Sound Radiation and Caustic Formation from a Point Source in a Wall Shear Layer
Sound Radiation and Caustic Formation from a Point Source in a Wall Shear Layer
复制标题
壁剪切层中点源的声辐射和腐蚀形成
作者：
I. Abrahams;G. Kriegsmann;E. Reiss
研究了横切层中高频点源的声波传播问题。使用先前开发的方法，得到解的渐近展开式为k = omegaD/c0—>∞。其中，ω为点源的圆频率，D为剪切层厚度，c0为环境声速。对静止源和运动源进行了分析。在光源下游和墙附近，通过光源光线的折射和随后的墙壁反射，形成了一个无限序列的腐蚀性表面。得到了焦散线前后的声场。声场的涡度在焦散表面上很大，但在远离焦散表面“中心”的地方衰减很快。也就是说，焦散上的声场能量是局域的。此外，腐蚀性表面被运动源“扫回”。运动的腐蚀性表面与实验确定的相干流体结构惊人的相似，它们起源于湍流边界层的层流亚层，是强涡度的局部区域，这表明先前提出的这些局部相干流体结构形成的机制可能是有效的。这些结构的破裂和流体从亚层猛烈喷出可能是边界层湍流产生的机制之一。
The propagation of acoustic waves from a high-frequency point source in a shear layer flowing over an infinite rigid plate is considered. Asymptotic expansions of the solution are obtained as k = omegaD/c0 --> infinity, using a previously developed method. Here, omega is the circular frequency of the point source, D is the thickness of the shear layer, and c0 is the ambient sound speed. Stationary and moving sources are analyzed. An infinite sequence of caustic surfaces are created downstream of the source and adjacent to the wall, by the refraction of rays from the source and their subsequent reflection from the wall. The acoustic fields on and off the caustics are obtained. The vorticity of the acoustic field is large on a caustic, but it decays rapidly away from the ''center'' of the caustic surface. That is, the energy of the acoustic field on a caustic is localized. In addition, the caustic surfaces are ''swept back'' by the moving source. The striking similarity between the moving caustic surfaces and the experimentally determined coherent fluid structures, which originate in the laminar sublayer of a turbulent boundary layer and are localized regions of intense vorticity, suggests that the previously proposed mechanism for the formation of these localized coherent fluid structures might be valid. The bursting of these structures and the violent ejection of the fluid away from the sublayer could be one of the mechanisms producing turbulence in the boundary layer.