非定常分离流动中拉格朗日湍流结构及其演化的实验研究

本课题以主动控制状态下的二维后台阶非定常分离流动为主要研究对象，采用高时间分辨率2D2C-PIV和tomo-PIV技术，对自然和人工激励状态下分离剪切流的流动特性进行基础实验研究。实验拟在天津大学低湍流度回流式水洞中进行。主动控制装置由电机配合活塞导管组成，在后台阶边缘引入流向小尺度的吹吸扰动。首先，采用2D2C-PIV测量后台阶流动拓扑结构，对比自然和激励状态下分离剪切层变化，确定不同雷诺数条件下典型激励频率，以及剪切层中旋涡集中的区域；进一步，采用tomo-PIV结合STB技术，测量剪切层内部三维三分量速度场；利用有限时间李雅普诺夫指数方法，提取拉格朗日相干结构，分析其变形、耗散和裹挟效应，研究其对流动分离控制作用的贡献。通过研究主动控制状态下流动剪切层中的拉格朗日相干结构，有助于深入理解分离流动中旋涡的时间-空间演化规律，对推动航空航天、船舶工程中的流动分离控制具有重要意义。

This project focuses an unsteady separated flow which is perturbed by an active flow control device downstream of a two-dimensional backward-facing step. The experimental investigation will be carried out in the low-turbulence water tunnel in Tianjin University. Time-resolved 2D2C-PIV and tomographic-PIV systems will be used to measure the separated shear layer in both natural and perturbed cases. The active flow control device will be consisted of a servo motor and a cylinder piston which enables to introduce periodic, streamwise, small-scale perturbations into the shear layer. First, the separated shear layer in both natural and perturbed cases will be measured by 2D2C-PIV in order to characterize the major flow topology behind the step and then find the effective perturbation frequencies for flow separation control under various Reynolds numbers. Furthermore, the three-dimensional three-component velocity fields within the shear layer will be obtained by tomo-PIV and STB technique in order to characterize the vortex generation, evolution and interaction. Finite-time Lyapunov exponent (FTLE) will be applied to characterize the Lagrangian coherent structures and analyze their deformation, dissipation, entrainment as well as contributions for flow separation control. This experimental investigation of Lagrangian coherent structures within the perturbed separated shear flow will improve the understanding of vortex spatio-temporal evolution in flow separation control, which may, moreover, lead to potential improvement of the flow separation control technology in the fields of aerospace and ship engineering.