To understand noise generation mechanisms by an airfoil, I have used Tomo-PIV measurements to measure three dimensional flow field. Figure on the left, from my previous works, shows three dimensional vortical structure can be accurately measured by Tomo-PIV. The acoustic measurements can be performed using either a single far-field microphone or an acoustic array (see figure above) to localize noise sources.
How airfoil generates noise ?
Airfoil trailing edge noise is a canonical noise problem. For a semi-finite plate fully immersed in a flow, the flow at the edge of the plate needs to respect unsteady Kutta condition. To achieve this pressure is induced at the trailing-edge of the airfoil to cancel the unsteady pressure jump of the incident boundary layer. This induced pressure propagates to far-field as noise.
Unsteady pressure implications
Unsteady pressure cancellation means even a non-lifiting airfoil (NACA 00 series at zero incidence) can generate noise. As such noise is not a consequence of steady lift but unsteady lift.
The modeling the wall-pressure fluctuations is particularly challenging not only because of detailed mathematical description required but also because of the abstruse nature and scarcity of data on the two-point velocity statistics that dictate them. The present state of wall-pressure modeling can be encapsulated in the words of Chase (1980) who said some 40 years ago “Attainment of a comprehensive, validated, satisfactory