  Direct numerical simulations are carried out to investigate the flow features
responsible for secondary tones arising in trailing-edge noise at moderate
Reynolds numbers. Simulations are performed for a NACA 0012 airfoil at
freestream Mach numbers 0.1, 0.2 and 0.3 for angle of incidence 0 deg. and for
Mach number 0.3 at 3 deg. angle of incidence. The Reynolds number based on the
airfoil chord is fixed at $Re_c=10^5$. Flow configurations are investigated
where noise generation arises from the scattering of boundary layer
instabilities at the trailing edge. Results show that noise emission has a main
tone with equidistant secondary tones, as discussed in literature. An
interesting feature of the present flows at zero incidence is shown; despite
the geometric symmetry, the flows become non-symmetric with a separation bubble
only on one side of the airfoil. A separation bubble is also observed for the
non-zero incidence flow. For both angles of incidence analyzed, it is shown
that low-frequency motion of the separation bubbles induce a frequency
modulation of the flow instabilities developed along the airfoil boundary
layer. When the airfoil is at 0 deg. angle of attack an intense amplitude
modulation is also observed in the flow quantities, resulting in a complex
vortex interaction mechanism at the trailing edge. Both amplitude and frequency
modulations directly affect the velocity and pressure fluctuations that are
scattered at the trailing edge, what leads to secondary tones in the acoustic
radiation.
