  This paper presents a detailed aeroacoustic investigation of a
Controlled-Diffusion airfoil at near stall condition. The study aims at
answering two research questions: identify the flow mechanism responsible for
separation noise for an airfoil near stall conditions and whether the noise is
generated by a dipole for airfoil close to stall and can be quantified by
Amiet's diffraction theory. The study uses synchronized PIV, RMP and far-field
microphone measurements to perform experiments at two chord based Reynolds
numbers of about 150,000 and 250,000. The results show that when the airfoil is
placed at a higher angle of attack, such as $15^{\circ}$, strong amplification
of flow disturbance is seen, resulting in the rolling up of the shear layer in
the aft-region of the airfoil, forming large coherent structures. While these
rollers play a central role in the increase in noise due to flow separation,
the flapping of shear layer does not contribute to the separation noise. The
present study conclusively shows that separation noise is dipolar in nature,
and that the quadrupolar contribution for low-speed airfoils at near-stall
conditions can be neglected. However, the increase in flow disturbances
measured close to the trailing-edge of the airfoil implies that the assumption
of small amplitude disturbance is no longer valid, which is the central premise
of the thin linearized airfoil theory. Outside the frequency range at which
flow separation operates, Amiet's theory is able to predict the far-field noise
even at high angles of attack.
