Three-dimensional flow visualization with picosecond Mie scattering and streak-camera detection

Two-dimensional images of Mie scattered light from a water aerosol have been recorded by a streak camera with a time resolution of a few picoseconds. The laser pulse, which is 35 ps long, is repeatedly reflected and refocused in the probe volume. On the output phosphor screen of the streak camera, the images of the scattering appear as adjacent, separated pictures on the temporal axis of the streak camera. Because the pictures obtained in this way are separated in time by less than 3 ns, which is much shorter than the typical time scales of the turbulent gas flow, and because they are separated in space by fixed intervals, the resulting images can be used to compose a three-dimensional picture of the aerosol distribution.     

An automatable algorithm for measurement of fluid flow during laser fusion cutting using in‐situ trim cut analysis

Irregularities inside the fluid flow in laser fusion cutting of sheet metals cause quality defects like large striation depths or adhered dross. Trim‐cut analysis is used to measure the flow velocities of the molten material by high speed imaging. The so‐called streak analysis creates a repetitive pattern for multiple melt waves and the angle of these patterns is assignable to specific flow velocities. In this article, a discrete fourier transform is used to automatically determine the spatial distribution of flow velocities. The results show that identifying the velocity domains and assigning them to the parameters of the cutting process and quality characteristics is a promising approach to enhance the understanding and to achieve higher quality cuts.     

Boundary layer transition experiments with embedded streamwise vortices

Experiments were conducted with a counter-rotating, streamwise vortex pair embedded in flat plate boundary layers, in a low-turbulence wind tunnel, to understand the role of local separation on transition. Steady, streamwise vortices were generated downstream of gaps in spanwise-uniform, smooth hills (of height h) affixed to the plate, 175 mm from its leading edge. The flow between is directed away from the plate. At the four tunnel speeds 1.8–3.5 m/s considered, the Reynolds numbers based on displacement thickness at this location varied from 248 to 346. Small, medium and large gaps of 2, 4 and 8 mm, respectively, were set up; they were about a third to twice the boundary layer thickness (\(2/3< b/h < 8/3\)). With the closest vortex pairs, transition was observed at all freestream speeds considered. With larger spacing, transition occurred at the highest speed only. The vortex pair caused the flow to separate in all but the largest-gap cases. Separation was steady and re-attachment unsteady in all cases. Velocity fluctuations grew slightly upstream of re-attachment in transitional cases. No evidence was found for separation or re-attachment as a direct cause for transition; transition occurred even without separation. Instead, whenever transition was observed, its origin could be traced to instability of a streak of sufficient amplitude that had been created by the vortex pair. Streak instability appeared as fluctuations growing along its sides and spreading. Anomalous behaviour was also observed with moderate spacing, where transition did not occur in spite of flow separation and streak amplitudes in excess of known thresholds for streak instability.     

Streaks and vortices in near-wall turbulence

This paper presents evidence that organization of wall-normal motions plays almost no role in the creation of streaks. This evidence consists of the theory of streak generation not requiring the existence of organized vortices, extensive quantitative comparisons between the theory and direct numerical simulations, including examples of large variation in average spacing of the streaks of different scalars simultaneously present in the flow, and an example of the scalar streaks in an artificially created purely random flow.     

时间分辨条纹相机技术的发展及相关研究展望

梳理了瞬态科学超快诊断技术发展更替脉络,以时间分辨率为主线回顾了时间分辨条纹相机技术的发展历程,从相关应用需求角度分析了条纹相机技术的发展趋势,重点阐明了双色场泵浦-探测阿秒条纹相机技术相对于基于变像管的条纹相机诊断技术在测量机制方面的创新。同时对阿秒条纹相机技术对微观超快现象向更深层次拓展的推动作用作了前瞻性的论述。     

Development of picosecond time resolution optical and X-ray streak cameras

We describe the development of an optical and an X-ray streak camera with picosecond time resolution. The entire peripheral electronics and testing systems have been developed indigenously. Both the streak cameras provide ∼ 15 mm/1 ns streak rate with a sweep voltage of ∼ 1 kV amplitude and rise time of 1 ns. The time and spatial resolution of the optical streak camera have been found to be ∼ 17 ps and 100 μm respectively. The sweep pulse generator developed for this purpose provides a step pulse of rise time ≦1 ns and amplitude ∼ 2 kV. The laser diode used for testing the optical streak camera provides multiple pulsation when the pump current is increased beyond a critical threshold.     

