Trailing-edge scattering of spanwise-coherent structures

Abstract

© 2016, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.We study mechanisms of noise generation by an airfoil at zero angle of attack using flow-acoustic correlations. We use a large-eddy simulation (LES) of the compressible flow around a NACA0012 airfoil for M∞ = 0.115 and Rec = 408000. With this simulation we analyse flow fluctuations around the airfoil, such as pressure and velocity, and relate them to the far-field sound using standard correlation techniques. Similar to the results of Cava- lieri et al.,3 who have obtained more significant flow-acoustic correlation for turbulent jets when the axisymmetric mode was isolated, we have noticed much higher correlations in the present problem when the two-dimensional mode, i.e. spanwise-averaged uctuations, was isolated from the turbulent flow and subsequently correlated to the acoustic pressure. This result is justified theoretically by an analysis of the tailored Green's function for a half plane, where we find that a necessary condition for trailing edge scattering is ǀkzǀ < k, where kz is the spanwise wavenumber of turbulent disturbances and k is the acoustic wave number. Two-dimensional perturbations, associated to kz = 0, are always radiating. Isolation of spanwise-coherent disturbances is thus a means of filtering non-radiating structures. Another feature that tends to increase correlation coefficients is the use of the difference between uctuations in the upper and lower surfaces of the airfoil, which again is in line with theory: when disturbances are in phase opposition between the two sides of the airfoil acoustic scattering is maximal.