Characterization of noise sources in two- and three-element high-lift airfoil configurations

To improve the current understanding of flap noise generation mechanisms in high-lift devices used by small- to large-size commercial aircraft, a detailed study of both two- and three-element configurations of the canonical high-lift 30P30N airfoil has been conducted. Results from unsteady Reynolds-Averaged Navier-Stokes simulations are compared with aerodynamic and acoustic measurements in the hybrid anechoic wind tunnel at the University of Toronto to gain better physical insights into the flow phenomena associated with the noise sources in multi-element configurations. It is shown that the slat element has little impact on the flow in and around the flap and flap gap when the main wing is at the same orientation relative to the incoming flow. Tonal peaks are found both in the flap cove and on the flap, matching the tonal noise produced by the slat when in the three-element configuration. These peaks are present at the flap due to pressure waves that emanate from the slat and propagate downstream. Analysis of the frequency wavenumber spectra of the unsteady pressure along the main element pressure surface shows the existence of forward- and backward-traveling acoustic waves, providing evidence of a feedback loop between the two coves, which amplifies the narrowband tonal peaks in both coves. These tones disappear from the flap when the slat is removed in a two-element configuration, with the flap-cove overall broadband pressure fluctuation levels remaining unchanged.