MHz-rate nitric oxide planar laser-induced fluorescence imaging in a Mach 10 hypersonic wind tunnel

Nitric oxide planar laser-induced fluorescence (NO PLIF) imaging at repetition rates as high as 1 MHz is demonstrated in the NASA Langley 31 in. Mach 10 hypersonic wind tunnel. Approximately 200 time-correlated image sequences of between 10 and 20 individual frames were obtained over eight days of wind tunnel testing spanning two entries in March and September of 2009. The image sequences presented were obtained from the boundary layer of a 20° flat plate model, in which transition was induced using a variety of different shaped protuberances, including a cylinder and a triangle. The high-speed image sequences captured a variety of laminar and transitional flow phenomena, ranging from mostly laminar flow, typically at a lower Reynolds number and/or in the near wall region of the model, to highly transitional flow in which the temporal evolution and progression of characteristic streak instabilities and/or corkscrew-shaped vortices could be clearly identified.     

Coherent detection in Doppler global velocimetry: a simplified method to measure subsonic fluid flow fields

The principles of an entirely new method to demodulate Doppler signals in Doppler global velocimetry (DGV) are discussed. The method makes use of coherent detection and streak imaging to record both temporal and spatial information on a single image. The method retains the simplicity of the basic DGV technique yet increases its applicability to subsonic flow regimes. The combination of a straightforward optical configuration and the ability to collect large data sets efficiently make this technique particularly useful in high-cost experimental facilities such as wind-tunnel testing where large quantities of data must be collected in a relatively short time period.     

合金化热镀锌IF钢表面亮条纹缺陷分析

利用光学显微镜、扫描电镜和能谱分析仪分析了合金化热镀锌IF钢板表面的亮条纹缺陷。条纹部位与正常部位相比,镀层的表面形貌、截面特征以及侵蚀锌层后的钢板表面形貌均有不同。造成条纹的直接原因是钢板表面的微裂纹,而引起微裂纹的原因是Al2O3颗粒。     

Jitter Measurement Technique for Image-Converter Streak Cameras

Measurement Techniques - The phenomenon of jitter is examined in image-converter (electro-optical) streak cameras used for studies of high-speed processes. A method for measuring jitter in streak...     

钛合金β相向ω相转变产生的漫散射条纹特征

利用透射电子显微镜研究了钛合金β相的选区电子衍射花样中漫散射条纹特征。花样中的漫散射条纹互相交叉形成网格,对于大多晶带轴,几段条纹互相平行,但不在一条直线上。漫散射条纹位于{112}β面上,在晶带轴为[uvw]β时,条纹方向为[uvw]β×{112}β。解释了漫散射条纹不在一条直线上的原因。漫散射条纹的出现与{112}β面上的声子模软化有关。     

"丝状斑迹"缺陷的成因

应用扫描电镜、电子探针微分析仪等设备对电镀锌钢板丝状斑迹缺陷形貌特征进行了分析测试研究。研究发现,镀锌板正常部位与缺陷部位表面差别很大,去除锌镀层后发现对应部位的显微织构也有明显的不同,缺陷部位铁{110}＜001＞取向的晶粒沿轧向分布,与“丝状斑迹”缺陷分布一致,而正常部位铁{110}＜001＞取向的晶粒分布无规则。提出“丝状斑迹”缺陷与钢板表面的高斯织构沿轧向排列有对应关系。     

Measurement of the components of the gas flow behind a shock front by streak photography of the spectrum of interference bands

This article is the first of several in a continuing series on opticophysical methods for the investigation of inhomogeneities, begun in the preceding issue and in the journal Metrologiya, No. 4 (1994). Here a method is proposed for determining the composition of an ionized gas from data obtained in streak photography of interference fringes produced by oblique transmission of the sensing light beam across a shock front. The author gives a comparative estimation of the errors of the results in normal and oblique transmission of a monatomic gas flow containing singly charged ions.Translated from Izmeritel'naya Tekhnika, No. 7, pp. 19–21, July, 1994.     

Chirp measurement of multimode q-switched laser diode pulses by use of a streak camera and a grating monochromator

We discuss spectrotemporal measurements of laser diode pulses performed with a streak camera and a grating monochromator to yield a precise evaluation of chirping effects in Q-switched multimode emissions. We experimentally illustrate several causes of errors, depending on the grating size and period as well as on the adjustment of the collimating lens at the monochromator output. An analytical formula is derived that allows us to relate the chirp amplitude to the inclination of the modal structures in the streak image. Two configurations are proposed for a practical determination of the chirp amplitude in multimode emissions. Illustrations are provided with Q-switched AlGaAs laser diodes that exhibit chirp amplitudes larger than the mode spacing.     

