Acoustic Resonances in Open and Closed 3D Cavities

Airframe noise is a significant component of the total noise radiated by an aircraft on approach. Although these tones are generated as the result of a complex interaction between the turbulent air flow and the cavity, it is expected that the acoustic resonances of the cavity approach the noise tones at low Mach numbers. Moreover, an accurate design of noise control systems of these cavity tones can be facilitated through an advanced knowledge of the acoustic resonances of the open cavity. The present work presents a computational method to predict the acoustic resonances of a 3D open cavity. A multi-dimensional Helmholtz equation closed with appropriate Perfectly Matched Layer absorbing boundary conditions is solved. Results of the method in the presented three dimensional cavity configuration are shown, the results of which have been compared with experimental measurements in a three-dimensional (cubic) open cavity. Moreover, the noise generated by an electric machine inside of a wind turbine is also studied as direct application of the numerical methodology dealing with very complex geometries.