Flow Organization and Trailing-Edge Tonal Noise: A Full-Span Experimental Study

The spanwise organization of the flow over a NACA 0012 airfoil and the associated trailing-edge tonal noise are studied experimentally at moderate incidence angles and at a Reynolds number of about 500,000. The sound field is investigated using a planar array of 208 microphones and frequency beamforming. At 12 deg of incidence, the acoustic spectrum exhibits one fundamental peak and its first harmonic, while at 8 deg of incidence, three main peaks are reported, whose sources are localized at three respective spanwise positions. The velocity field in the near wake is measured synchronously with the sound field using time-resolved particle image velocimetry (TR-PIV) over a plane extending almost 3 chords spanwise and 1.5 chords streamwise. This allows us to estimate the spanwise coherence length of the flow for the two cases of incidence. A good consistency is found between the results given by the acoustic measurements and those given by the velocity measurements. Namely, for the peak frequencies, the sound sources are localized by the array technique very close to the spanwise positions where the coherence length is maximum. Further analyses show that the multiple source locations and frequencies can be associated with weak spanwise inhomogeneities of the ambient flow and end effects from the wind-tunnel mixing layers.