Experimental Study of Far-Field Radiated Noise Characteristics for the Leading-Edge Slat of a 30P30N Airfoil

Abstract

To study the far-field aerodynamic noise of the leading-edge slat of a high-lift airfoil configuration, an experimental investigation of the BANC-II/30P30N airfoil model was carried out in the D5 areo-acoustic wind tunnel at Beihang University. A novel Kevlar-walled closed test section design was applied, as a way of upgrading the capabilities of a conventional aerodynamic wind tunnel to include far-field acoustic measurements for larger models at real Reynolds numbers. After the flap was retracted and the slat was deployed to isolate slat noise from other possible sources, the surface pressure distribution and the far-field sound pressure signals under different operating conditions were obtained by varying flow speed and angle of attack. The slat noise spectrum characteristics and the dependence of far-field noise on free stream velocity and airfoil incidence angle were acquired by the fast Fourier transform. The joint time-frequency analysis methods (short-time Fourier transform and continuous Morlet wavelet transform) were employed to process the far-field unsteady acoustic signal to identify the inherent physical mechanisms involved in the generation of multiple low-frequency tonal peaks emerging from the broadband component of the slat noise spectra. Results of the short-time Fourier transform (STFT) and wavelet analysis clearly showed that the dominant acoustic energy of these tonal noise was shared between the primary modes through rapid switching in time. Furthermore, the analysis results also indicated that an amplitude-modulation phenomenon exists in the dominant modes. The joint probability density function method was introduced to quantify the mode-switching phenomenon, and the results provided strong evidence that mode-switching occurs between the dominant modes of these low-frequency tones.