA-0012. The measurements also confirmed the AFL-induced \"ladder-type\" structure featured when plotting the variation of the dominant tone with increasing flow speed, for both airfoils. [PERSON] et al. [15] also categorized several regimes of tone emission, namely a switching regime between two tones that are not observed simultaneously, a regime with a single tone or two simultaneous tones, and a regime of intermittency with multiple unstable tones.  
Concurrently and in close collaboration with experimental efforts of [PERSON] et al. [15], [PERSON] et al. [18] conducted a series of high-fidelity numerical parametric investigations of the effects of AoA and _Re_ that confirmed that the sustainment of AFL was dependent on the formation of LSB. Furthermore, using a combination of high-fidelity and linear stability analyses, they substantiated the switch from viscous T-S to inviscid K-H instabilities (facilitated by the presence of LSB near TE) as a necessary condition for AFL existence. These and some other recent studies are further detailed below in the discussion of the  
Figure 4: Integrated plot adapted from [PERSON] et al. [19] and [PERSON] et al. [20] indicating NACA-0012 tone-producing and no-tone-producing regions according to experimental and numerical studies of [PERSON] et al. [16] (squares), [PERSON] et al. [19] (diamonds), [PERSON] et al. [20] (circles), [PERSON] et al. [1] (triangles), and [PERSON] [10] (inverted triangles). Filled markers represent tone-producing regimes, while unfilled markers represent no-tone-producing regimes. Tonal envelope and maximum tonal amplitude (dashed) line are from experiments by [PERSON] et al. [11]. Red lines indicate regimes investigated by [PERSON] et al. [18].  
Figure 5: NACA-0012 and SD7003 airfoils employed in experimental study of [PERSON] et al. [15].  
investigation methods used and the physical mechanisms found responsible for the genesis of AFL and the observed transition between tone- and no-tone-producing regimes.