Direct numerical simulation of noise from an airfoil in a uniform flow

Acoustic noise generated from a two-dimensional NACA 0012 airfoil in a uniform flow is investigated by a direct numerical simulation. The compressible Navier-Stokes equations are solved by a finite difference method over the whole domain from a near-vortical field to a far-acoustic filed. The Reynolds number based on the chord length is prescribed to be Re = 5000 and the Mach number of the uniform flow is fixed at M = 0.2. The angle of attack, a, is varied from 5° to 10°. The generated sound is enhanced drastically by increasing α from 5° to 7°, while the amplitude of sound at higher-α remains the same order of that at α = 8°. A pure tone is generated at α up to 6° and the 2nd-order high-harmonic appears around α = 7° and 8°. It is also found at α ≥ 8° that, althogh both amplitudes and directivities do not differ so much, an acoustic wave with high-harmonics up to fourth-order is generated at α = 10° while almost a pure tone is generated at α = 9°.