# Recent Advances in Acoustics of Transitional Airfoils with Feedback-Loop Interactions: A Review
## 4 Discussion of AFL Frequency Selection Mechanism and Airfoil Tonal Acoustic Structure
The argument was extended to suggest that the vortex shedding behavior while important to noise production, was not directly caused by the AFL process but rather was a result of a global instability that may occur in the absence of AFL, the point certainly in agreement with [PERSON] and [PERSON] [6]. It was also hypothesized that AFL could act as a frequency selection mechanism for the pre-existing vortex shedding behavior, which led to the previously stated argument of AFL \"regularizing\" the shedding behavior and producing the narrow tonal content when shedding and AFL-selected frequencies matched. In their parametric study of NACA-0012 airfoil at \(Re\) = 180,000, [PERSON] et al. [18] also considered frequency variation with AOA of the most amplified instability mode and the peak radiated acoustic tone. Figure 20 compares the LSA-predicted frequencies corresponding to the instability waves with maximum amplification (dashed blue lines) against the peak frequencies of calculated acoustic spectra (dashed black lines). In addition, the airfoil surface location where each frequency mode reaches its maximum is shown by the solid green line. Note that for AOA of 0\({}^{\circ}\), the predicted frequency of the most amplified instability mode agrees well with the observed far-field frequency. For other cases, however,a substantial difference was seen, and was also observed between the predicted peak amplifications for the suction and pressure surfaces. Overall, as the angle of incidence increases, the disparity between the peak LSA-predicted and acoustic tonal frequencies appears to grow. This is illustrated in Figure 21 also showing that the vortex shedding frequency agreed well with the observed acoustic peak tone. Such results are in agreement with those in [17], and it is reasonable to echo some of the scenarios proposed in [17] including the hypothesis of resonant interaction between AFL and shedding mechanisms leading to tonal response with greatly amplified amplitude, compared to the weak pure shedding tone in DNS study of [PERSON] and [PERSON] [6]. It also seems plausible that from the multitude of LSA-identified amplified instability modes, the AFL mechanism described by Equation (11) selects and elevates only those modes in the observed acoustic spectrum that appear the closest to the shedding frequency. We recall that [PERSON] and [PERSON] [6] identified the shedding frequency with that of the most amplified near-wake K-H instability in the free shear layer. The TE scattering of such near-wake instability was claimed as the source of the shedding tone in the absence of AFL. As hypothesized in [18], such wake instability mechanism may superimpose on the AFL mechanism in airfoil tonal noise production, both related to TE scattering of saturated shear-layer vortical structures. To summarize, when the airfoil flow is fully laminar with no AFL present, the remaining shedding tone appears very weak, as demonstrated by [PERSON] and [PERSON] [6]. We have also seen that when the AFL mechanism is suppressed (due to BL tripping or a low-intensity upstream turbulence), the acoustic spectrum still reveals a broadband hump centered around the shedding frequency [14,15]. With the AFL present, the shedding process still provides with the natural selection of the spectral region amplified due to the mutual resonant interference of both mechanisms leading to the observed multi-tonal response. The above interpretation of the AFL frequency selection mechanism finally allows addressing the dual-ladder-type tonal structure first observed by [PERSON] et al. [1] (Figure 1). In [18], the results obtained from LSA and numerical predictions over a range of flow velocities were analyzed and compared well (Figure 22) with experimental results of [1,45] to show the expected overlap of the shedding and AFL sound generation mechanisms corresponding to 1.5 and 0.8 in powers of velocity dependence, correspondingly.