【作者】 赵玉新；

【导师】 易仕和；

【作者基本信息】 国防科学技术大学 ， 力学， 2008， 博士

【摘要】 本文设计了层流化、低噪声运行的超声速混合层风洞,开发了一种新的超声速流动定量成像技术。以该风洞和成像技术为基础,从时空结构、信号特征、动力学行为和光学传输特性的角度对混合层流场进行了深入的研究。提出了超声速混合层风洞的气动与结构设计方法。为保证风洞喷管的层流化运行,提出了基于B-样条曲线的可调型面喷管设计方法,并以数值模拟技术验证了设计方法的可靠性。采用纹影仪和压力测量系统对所设计的风洞进行了调试与流场校测,结果表明:风洞低速层压力可连续调节,形成了层流化、压力匹配的超声速混合层,且实验段的结构便于定量流动成像技术的实施。为解决当前超声速混合层精细结构测量和流动显示所面临的问题,提出了基于纳米粒子的平面激光成像技术(NPLI)并进行了系统的研究。该项技术采用纳米粒子作为示踪粒子,以脉冲平面激光作为光源,能够实现超声速复杂流场结构的高时空分辨率和高信噪比成像。对影响NPLI系统性能的因素进行了深入探讨。研究了纳米粒子在超声速流场中的跟随性,并采用斜激波校准方法测量了粒子直径。根据光散射理论,分析了纳米粒子的激光散射性能。结果表明,NPLI方法比其它粒子成像方法有更好的示踪性能,比基于分子成像的方法有更高的光散射强度,满足了超声速流动成像对跟随性和信噪比的双重要求。NPLI技术和设备为超声速混合层实验研究提供了重要的技术支持。流向NPLI实验图像展现了超声速混合层从层流到涡结构产生、发展直至破碎的整体结构,并且在展向NPLI图像中发现了一种新的涡结构。通过对大量NPLI图像的比较与分析,深入研究了混合层流场的空间结构和时间演化特性。研究结果表明:来流流场品质对混合层流向结构有十分重要的影响;混合层的三维涡结构具有快运动、慢变化的特征;压力不匹配混合层流场中存在激波与边界层、涡结构相互作用的现象。流动控制实验表明,单波长和三维扰动片具有明显的混合增强功能。根据NPLI技术的特征,提出了一种超声速流动密度场测量的新方法,实现了混合层密度场的高分辨率定量测量,并根据混合层涡结构的空间特征对密度场进行了三维近似重构。采用傅里叶变换和小波变换技术,实现了混合层密度脉动信号的频谱与多分辨率分析,结果表明:信号的频域特性与流场的涡结构相对应,不同层面的离散小波系数反映了不同尺度下信号与图像的细节和平滑近似特征,连续小波变换系数图定性的反映了信号的自相似特征。NPLI图像的高分辨率特征为超声速混合层的分形研究提供了有力的实验数据支持。研究结果表明:混合界面在不同尺度下具有很好的自相似性;分形维数可定量的度量混合界面的破碎程度;在完全发展的混合层中,虽然流场结构各不相同,但其分形维数基本不变。分析了超声速PIV系统的硬件及算法应具备的性能。利用NPLI系统的粒子发生系统解决了超声速PIV技术的粒子跟随性问题。以该系统为基础,研究了超声速混合层的速度场结构,结果表明:超声速混合层前段主要受来流边界层的影响,体现为速度的剪切;中段涡卷起的区域具有周期性的结构;涡结构向混合层两侧翻转的速度是非对称的;相对速度矢量图和对应的流线图所反映的拓扑结构与原始粒子图像相对应。背景纹影技术(BOS)是新近提出的一种定量流动成像技术,本文通过分析BOS系统的基本原理、工作方式和误差来源,开发了可用于超声速流场结构及其气动光学特性分析的BOS装置。以该装置为基础,研究了混合层密度梯度场的空间结构和时间演化特征。结果表明:不同对流马赫数的混合层流向和展向BOS图像均具有周期性的结构,体现了流场造成的气动光学畸变特征;时间相关BOS图像反映了涡运动造成的气动光学抖动效应。更多还原

