Identification of structures driving trailing-edge noise. Part I -- Experimental investigation

TL;DR

This paper experimentally investigates the flow structures responsible for trailing-edge noise on airfoils, revealing that large spanwise wavelength coherent structures significantly contribute to broadband noise emission.

Contribution

It identifies the dominant spanwise-coherent flow structures responsible for TE noise and demonstrates their correlation with acoustic radiation, advancing understanding of aeroacoustic mechanisms.

Findings

Large spanwise wavelength structures (>60% chord) drive TE noise.

Only spanwise waves with wavenumber below the acoustic wavenumber radiate sound.

Strong correlation between zero-wavenumber flow structures and acoustic emission.

Abstract

Trailing-edge (TE) noise is the main contributor to the acoustic signature of flows over airfoils. It originates from the interaction of turbulent structures in the airfoil boundary layer with the TE. This study experimentally identifies the flow structures responsible for TE noise by decomposing the data into spanwise modes and examining the impact of spanwise coherent structures on sound emission. We analyse a NACA0012 airfoil at moderate Reynolds numbers, ensuring broadband TE noise, and use synchronous measurements of surface and far-field acoustic pressure fluctuations with custom spanwise microphone arrays. Our results demonstrate the key role of coherent structures with large spanwise wavelengths in generating broadband TE noise. Spanwise modal decomposition of the acoustic field shows that only waves with spanwise wavenumbers below the acoustic wavenumber contribute to the radiated acoustic spectrum, consistent with theoretical scattering conditions. Moreover, a strong correlation is found between spanwise-coherent (zero wavenumber) flow structures and radiated acoustics. At frequencies corresponding to peak TE noise emission, the turbulent structures responsible for radiation exhibit strikingly large spanwise wavelengths, exceeding $60\%$ of the airfoil chord length. These findings have implications for numerical and experimental TE noise analysis and flow control. The correlation between spectrally decomposed turbulent fluctuations and TE noise paves the way for future aeroacoustic modelling through linearized mean field analysis. A companion paper further explores the nature of the spanwise-coherent structures using high-resolution numerical simulations of the same setup.