Experimental Evidence of Tonal Noise and Its Control in Tip-Leakage Flows

The present study is first to provide experimental evidence of tonal noise and acoustic feedback by a stationary airfoil with a tip-gap placed between two side plates. Tones are linked to an aero-acoustic coupling using synchronized measurements between Particle Image Velocimetry and far-field microphone probes. To aid in the correlation analysis, the wall boundary condition on the pressure side of the airfoil was modified by introducing surface roughness. For tip leakage flows without roughness elements, the far-field acoustic spectra show peaks in the acoustic autospectra. These peaks are present regardless of the flow speed studied. The findings reveal that tones associated with tip-gap extend till 16.5 kHz in a low-noise facility. Roughness alters cross-flow velocity along the airfoil chord, influencing acoustics at matched Reynolds and Mach numbers. Roughness induces cancellation of spectral peaks and changes in the velocity and acoustic pressure correlation pattern between airfoil pressure and suction side. Modal shapes confirm the presence of coherent structures in the tip-gap region, with smooth surfaces exhibiting confined flow instabilities and non-dipolar noise sources around and beyond 10 kHz. At these high frequencies, the sound radiation mechanism can be uniquely attributed to neither a jet-like noise source nor these acoustic peaks can be uniquely attributed to coherent structures in the tip-gap region. On the contrary, coherence between airfoil sides is required for noise peaks, which stresses the importance of diffraction on the airfoil pressure side and acoustic feedback.