Aftwall Inclination in Surface-Embedded Airfoil Cavities: A Review of Aeroacoustic Mechanisms and an Unexplored Design Frontier for Passive Noise Control

Airframe trailing-edge noise is a dominant source during aircraft approach, particularly at low Reynolds numbers (Re ~ 104–5×105) relevant to UAVs and wind turbines, where discrete tonal noise arises from a self-sustaining aeroacoustic feedback loop involving Tollmien–Schlichting instabilities, trailing-edge scattering, and upstream acoustic propagation [1,2]. Surface-embedded cavities (recessed into the airfoil pressure surface) offer a passive, zero-energy means of disrupting this loop, making them a practically attractive noise reduction strategy.

Prior numerical and experimental studies [3,4] identify cavity depth and chordwise position as the primary governing parameters: shallow, mid-to-aft chord cavities sustain a single coherent recirculation zone that energises the boundary layer and yield the most favourable noise-to-drag tradeoff, while deeper or leading-edge- proximate cavities amplify tonal noise through multiple stacked recirculation zones. Separately, the standalone open-cavity literature demonstrates that aftwall geometry modification (slanted or chamfered walls) reduces overall sound pressure levels by 5–10+ dB at subsonic conditions by distributing vortex impingement energy [5,6].

Despite this, aftwall inclination has never been explored as a design variable in the surface-embedded airfoil context; no study to date has investigated angled or inclined aftwall geometries, whether at 45° or any other inclination. This review identifies aftwall inclination as a clearly unexplored design variable in this context and develops the physical rationale for a 45° configuration: the inclined surface deflects rather than reflects the arriving vortex, distributing impingement energy, attenuating the peak acoustic pulse, and promoting a stable single- recirculation flow structure, consistent with bio-inspired noise reduction principles [7]. Future work is directed towards scale-resolving CFD across inclination angles (30°–75°), frequency-resolved acoustic characterisation of individual Rossiter mode suppression, and extension to continuously curved aftwall profiles, collectively advancing passive aeroacoustics for UAVs, urban air mobility, and approach noise reduction.