Effect of dual vortex shedding on airfoil tonal noise generation

To improve understanding of dual vortex shedding over a trailing edge with regard to airfoil tonal noise generation, synchronized velocity and noise measurements are conducted on a NACA (National Advisory Committee for Aeronautics) 0012 airfoil at angle of attack near zero and a Reynolds number of 220 000. Instantaneous flow fields obtained by particle image velocimetry show the development of separated shear layer, vortex roll-up, and vortex breakup near the airfoil trailing edge. The time-averaged flow fields feature separation bubbles on both sides, and the root mean square values of streamwise velocity fluctuations show triple peak structures. The velocity spectra agree well with the noise spectra in terms of broadband humps and discrete tones. Through proper orthogonal decomposition (POD) analysis, the most energetic modes are identified, which represent global structures in the flow field. The trailing edge noise being an integral effect of the velocity fluctuation near the trailing edge, the POD analysis provides an alternative view for understanding the noise generation mechanism. The first and second modes are dominated by out-of-phase vortex shedding along the airfoil surface and in the near wake, which dominates the high amplitude noise emission process. The third and fourth modes represent in-phase vortex shedding, which dominates the low amplitude noise emission process. The noise source region is determined by the correlation between the velocity and sound pressure, which shows approximately the same periodic pattern as the first and second POD modes.