Noise generation mechanisms of a micro-tube porous trailing edge

The noise generation and flow characteristics of a forced-transitioned NACA 0012 airfoil with a micro-tube porous trailing edge and its solid counterpart have been numerically investigated. The near-field flow dynamics and far-field noise predictions are obtained using compressible large-eddy simulations and the Ffowcs-William and Hawkings acoustic analogy, respectively. The computed far-field noise levels agree with experimental data, showing that the micro tube structure reduces low-frequency trailing-edge noise without noticeably altering the noise directivity but induces additional high-frequency noise along the direction perpendicular to the airfoil chord. An in-depth analysis of the trailing-edge flow and surface pressure fields reveals that the noise reduction is largely linked to the 'cross-jet-like' flow permeation through the micro-tube structures. The interaction between the flow permeation and the boundary layer turbulence leads to a reduction in the strength of the Reynolds-stress source term in the pressure-side flow field and a reduction in the magnitude of the source terms in Amiet's trailing-edge noise model. Spectral proper orthogonal decomposition is performed to identify flow structures that are spatially and temporally coherent within the micro-tube geometries and to link the flow features to the attenuation in noise scattering efficiency. The effect of the micro-tube structure on the wavenumber-frequency spectra of the trailing-edge surface pressure field is quantitatively analysed. Additional high-energy regions exist in the wavenumber- frequency spectra of the porous trailing edge, resulting from the turbulent eddies that permeate through the micro-tubes. Further, the high-frequency noise increase mechanism is identified as the acoustic dipole noise arising from interactions of edge-induced flow separation with the suction-side surface.

Automated summary

This study numerically investigates the noise generation and flow characteristics of a forced-transitioned NACA 0012 airfoil with a micro-tube porous trailing edge. The results show that the micro tube structure reduces low-frequency trailing-edge noise and induces additional high-frequency noise through flow permeation. The interaction between flow permeation and boundary layer turbulence is the main cause for the noise reduction.