自由剪切层中的三维不稳定性

本文是在文1]的基础上研究自由剪切层中由Kelvin-Helmholtz不稳定波发展而形成的展向大涡结构的三维不稳定性。以大涡结构为基本流动,将稳定性分析归结为二维特征值问题,用pseudo spectral(伪谱)方法数值求解。研究发现:在没有亚谐波存在的情况下,大涡结构的最不稳定的扰动波是流向波长和其相同。展向波数较高,有对流特性的三维扰动波。它在剪切层中的发展与展向涡量的分布有关,大涡结构的涡核不稳定性和辫子不稳定性是流向涡形成的主要力学机制。本文还给出了不同雷诺数下三维扰动波增长率与展向波数的关系,这些结论与实验及数值模拟结果基本一致。

Three-Dimensional Instability of Free Shear Layer

This paper is to study three-dimensional instability of two-dimensional full nonlinear quasi-equilibrium spanwise vortex rolls which result from evolution of Kelvin-Helmholtz wave in a free shear layer. This instability process seems to be a necessary step by which the flow undergoes transition to turbulence and chaos.We examine its characteristics by three-dimensional linear eigenvalue calculation using pseudospectral method.The behaviors of three-dimensional spanwise peturbation wave and vorticity dynamics of instability are discussed.It has shown small-scale instability exist for spanwise scales. These modes extract their energy from the base flow and grow on convective timescales. The growth rate, wavenumber of the most unstable wave approach finite limit as Reynolds number increases.So three-dimensional instability is invisid in character. It's also shown that three-dimensional instability highly depends on the distribution of spanwise vorticity. Stretching and tilting effects of spanwise vortex are main mechanisms of the formation of streamwise vortex.These conclusions are similar to that found in experiment,nume-rical simulations and other analysises.