【Abstract】 In this paper, laminarized and low noise supersonic mixing layer wind tunnels were designed and a new quantitative supersonic flow imaging technique was developed, with which the spatiotemporal structure, signal characteristic, dynamic behavior, and light transmission characteristic of supersonic mixing layer were studied in detail.The aerodynamic and structural design methods of supersonic mixing layer wind tunnel were proposed. In order to ensure the laminarization of nozzle outflow, an adjustable contour nozzle design method based B-Spline curve was proposed, the reliability of which was validated by numerical simulation method. The performance of the wind tunnel was tested and calibrated with schlieren and pressure measurement system, the result shows that the pressure of low speed layer can be adjusted continuously, laminarized and pressure matched supersonic mixing layer can be realized, and the structure of test section is convenient for quantitative flow imaging.In order to solve the problems encountered in fine structures measurement and flow visualization of supersonic mixing layer, the nanoparticle-based planar laser imaging (NPLI) method was developed and systematically studied. In NPLI, nanoparticles and pulsed planar laser were used as tracer and illumination respectively, with which the complex supersonic flow structures can be imaged at high spatiotemporal resolution and high signal-to-noise ratio (SNR). The factors that influence the performance of NPLI were investigated in detail. The flow-following ability of nanoparticles in supersonic flows was studied, and the diameter of nanoparticle was measured based on oblique shock wave calibrating method. The laser scattering performance of nanoparticles was analyzed according to light scattering theory. The results show that the NPLI technique yields better flow-following ability than particle-based method and more intensive scattered light than molecule-based method, which meets the requirements of supersonic flow imaging on the flow-following ability and SNR.The NPLI technique and apparatus facilitate the experimental studies of supersonic mixing layer. The streamwise NPLI images reveal the whole structure of supersonic mixing layer from laminar region to the generation and breakdown of vortex. A new type vortex structure was discovered in spanwise NPLI images. By comparing and analyzing of the NPLI images, the spatial structures and temporal evolution of mixing layer were investigated. The results show that the quality of the inflow severely influences the streamwise structures; the three-dimensional (3D) vortex structures reveal characteristics of rapid translation and slow distortion; the interaction between shock wave, boundary layer and vortexes exists in pressure-unmatched flow field. The flow control experiments show that single wavelength and 3D disturbance evidently enhance the mixing efficiency.A new density measuring method of supersonic flow fields was proposed, with which the density distribution of mixing layer can be measured quantitatively at high spatiotemporal resolution. The 3D density distribution of mixing layer was reconstructed according to the spatial characteristic of vortexes. By utilizing Fourier and wavelet transformation, the frequency spectral and multi-resolution analysis of density signal fluctuation in mixing layer were realized. The results show that the frequency characteristic corresponds with vortex structures; the detail and approximate characteristics of signal are reflected by wavelet coefficient at different scales; the coefficient map of continuous wavelet transformation reveals the self-similarity of the signal. The high spatiotemporal resolution characteristic of NPLI images facilitates studying fractal characteristics of supersonic mixing layer. The results show that the interface of mixing layer exhibits excellent self-similarity at different scale; the fractal dimension can quantify the fragmental characteristic of mixing layer; the fractal dimension of fully developed mixing layer does not varies with different flow structures, which also reveals the self-similarity.The performance of hardware and algorithm of supersonic PIV system were analyzed. The flow-following ability problem of tracer particles encountered in supersonic PIV measurement was solved by utilizing particle generator of NPLI. The velocity distribution of supersonic mixing layer was studied with this PIV system. The results show that the laminar section of supersonic mixing layer is mainly influenced by the boundary layer of inflow and dominated by streamwise shear of velocity; the region dominated by vortexes reveals periodical structure; the rolling velocity of vortexes is asymmetric; the vector field of relative velocity and the relative streamline correspond with topological structure of particle imagesBackground oriented schlieren (BOS) is a kind of newly developed quantitative flow imaging technique. By analyzing the principle of BOS, a BOS apparatus was developed, which can be used to study the supersonic flow structures and its aero-optic characteristics. With this apparatus, the spatial structure and temporal evolution characteristic of supersonic mixing layer were studied. The results show that the periodic structures widely exist in streamwise and spanwise BOS images of mixing layer at different convective Mach number, which embodies the aero-optic aberration of the flow field. The time-correlated BOS images reveal the aero-optic dithering characteristics that influenced by the movement of vortexes.更多还原