Study on free shear layrs control by mians of direct numerical simulations.

本文用直接数值模拟的方法研究了尾迹、混合层和喷流等自由剪切层的转折机制和结构演化。自由剪切层被由线性稳定性理论得到的时间/空间发展的不稳定模式所扰动。采用PADE有限差分近似和映射拟谱傅立叶方法/谱Galerkin方法对不可压含时N-S方程进行空间离散。采用紧凑的三阶Runge-Kutta格式对方程进行时间推进。当基模被放大时，平面尾迹中稳定涡街的发展出现在下游。当进口气流受到一个基模和两个次谐波的强迫时，也会出现涡街，但涡街的形状是扭曲的。基频附近的相位抖动在下游产生任意形状和间距的涡旋中起着关键作用。弱背景噪声对尾迹演化影响不大，二维双滚是尾迹的主要特征。研究还发现，小波长斜模的增长导致展向滚子的破裂，并在下游出现了反向旋转的流向涡结构。此外，还研究了可压缩性对自由剪切层中失稳模式增长的影响。数值结果表明，随着对流马赫数的增加，斜模在混合层中起着重要的作用。在平面尾迹和喷流中，马赫数越高，二维不稳定模的增大率越大。最后，利用三维临界点方法对数值模拟的可压缩混合层内的流型几何进行了研究。与高耗散率相对应的运动被发现由结点/鞍点/鞍点和U形结点/鞍点/鞍点类型的三维应变速率拓扑来表征。

In the present report, the transition mechanism and the structure evolutions in free shear layrs such as wakes, mixing layrs and jets are studied by means of direct numerical simulations. The free shear layrs are perturbed by temporally/spatially developing instability modes obtained from linear stability theory. The incompressible time-dependent Navier-Stoles equations were solved using pade finite difference approximations and a mapped pseudospectral Fourier method/spectral Galerkin method for spatial discretization. The equations were advanced in time using a compact third-order Runge-Kutta scheme. When a fundamental mode is amplified, the development of a stable vortex street in a plane wake appears downstream. When the inlet flow is forced by a fundamental mode and two subharmonics, a vortex street also appearsm, but the shape of the vorticies is distorted. The phase jitter around the fundamental frequency plays a critical role in generateing vorticies of random shape and spacing downstream. The weak back ground noise has little effect on the evolution, the two-dimensional double rollers are the dominant features of the wake. It is also found that the growth of small wavelength oblique modes leads to the breakdown of the spanwise rollers and counter-rotating streamwise vortical structures appear downstream. Furthermore, the effect of compressibility on the growth of instability modes in free shear layrs was investigated. Numerical results reveal that oblique modes are significant in the mixing layrs at the convective Mach number increases. In plane wakes and jets, two-dimensional instability modes show the most amplification rates for higher Mach numbers. Finally, a study of the geometry of flow patterns in numerically simulated compressible mixing layr was carried out using 3-D critical point methodology. Motions corresponding to high rates of dissipation are found to be characterized by a 3-D rate-of-strain topology of types nustable node/saddle/saddle and u