超声速混合层中扰动增强混合实验

以基于纳米技术的平面激光散射(nano-based planar laser scattering, NPLS)流动显示技术定性研究了隔板扰动对超声速混合层($Mc=0.5$)的混合增强效果. 首先通过系列实验优化设计了扰动参数. 实验结果表明,超声速混合层对于从隔板引入的扰动非常敏感. 二维扰动的混合强化机制是提前混合层失稳位置,增厚混合层;而三维扰动的混合强化机制主要是通过诱导流向涡和展向运动,促进流动三维性质的发展. 总体而言,三维扰动的混合强化效果优于二维扰动. 由于是超声速混合层,隔板上的扰动片虽然很薄,但同样会引起激波的产生,是该方法中总压损失的主要原因.      

不同光线入射角度下超声速湍流边界层气动光学效应的实验研究

利用基于纳米粒子的平面激光散射技术获取超声速(Ma=3.0)湍流边界层的密度分布,采用光线追迹方法计算其对应的光程差分布,并结合边界层气动光学相似律验证实验结果的可靠性.着重研究了光线入射角度对超声速湍流边界层气动光学效应的影响,并对其内在机理进行了分析.研究表明,气动光学效应对光线入射角度的依赖性源于光线在流场中的传输路径,传输路径的不同导致了光线在流场中的传输距离以及对应密度脉动互相关结果的差异.光线倾斜入射导致其在流场中传输距离增长,进而气动光学效应出现恶化.光线入射方向与壁面垂直方向之间的夹角越大,气动光学效应越显著,而且不同时刻的差异性增加,气动光学效应校正的难度增加.超声速湍流边界层中大量具有特定方向的涡结构导致了湍流边界层气动光学效应的各向异性.当光线倾斜向下游入射时,光线传播方向与流场中的涡结构具有较好的一致性,体现为此方向上密度脉动互相关系数较大,故气动光学效应比较严重.而当光线倾斜向上游入射时,相关系数较小,故气动光学效应较弱.     

Simultaneous density and velocity measurements in a supersonic turbulent boundary layer

A novel technique for simultaneous measurements of instantaneous whole-field density and velocity fields of supersonic flows has been developed.The density measurement is performed based on the nano-tracer planar laser scattering(NPLS) technique,while the velocity measurement is carried out using particle image velocimetry(PIV).The present experimental technique has been applied to a flat-plate turbulent boundary layer at Mach 3,and the measurement accuracy of the density and velocity are discussed.Based on this new technique,the Reynolds stress distributions were also obtained,demonstrating that this is an effective means for measuring Reynolds stresses under compressible conditions.     

Imaging of supersonic flow over a double elliptic surface

The coherent structures of flow over a double elliptic surface are experimentally investigated in a supersonic lownoise wind tunnel at Mach number 3 using nano-tracer planar laser scattering(NPLS)and particle image velocimetry(PIV)techniques.High spatiotemporal resolution images and velocity fields of both laminar and turbulent inflows over the test model are captured.Based on the time-correlation images,the spatial and temporal evolutionary characteristics of the coherent structures are investigated.The flow structures in the NPLS images are in good agreement with the velocity fluctuation fields by PIV.From statistically significant ensembles,spatial correlation analysis of both cases is performed to quantify the mean size and the orientation of coherent structures.The results indicate that the mean structure is elliptical in shape and the structural angles in the separated region of laminar inflow are slightly smaller than that of turbulent inflow.Moreover,the structural angles of both cases increase with their distance away from the wall.     

纳米示踪平面激光散射技术在激波复杂流场测量中的应用

在激波以及激波边界层相互作用这类含激波的复杂流场中,流场结构具有明显的三维特征.研究这类流场,采用纹影、阴影和干涉等传统流动显示技术空间分辨率较低,难以分辨流场的三维特性.基于纳米示踪的平面激光散射技术(nano-tracer planar laser scattering,NPLS),是作者近年来开发的一种新的研究超声速流场的测试与显示技术,可对超声速复杂三维流场进行高时空分辨率流动显示与测量.NPLS技术的特点使其成为测量激波复杂流场的有力手段.近年来,作者以NPLS技术为主要手段,对航空航天领域典型的激波复杂流场进行了试验研究,包括超声速弹头绕流、超声速混合层、超声速边界层,以及激波边界层相互作用流场,显示出NPLS技术在激波复杂流场精细测试与流动显示中优势.本文简要介绍NPLS技术在激波复杂流场测量中应用的研究进展.      

