  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.
