壁湍流发卡涡包结构奇异点动力系统模型及其减阻控制的TOMO-TRPIV实验研究

It is widely agreed that the topological mode of coherent structure in wall-bounded turbulence is manifested as a cluster of hairpin vortex packet with envelope lines which are downstream inclined to the flow. Recently, it has been found that the topological mode of their streamlines of the coherent structure presents as a singular dynamics system with a family of (generally three) typical unstable focus points and saddle points that alternately appear in our time-resolved PIV(TRPIV) experimentally investigations on the wall-bounded turbulence. According to the nonlinear dynamics theory, the singular dynamics system with the unstable focus points and saddle points can easily lead to the chaos, which is in agreement with the coherent structure bursting phenomenon. The bursting is able to transiently produce a large number of turbulent fluctuations, which contains several fluctuating ejections. Accordingly, the proposed project aims to establish a singular dynamics system with a group of parameters for hairpin vortex packet structures based on the previous interesting experimental results, which is constituted of several unstable focus points and saddle points. Variations of the parameters may have certain effects on the singular dynamics system. The relationship between the singular dynamics system model and the solutions of Navier-Stokes equations will be clarified; meanwhile, whether the previous traditional active/passive controlling techniques and their effects on the coherent structure can be reflected by the variations of these parameters will be studied by the tomographic TRPIV experiments in the water channel; lastly, a realizable technique in physical to control the hairpin vortex packet by effectively controlling the parameters of the singular dynamics system will be proposed and its feasibility and reliability will be further validated by the tomographic TRPIV experiments in the water channel.

已经公认：壁湍流相干结构表现为一簇包络线向下游倾斜的发卡涡包结构。最近我们在壁湍流TRPIV实验中发现，发卡涡包的流线图表现为一簇典型的不稳定焦点和鞍点交替组成的奇异点动力系统。根据非线性动力学理论，这种焦点和鞍点组成的奇异点动力系统及同（异）宿轨道预示着混沌的产生，这在物理上符合壁湍流相干结构的猝发瞬间产生大量湍流脉动的现象，而且一次猝发包含多次喷射。基于以上认识，希望为壁湍流发卡涡包结构建立一组含参数的、由多个交替不稳定焦点和鞍点组成的奇异点动力系统模型，其中参数的变化对该动力系统有一定的控制作用；研究该模型与N-S方程的解的关系；通过Tomographic TRPIV水槽实验，检验以往的相干结构主/被动控制技术及效果是否在控制该参数上有所体现。在此基础上，研制一种物理上可实现的基于有效控制该参数的发卡涡包控制技术；最后通过Tomographic TRPIV水槽实验验证该技术的有效性。

在风洞和水洞中开展壁湍流发卡涡包奇异动力系统及其主被动控制的实验研究。通过利用层析高时间分辨率粒子图像测速(Tomo-TRPIV)等先进的流动测量技术，对壁湍流相干结构时-空发展演化特征进行了实验研究。用条件采样及空间相位平均技术提取了不同法向位置发卡涡展向涡头周围流线的空间拓扑，发现发卡涡的空间流线拓扑表现为鞍点-焦点不稳定动力系统。发卡涡包的流线图表现为一簇典型的不稳定焦点和鞍点交替组成的奇异点动力系统，在物理上符合壁湍流相干结构的猝发瞬间产生大量湍流脉动的现象。这个系统的重要参数之一是湍流边界层近壁区多尺度成分的功率随尺度（频率）的分布中的功率峰值对应的尺度（频率），自主设计了单/双压电振子同/异步振动开/闭环主动控制壁湍流对策及其实验装置，通过控制压电振子的振动频率接近相干结构功率最大尺度对应的频率，实现了双压电振子的同步和异步振动对壁湍流发卡涡包结构的有效主动控制。针对舰船、潜艇、鱼雷、水下航行器等多种水面/水下航行体及管道输运等工程减阻需求，开展超疏水壁面、沟槽壁面等减阻壁面湍流边界层相干结构特征与被动控制减阻机理的实验研究。在上述基础上，发明了一种新型的沟槽-超疏水二重尺度结构复合壁面高效被动减阻技术。这种被动高效减阻技术不需要任何额外的能量注入和能量消耗，对湍流边界层就能实现和主动减阻技术一样的高减阻率，减阻率高达20%以上。