后掠翼三维边界层感受性机制的直接数值模拟

Prediction and control of the boundary layer transition is one the most important issues of the fluid mechanics. The receptivity is the key stage of three-dimensional boundary layer transitions which are often seen in engineering practices, and it determines the positions, paths and the most unsteady modal types of the transitions. In this research,direct numerical simulation(DNS) is used to study receptivity triggered by the free stream turbulence, wall roughness, surface curvature of aerofoil leading edge and interactions between them in three-dimensional boundary layer on a swept wing, and to research upon natures of initial perturbation generated from different free stream circumstance and initial conditions in three-dimensional boundary layer. The connection between the receptivity of the three-dimensional boundary layer and the free stream turbulence, wall roughness, surface curvature of aerofoil leading edge and interactions are established，that the different external environment disturbances are induced the formation of the disturbance of the most unstable mode is made clear, and what type of disturbance modes occupy the dominant position in the transition process is determined. Consequently, this research can reveal mechanics of receptive processes in three-dimensional boundary layer and provide basis for improvement of turbulent and hydrodynamic stability theory. Therefore, there are great values in theory and wide prospects in engineering to study receptivity mechanics in three-dimensional boundary layer on a swept wing.

边界层流动转捩的预测与控制一直是流体力学研究中的一个很重要的理论课题。三维边界层流动在实际工程中十分常见，而感受性阶段是三维边界层流动转捩的最关键性过程，决定着流动转捩位置、路径以及最不稳定扰动模态的类型等。本课题拟采用直接数值模拟方法，研究自由来流湍流度、壁面粗糙度、机翼前缘表面曲率以及它们之间的相互作用激励后掠翼三维边界层感受性过程的内在机理，探讨不同来流环境、初始条件激发三维边界层内产生初始扰动的物理本质，建立三维边界层感受性机制与自由来流湍流度、壁面粗糙度、机翼前缘表面曲率以及它们之间相互作用的内在联系，弄清楚不同外部环境扰动诱导形成何种最不稳定的扰动模态，确定什么类型的扰动模态在转捩过程中占据主导地位，从而揭示三维边界层感受性过程的基本机制，为完善湍流与流动稳定性理论提供依据。因此，研究后掠翼三维边界层感受性机制不仅具有重要的理论价值，而且在工程实际中还有着广泛地应用前景。

层流向湍流转捩的预测与控制问题一直是人们研究的前沿热点课题之一，其感受性阶段是转捩过程中的初始阶段，决定着湍流形成的物理过程。本课题采用紧致有限差分和高精度的有限差分与傅里叶展开相结合的数值方法，详细研究了后掠翼平板三维边界层感受性过程的理论机制；首先，在自由来流湍流、三维局部粗糙以及它们两者之间相互作用下，三维边界层内分别发现了非定常横流扰动模态、定常横流扰动模态以及一种新的非定常横流扰动模态，且新发现的非定常横流扰动模态的展向波数满足正、负自由来流湍流脉动的展向波数与在三维壁面局部粗糙作用下三维边界层内被激发出某种定常横流扰动涡模态的负、正展向波数之和的组合关系；当自由来流湍流度较低时，三维边界层内被感受出的定常横流扰动模态占主导地位；反之，被诱导出的非定常横流扰动模态绝对占据主导作用；还揭示出自由来流湍流度、方向和各向异性程度以及三维局部粗糙的几何形状、分布位置和类型与三维边界层感受性问题之间的内在规律。其次，研究了前缘椭圆曲面的改变分别对二维和三维边界层感受性问题的影响，分别重点揭示出不同椭圆形前缘曲率变化对三维边界层内被诱导出非定常和定常横流扰动涡模态的作用以及建立不同椭圆形前缘曲率与三维边界层内被激发出非定常和定常横流扰动涡模态的幅值、相位、波长以及增长率之间的内在机制；确定具有不同椭圆形前缘后掠翼平板边界层内被诱导出定常横流扰动涡模态的感受性系数与三维局部粗糙的几何参数、位置和个数之间的联系以及前缘曲率对三维边界层内被激发出非定常横流扰动涡模态的扰动波波包传播速度、传播方向、分布规律、感受性系数以及分别提取获得的一组扰动波幅值、色散关系和增长率等关键因数的影响；最后，还分别研究了有压力梯度对二维和三维边界层感受性问题的作用，取得了一些有意义的成果。通过该項目的研究将有益于拓展和完善流动稳定性理论，为三维边界层内层流向湍流转捩的预测与控制提供理论依据。