What happens to aerofoil noise when the aerofoil undergoes flow separation and stall?

Aerofoil flow separation and stall is a significant source of self-noise for many engineering applications, particularly when operating at high angles of attack in unsteady inflow conditions. After many years of studies on aerofoil noise, several questions remain regarding both the quantification and physical mechanisms of separation noise. One of the most fundamental questions yet to be determined is the role of quadrupole sources in contrast with dipoles. The majority of previous aerofoil noise investigations have considered low angles of attack, where the dipole noise generated by a turbulent boundary layer scattered at the trailing edge (TE) is usually considered to dominate the noise production. However, in the stall regime, the highest intensity flow structures typically appear away from the wall, indicating that the TE will generate sound less effectively. The conventional wisdom that dipole sound should dominate over quadrupole sound may therefore not apply for high angle of attack cases. This talk provides insight into the role of quadrupole sources due to separated shear layers at low Mach numbers, based on the results of direct numerical simulations. Image credit: [Yzy Pop (Unsplash)](https://unsplash.com/photos/a-view-of-a-mountain-range-from-an-airplane-window-SmrhgI5upFg).