Recent Advances in Acoustics of Transitional Airfoils with Feedback-Loop Interactions: A Review

We discuss herein recent experimental and numerical studies examining resonant flow-acoustic feedback–loop interactions in transitional airfoils (i.e., possessing a notable area of laminar-to-turbulent boundary-layer transition) characteristic of low-to-medium Reynolds number flow regimes. Such interactions are commonly attributed to the viscous dynamics of the convected boundary-layer structures scattering into acoustic waves at the trailing edge which propagate upstream and re-excite the convected vortical structures. While it has been long suspected that the acoustic feedback mechanism is responsible for the highly pronounced, often multi-tonal response, the exact reason of how the boundary-layer instability structures could reach a sufficient degree of amplification to sustain the feedback-loop process and exhibit specific tonal signature remained unclear. This review thus pays particular attention to the critical role of the separation bubble in the feedback process and emphasizes the complementary roles of the experimental and numerical works in elucidating an intricate connection between the airfoil radiated tonal acoustic signature and the properties of the separation zones as determined by airfoil geometry and flow regimes.