Effect of a Variable-Bend Slat on Tones Due to the Cove’s Self-Excited Oscillation

Abstract

To study the noise reduction effect of a variable-bend slat, both experimental and numerical investigations are carried out on a two-dimensional high-lift configuration with a stowed flap at varying angle of attack. With respect to the base configuration, the different geometrical settings of the variable-bend slat at varying bending angles of the trailing edge are described, and the gap between the slat and the main wing is completely sealed with a bending angle of 18°. At smaller bending angles, the slat noise is reduced, especially for tones generated from the self-excited oscillation within the leading-edge slat cove. With the increase of the bending angle, the tonal frequency in the same mode gradually decreases, which agrees well with the predicted formula proposed by Terrocal. Moreover, the noise reduction effect of the bending slats decreases with the increase of the angle of attack. To provide further understanding of the noise reduction effect on tones at varying bending angles, flow parameters related to the self-excited oscillation are extracted using numerical calculation results. It is found that change in self-excited oscillation is mainly a consequence of change of shear layer length Lv and average vortex convection velocity along the shear layer Uv. As the bending angle increases, the main mode of self-excited oscillation gradually switches to a higher mode, which agrees well with the relationship between the flow parameters and the main mode of traditional nondeformable slat.