Numerical prediction of the unsteady flow and radiated noise from a 3D lifting airfoil

Abstract

The numerical prediction of the aerodynamic noise radiated by an isolated airfoil is performed using a Computational AeroAcoustics (CAA) method. This hybrid method combines (i) a compressible three-dimensional Large Eddy Simulation (LES) of the nearfield unsteady flow and (ii) a Kirchhoff integration providing the noise radiated in the far field. This process is applied to a symmetrical NACA0012 airfoil with a constant section and a blunted trailing edge (TE), at a Mach number of 0.205 and an angle-of-attack of 5°. The Reynolds number based on the airfoil chord is 2.86 millions. The computational domain has a spanwise extent representing 3.3 % of the chord. The unsteady flow computed via LES exhibits a superimposed pressure fluctuation field which presents the qualitative and quantitative features of the TE noise generated by the acoustic scattering of (i) the turbulent boundary layers convected on both airfoil sides (broad-band noise) and (ii) the alternated vortex shedding generated by the TE bluntness (narrow band component). Due to the strong stretching of the LES computational grid, which acts as an acoustic low-pass frequency filter, this acoustic field cannot radiate farther than a half-chord from the body. Consequently, the LES must be relayed by an acoustic propagation method to correctly simulate the farfield noise, which is done using a Kirchhoff integral code. This method necessitates a careful parametric study of the position of the control integration surface enclosing the airfoil and its wake. Farfield noise predictions are compared with (published experimental data obtained in an anechoic facility with the same airfoil geometry (but much larger span). The overestimation of experimental data by predicted levels is discussed.