Influence of incident beam divergence on the intensity of diffuse streaks

The influence of incident beam divergence on the length of the streak intercepted by the Ewald sphere is considered, as a relpHK·L of a faulted hexagonal crystal, mounted about itsc-axis on the goniometer head attached to the ø-circle, is brought into diffracting condition for the bisecting setting of a 4-circle diffractometer. For the special crystal mounting correction factors required to convert the measured intensities corresponding to a fixed length of the streak are derived. A procedure for experimentally verifying the mathematical approach employed in these derivations is also presented.     

基于混合蛙跳优化的条纹管图像自适应增强

针对条纹管相机荻取的条纹图像,提出一种将混合蛙跳算法和非完全Beta函数结合的自适应增强方法.利用混合蛙跳算法中局部信息交换和全局信息交换的寻优机制,增加寻优稳定性和加快收敛度.通过混合蛙跳优化算法自动搜索最佳灰度变换参数,从而得到一条最佳灰度变换曲线,实现对条纹图像增强处理.实验证明该算法能降低条纹图像中背景对目标的...     

Overall comparison of subpicosecond electron beam diagnostics by the polychromator, the interferometer and the femtosecond streak camera

Measurements of longitudinal bunch length of subpicosecond and picosecond electron beams have been performed by three methods with three radiation sources at the 35 MeV S-band twin liner accelerators at Nuclear Engineering Research Laboratory, University of Tokyo. The methods we adopt are the femtosecond streak camera with a nondispersive reflective optics, the coherent transition radiation (CTR) Michelson interferometer and the 10 ch polychromator that detects the spectrum of CTR and coherent diffraction radiation (CDR). The measurements by the two CTR methods were independently done with the streak camera and their results were consistent with one another. As a result, the reliability of the polychromator for the diagnostics of less than picosecond electron bunch and the usefulness of the diagnostics for the single shot measurement were verified. Furthermore, perfect nondestructive diagnostics for subpicosecond bunches was performed utilizing CDR interferometry. Then the good agreement between CDR interferometry and the streak camera was obtained.     

A deterministic model for the sublayer streaks in turbulent boundary layers for application to flow control

We present a relatively simple, deterministic, theoretical model for the sublayer streaks in a turbulent boundary layer based on an analogy with Klebanoff modes. Our approach is to generate the streamwise vortices found in the buffer layer by means of a vorticity source in the form of a fictitious body force. It is found that the strongest streaks correspond to a spanwise wavelength that lies within the range of the experimentally observed values for the statistical mean streak spacing. We also present results showing the effect of streamwise pressure gradient, Reynolds number and wall compliance on the sublayer streaks. The theoretical predictions for the effects of wall compliance on the streak characteristics agree well with experimental data. Our proposed theoretical model for the quasi-periodic bursting cycle is also described, which places the streak modelling in context. The proposed bursting process is as follows: (i) streamwise vortices generate sublayer streaks and other vortical elements generate propagating plane waves, (ii) when the streaks reach a sufficient amplitude, they interact nonlinearly with the plane waves to produce oblique waves that exhibit transient growth, and (iii) the oblique waves interact nonlinearly with the plane wave to generate streamwise vortices; these in turn generate the sublayer streaks and so the cycle is renewed.     

纳秒级紫外/软X射线条纹相机

纳秒级紫外/软X射线条纹相机是一种利用高速扫描电路实现将纳秒级光信号转化为图像信 号的高速图像设备,它主要包括了条纹管、高速高压扫描单元、高压单元以及控制记录单元等部分。系统设计中采用近贴聚焦电子光学系统和雪崩管电路使得目前所能达到的指标为:能谱响应范围紫外～1keV,空间分辨力静态15Lp/mm,动态5Lp/mm,时间分辨力为500ps。     

Interferometric determination of the quadratic electro-optic coefficient of nitrobenzene

An interferometric method that was used to measure the electro-optic s(11) coefficient of nitrobenzene is presented. The method uses a Mach-Zehnder interferometer arrangement that produces a finite fringe interferogram. A nitrobenzene Kerr cell is placed in one arm of the interferometer and a pulsed high voltage is applied to the two rectangular electrodes. The voltage application causes the fringe image to shift in time, and this fringe shift is measured by a streak camera that streaks the fringe image across a slit. The streak start time is chosen before the application of the voltage pulse and ends after the voltage pulse has been applied. Thus continuous observation of the event is possible. By accurately measuring the fringe shifts the electro-optic s(11) coefficient can be determined. The experimental arrangement is described, and the results obtained are discussed.