A strong, normal shock wave, terminating a local supersonic area located at the tip of a helicopter blade, not only limits the aerodynamic performance, but also constitutes an origin of the High-Speed Impulsive (HSI) noise. The application of a passive control device (a shallow cavity covered by a perforated plate) just beneath the interaction region weakens the compression level, thus reducing the main source of the HSI noise. The numerical investigation based on the URANS approach and Bohning/Doerffer (BD) transpiration law (SPARC code) confirms a large potential of the new method. Two exemplary implementations, adapted to model helicopter rotors tested at NASA Ames facility in transonic conditions: Caradonna-Tung (lifting, transonic hover) and Caradonna-Laub-Tung (non-lifting, high-speed forward flight), demonstrate the possible gains in terms of the reduction of acoustic pressure fluctuations in the near-field of the blade tip. The CFD results are validated against the experimental data obtained for the reference configurations (no control), while the analysis of the passive control arrangement is based on a purely numerical research. The normal shock wave is effectively eliminated by the wall ventilation exerting a positive impact on the generated level of the HSI noise.

Rod vortex generators are known to mitigate flow separation, but their effect on noise emission is not fully understood. This study examines their impact on both flow structures and acoustic characteristics for an airfoil in a complex flow environment. An experimental campaign is conducted for the DU96-W-180 airfoil equipped with rod-type vortex generators. The flow characteristics developed over the airfoil (natural transition) are observed through oil flow visualization and compared for the airfoil with/without flow control. A laminar separation bubble close to the leading edge, followed by transition and reattachment of the flow to the surface, is observed at all inflow angles. At higher angles, turbulent separation develops, starting at the trailing edge. The streamwise vortices generated by the rods energize the boundary layer, thus delaying separation. Among the analyzed inflow angles, a 45% reduction of turbulent separation area is observed at AoA = 13°. Acoustic measurements for the airfoil with/without rods are conducted using a microphone array and processed using beamforming techniques for selected inflow angles (AoA = 0°, 4°, 8°, and 11°). Results show that the overall sound pressure levels increase with increasing inflow angles. Tonal noise components are observed at low frequencies for all angles. The rods decrease the total noise emitted by the airfoil at all presented inflow angles with a maximum reduction of ∼ 8 dB at AoA = 11°. Notably, the rods also reduce noise at low inflow angles, even without turbulent flow separation. Thus, in addition to enhancing aerodynamic performance by reducing turbulent separation, the rods do not impose any acoustic penalty and, in this particular configuration, further decrease noise emissions.