后向台阶流动稳定性和转捩的动力学模态研究

Transition of separated shear layer is one of the most important aspects of flow instability and transition. Supported by the previous foundation, we have experimentally measured the flow-field of transitional backward-facing step flow. The Reynolds number dependency of the shear layer transition characteristics was investigated. A strong correlation between the rolling-up of large-scale spanwise vortex and the transition onset of the separated shear layer was found. Moreover, both the approach of dynamic mode decomposition (DMD) and the technique of introducing controlled 2D/3D disturbance were studied in detail. .Based on such progress, the intended project is devoted to a detailed understanding of the shear layer instability characteristics and transition mechanisms, with the primary aim of casting some light onto two fundamental problems: the receptivity mechanisms of Kelvin-Helmholtz instability and the complex dynamics of reattached large-scale vortical structures. This project is based on the framework of DMD-based stability analysis, and involves the techniques of the 2D/3D PIV measurement, the controlled disturbance exciting method, and the synthetic usage of vortex identification methods. The disturbance growth in the natural case will be studied at first, following by an investigation of the response of the shear layer to controlled 2D/3D disturbance, which provides a direct comparison. Finally, the vortex dynamics around the reattachment region will be statistically analyzed. We hope that recent theoretical/numerical work can be verified by such experimental attempts. Moreover, the understanding on the instability characteristics and transition mechanisms can be enhanced, and new ideas about the control of separation-reattachment flow can be stimulated.

分离剪切层转捩是流动稳定性与转捩的重要研究内容。在青年基金的资助下，前期完成了对后向台阶转捩流场的二维PIV测量，研究了雷诺数对剪切层转捩特性和大尺度旋涡卷起的影响规律，并针对动力学模态分解（DMD）方法、二维/三维受控扰动产生方法等进行了技术储备。.在此基础上，拟继续深化对分离剪切层流动稳定性和转捩机理的研究，以基于DMD的流场稳定性分析方法为基础，围绕“K-H不稳定性对初始扰动的选择机制”和“再附区大尺度旋涡的复杂动力学行为”两个科学问题，综合运用二维/体式PIV测量、受控扰动激励技术和旋涡辨识技术等多种研究手段，研究后向台阶流动的低维动力学模态特性，包括：无受控条件下的扰动增长特性、二维扰动对K-H不稳定性的影响、分离剪切层对三维扰动的响应、再附区旋涡的形态学和运动学特性。从而加深对剪切层流动稳定性和转捩的认识，检验近期的部分理论和数值研究，并尝试探索分离-再附控制新技术。

后向台阶流动是一种最简单的二维分离-再附流动，但却包含了分离-再附流动的所有主要特征，能够代表工业应用中的多种典型分离-再附流动，因此研究后向台阶流动具有重要的学术研究意义和工业应用价值。本项目对后向台阶分离剪切层流动稳定性和转捩机理进行实验研究,以基于DMD和POD的流场稳定性分析方法为基础，围绕“K-H不稳定性对初始扰动的选择机制”和“再附区大尺度旋涡的复杂动力学行为”两个科学问题，综合运用二维/体式PIV测量、受控扰动激励技术和旋涡辨识技术等多种研究手段，研究了后向台阶流动的低维动力学模态特性，包括：无受控条件下的扰动增长特性、二维扰动对K-H不稳定性的影响、分离剪切层对三维扰动的响应、再附区旋涡的形态学和运动学等研究内容。研究发现，在过渡区中，分离后的层流剪切层在稳定性机制作用下在再附之前即发生向湍流状态的转变，其流态与层流分离层流再附及湍流分离湍流再附的情况明显不同。该流动的非定常性体现在剪切层和再附点的拍动，流动的三维化表现为剪切层的三维演化，进一步从涡-涡相互作用的角度出发，研究了剪切层和再附点的拍动特性、拍动的产生机理。为弄清层流分离剪切层转捩现象中的剪切层不稳定性、旋涡相互作用、湍流自维持等问题提供新的依据。此外，还在DMD不确定度分析、PIV快速后处理算法、壁面摩阻测量方法、新的模态辨识和分解方法等方面进行了有益的研究，发展了新的流体力学实验和数据分析方法。本项目的相关成果发表SCI论文9篇、EI论文2篇（均标注基金资助），获批国家发明专利2项、软件著作权5项。