NUMERICAL SIMULATION OF NOISE GENERATED BY FLOW PAST AN AIRFOIL USING ACOUSTIC ANALOGY
基于比拟理论的翼型扰流声场数值模拟

The numerical dispersion and anisotripic effects of several finite difference schemes are assessed by Fourier analysis from the full-discretized, two-dimensional model equations. It is found that there are three kinds of numerical waves propagating in the numerically dispersive medium,namely, the flow velocity waves and two acoustic waves superimposed on the flow velocity. The investigation shows that the upwind compact difference scheme suffers less significantly from the anisotropic effect than the symmetrical compact difference scheme with the similar arcurary and thus is more suitable for the simulation of aeroacoustic noise.Acoustic analogy computations of vortex shedding noise are carried out in the context of a two-dimensional flow past a NACA0012 airfoil. The near-field flow governed by the compressible Navier-Stokes equations is solved by 3-order upwind compact difference scheme combined with 3-order Runge-Kutta method. The unsteady flow generates aeroacoustic dipole and quadrupole sources. The radiated far-field noise is calculated based one Curle's extension to the Lighthill analogy. A method for reducing the spurious noise caused by eddies crossing the exit boundary is checked. Numerical results confirm that the quadrupole noise cannot be neglected compared with the noise due to lift and drag dipoles when the freestream Marh number is high. The far-field sound pressure has the directivity and spectra show that low frequency peaks shift toward higher frequency regions as the observation angle diverges from the flow direction.