关键词: 浸没边界法, 直接数值模拟, 椭圆, 尾迹涡, 噪声

Abstract:

Accurate numerical simulation of aerodynamic noise is of great significance for the study on flow-induced noise and vortex-acoustic resonance of aeroengine blades. This paper proposes an improved ghost-cell immersed boundary method based on local boundary refinement， which is combined with the high-precision finite difference method to successfully solve the fluid-solid coupling aeroacoustic problem at a low Reynolds number. The improvement in the current immersed boundary method includes the construction of a large number of intermediate nodes between the ghost points， and the establishment of relationship between the mirror image points in the fluid domain and the intermediate points in the solid domain according to the boundary conditions. Compared with the previous method， the dispersion of the solid wall boundary is significantly increased， thereby playing a role in local mesh refinement and leading to more accurate boundary description. Verification of the aerodynamic noise problem in the flow around a cylinder at a low Reynolds number shows that the characteristics of lift and drag， the evolution of wake shedding vortex and the near-field and far-field noise propagation characteristics obtained by the proposed method are in good agreement with the results in the literature. The numerical accuracy of the aerodynamic sound field is improved. The effects of different aspect ratios on the generation of aerodynamic sound sources and the propagation of noise in the elliptical flow are also studied. It is found that the wake shedding vortices are alternately arranged and evolve downstream. The noise field formed is symmetrically distributed about the long axis. Its flow pattern and noise characteristics are similar to those in the flow around a cylinder， but with significantly smaller lift drag fluctuation and noise intensity. Furthermore， through sound pressure decomposition， it is revealed that the contribution proportion of the dipole sound source to the overall sound intensity decreases with the decrease of the aspect ratio， while that of the monopole and quadrupole sound sources in the wake increases， directly causing the noise propagation direction to deflect upstream.

Key words: immersed boundary method, direct numerical simulation, elliptical cylinder, wake vortex, noise