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

本文采用直接数值模拟的方法，研究了尾迹、混合层和射流等自由剪切层的过渡机理和结构演变。自由剪切层受到由线性稳定理论得到的时间/空间发展的失稳模式的扰动。采用页有限差分近似和映射伪谱傅里叶法/谱伽辽金法进行空间离散，求解了不可压缩时变Navier-Stoles方程。利用紧凑的三阶龙格-库塔格式在时间上推进了方程。当基模被放大时，稳定涡街的发展出现在下游的平面尾流中。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，但涡街的形状会发生畸变。基频附近的相位抖动对下游产生随机形状和间距的涡旋起着至关重要的作用。弱背景噪声对尾迹的演化影响不大，二维双滚子是尾迹的主要特征。研究还发现，小波长斜模的生长导致展向辊的破坏，下游出现反旋转的顺流涡旋结构。进一步研究了可压缩性对自由剪切层中失稳模态发展的影响。数值结果表明，在对流马赫数增加时，斜模在混合层中显著存在。在飞机尾迹和射流中，二维不稳定模态在较高马赫数下表现出最大的放大率。最后，利用三维临界点方法对数值模拟的可压缩混合层的流态几何进行了研究。发现与高耗散率相对应的运动具有非稳态节点/鞍形/鞍形和u型的三维应变率拓扑的特征

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