Instantaneous and time-averaged flow structures around a blunt double-cone with or without supersonic film cooling visualized via nano-tracer planar laser scattering

In a Mach 3.8 wind tunnel, both instantaneous and time-averaged flow structures of different scales around a blunt double-cone with or without supersonic film cooling were visualized via nano-tracer planar laser scattering (NPLS), which has a high spatiotemporal resolution. Three experimental cases with different injection mass flux rates were carried out. Many typical flow structures were clearly shown, such as shock waves, expansion fans, shear layers, mixing layers, and turbulent boundary layers. The analysis of two NPLS images with an interval of 5 μs revealed the temporal evolution characteristics of flow structures. With matched pressures, the laminar length of the mixing layer was longer than that in the case with a larger mass flux rate, but the full covered region was shorter. Structures like K-H (Kelvin-Helmholtz) vortices were clearly seen in both flows. Without injection, the flow was similar to the supersonic flow over a backward-facing step, and the structures were relatively simpler, and there was a longer laminar region. Large scale structures such as hairpin vortices were visualized. In addition, the results were compared in part with the schlieren images captured by others under similar conditions.     

基于背景纹影波前传感技术的气动光学波前重构与校正

背景纹影波前传感(backgroud oriented schlieren based wavefront sensing, BOS-WS) 是利用背景纹影技术测量光学波前二维分布的新型实验手段, 可定量测量光线通过干扰场后产生的光学畸变并给出光程差. 为了利用BOS-WS技术获得光线因气动光学效应产生的畸变信息, 并通过已知畸变信息还原原始图像, 进而探索一种新型的超声速成像制导方法, 本文通过理论分析, 验证了利用背景纹影技术测量光学波前的方法, 探索了利用已知波前信息预测畸变位移场以及利用已知位移场进行波前重构的计算方法. 通过数值模拟比较了一阶梯形积分算法和Southwell方法在波前重构上的误差大小和结果合理性, 并通过误差分析证明了Southwell方法更加精确合理. 通过蜡烛火焰上方流场畸变实验和透镜对波前的扰动实验, 创造性地探索了利用已知光学光程差还原畸变位移场及用其校正畸变图像的方法, 并通过验证性实验证明了校正方法的有效性.     

压缩拐角流动中G?rtler涡特性及热流分布实验研究

对高超声速压缩拐角流动中G?rtler涡特性及热流分布进行了实验研究.开发了温敏漆（temperature sensitive paint,TSP）系统,简要介绍了TSP技术的原理、文章所用的TSP涂料的标定曲线、辅助设备参数、实验过程数据后处理过程,采用基于离散Fourier定律的热流算法.研究在Ma=6低噪声风洞中进行,采用TSP技术,得到压缩拐角斜坡板上的热流分布图像,并对高低热流条带现象做出解释,与G?rtler涡有对应关系.通过改变拐角角度及来流参数,获得了不同拐角和单位Reynolds数条件下的热流分布图像,分析得到压缩拐角斜坡上G?rtler涡特性及热流分布在变参数条件下的变化规律.研究发现:当增加拐角角度或增大单位Reynolds数时,G?rtler涡的波长减小,且涡的起始位置更靠近拐角;随单位Reynolds数增加,斜坡上热流值整体增加,热流峰值位置前移;峰值位置后,热流缓慢减小的区域与G?rtler涡位置相对应.      

超声速湍流边界层密度脉动小波分析

以基于纳米示踪平面激光散射技术的密度场测量方法获得的Ma=3.0平板湍流边界层密度场实验数据为基础,采用小波方法对湍流边界层密度脉动进行了多尺度与动态特性分析.研究表明,近壁区密度脉动概率密度呈偏离高斯分布,大尺度分量对湍流边界层密度脉动起主导作用,小尺度分量使概率密度呈"M"型分布;采用希尔伯特变换对幅度调制效应进行分析,结果表明超声速湍流边界层近壁处外层大尺度密度偏移会导致内层小尺度密度脉动的局部增强或减弱;采用基于小波变换的时变谱密度估计对边界层不同高度的密度脉动进行分析,结果表明脉动主要分布在1 MHz以内,主导频段的密度脉动间歇性明显;随着时间的发展,大部分脉动存在频率从高频过渡到低频,幅值先增加后减少的规律;随着高度的增加,对数区脉动主要分布在105 Hz以下,尾迹区则集中在105 Hz以上,边界层与主流交界处的脉动主要分布在两者相互作用形成的大尺度结构附近,脉动能量从近壁面到主流区呈现先升高后降低的变化规律.