基于超高频精细测试技术的可压缩混合层时空演化特性研究

The evolution mechanism of the supersonic mixing layer involves nonlinear problems such as system stability, transition and turbulence, and the compressible mixing layer plays an important role in applications such as scramjet engines, aero-optics and high-energy lasers. However, when taking experimental methods to study the supersonic mixing layer，problems such as the wind tunnel noise and low spatial-temporal resolution testing still exist. This project utilizes the low-noise supersonic mixing-layer wind tunnel invented by the applicant group, based on the flow field visualization of superhigh frequency NPLS system invented by the research group, to investigate the evolution process of the supersonic mixing layer changing from laminar flow to the state of turbulence. The time-resolved NPLS images of supersonic mixing layer from laminar flow to turbulent state are obtained experimentally. The frequency is up to MHz, and the unsteady characteristics and instability mechanism are analyzed. The time-resolved velocity distribution during the compressible mixing layer evolution can be obtained. Characteristics of mixing layers with different convection Mach numbers and various wind tunnel noise levels are investigated. This study will be the first time in the world to obtain the 8 time resolved images of fine flow structures as well as the velocity-field of supersonic mixing layer, with an time interval of 1 microsecond. The results will promote the understanding of the instability mechanism of the mixing layer, which provide experimental data supporting numerical methods as RANS and DNS. This project will push forward basic research and engineering applications of the compressible mixing layer.

超声速混合层失稳转捩过程的时空演化，涉及系统稳定性、转捩和湍流等非线性问题，而且可压缩混合层在超燃冲压发动机、气动光学和高能激光器等应用领域起重要作用。但是，实验方法研究超声速混合层，仍然存在风洞噪声高、测试技术时空分辨率低等问题。本项目拟采用课题组发明的国际先进的超高频纳米粒子激光散射（超高频NPLS）精细测试技术，在低噪声混合层风洞中研究混合层从层流到失稳转捩为湍流的过程。研究获取混合层从层流失稳到湍流状态全过程时间分辨的NPLS序列图像，频率达MHz，分析非定常特性和失稳机理；研究获取混合层演化的时间分辨速度场和变化规律；获取风洞湍流度和对流马赫数等参数对混合层失稳过程的影响。该研究将首次实验获得8张时间相关的超声速混合层精细结构图像及速度场分布，时间相关图像间隔达到1微秒，推动对混合层失稳机理的认识；为RANS和DNS等数值方法提供实验数据支撑；推动混合层基础研究和工程应用。

本项目改进升级了超声速混合层风洞，设计了不同对流马赫数的低噪声喷管。通过改进纳米粒子撒播装置，在混合层风洞上实现了MHz级超高频NPLS技术的应用。超高频NPLS技术由八腔激光器、多短曝光相机集成的超高帧频相机以及高精度多通道同步控制技术等组成。采用超高频NPLS技术研究了超声速混合层的时间演化特性，获得了不同对流马赫数、有无微型流动控制装置条件下的混合层流场序列图像，进一步采用图像互相关计算方法，获得了混合层速度分布。.对于Mc=0.17和Mc=0.26混合层流场，对比了混合层的层流段长度、失稳段、涡结构形态，选取了一些典型的涡结构作为参考，发现不同位置的涡结构随时间的发展存在明显差异。在较低的对流马赫数条件下，不稳定性发展阶段能维持更长的空间距离。混合层中段的不稳定性发展阶段，涡结构以平移和旋转为主，伴随一定的拉伸；混合层后段则以变形和破碎为主。剪切层诱发的小结构主要在后段破碎，上下层混合更强。受到剪切、大尺度结构以及小激波的影响，小尺度结构的运动存在高度非定常特性。.对于锯齿状涡流发生器流动控制，研究发现涡流发生器的高度越高，涡流发生器对超声速混合层的流动控制效果越显著，突出表现为对大尺度涡结构生成和发展的抑制作用；涡流发生器的凹槽和尖顶处超声速混合层的流场结构相似，即凹槽和尖顶对超声速混合层的流动控制效果相近。.对于涡结构特征明显的Mc=0.17混合层，基于互相关算法得到位移和流动速度，基于多张瞬态图像获取平均波长，发现该剪切流动的特征频率约为42.5kHz。从速度计算结果可以发现，在混合层发展的前段、中段和后段，其速度整体比较稳定、变化较小；随着时间的发展，同一组涡结构，其整体速度变化也是比较小的。

内容获取失败，请点击重试