Temperature fields in a liquid due to the thermocapillary motion of bubbles and drops

 Experiments were performed on the motion of isolated air bubbles and drops of Fluorinert FC-75 moving in a Dow-Corning silicone oil under the action of an applied temperature gradient in a reduced gravity environment aboard the Space Shuttle in orbit. The disturbance of the imposed temperature field due to the motion of the objects was studied optically using a shearing interferometer with a Wollaston prism and the results of a typical bubble run were compared with theoretical predictions. Also, the liquid velocity field surrounding the bubbles and drops has been qualitatively investigated in a few runs by the observation of tracer particles dispersed in the continuous phase fluid. The measurement techniques are described, and the results for the temperature and flow fields are presented and discussed. Received: 27 June 2000/Accepted: 17 November 2000     

A new moiré interferometer for measuring in-plane displacement

This paper proposes a new moiré interferometer to be used for determining in-plane displacement of a body deformed with small strains and small surface slopes. With this interferometer, the grating not only diffracts the incoming beam but also rediffracts the two first orders, each of which is reflected by a mirror, back into the path of the incoming beam. The two rediffracted beams are then directed to the viewer to generate an interference fringe pattern that depends only on the in-plane displacement. Theoretical analysis of this new interferometer is presented. It was determined that the sensivity of the interferometer is twice that of conventional interferometers, and the slightly warped beam wave front coming from the laser source does not affect the measurement result. An experiment was performed by adopting the proposed and conventional interferometers to determine the displacement of a specimen grating. The results confirm the advantages of using this new interferometer.     

Measurement of the mass diffusivity of miscible liquids as a function of concentration using a common path shearing interferometer

A common path-shearing interferometer was adapted, using a Wollaston prism, to measure the diffusion coefficient for mass transfer of transparent, miscible liquids. The mathematics was developed for the time-dependent, one-dimensional diffusion process in a test cell when the diffusivity is a function of the local concentration of the fluids, and a procedure was devised to determine the diffusivity from the measured concentration gradient profiles. The results show that for diffusion in a water/glycerine system, the diffusivity depends on the concentration in an almost linear fashion. In the case of the silicone oils, the diffusivity was found to be nearly independent of the local concentration.     

The generic nature of the absolute-retardation method of photoelasticity

The method of absolute-retardation measurements does not depend upon the use of any specific interferometer. Any one of the many extended field interferometers can be employed, including the Series, Michelson, Mach-Zehnder and Fizeau types. Where adequate monochromatic purity of light source is available, reflections from the two surfaces of the model can form the interferogram, thus dispensing with the necessity of any separate optical interferometer. The two-dimensional stress-optical relationships are derived in their most general form. The relationships are identical for all instruments or interference systems—only the calibration constants change with different systems. Interferograms from four distinctly different systems are shown. Interference-fringe sharpening is introduced and demonstrated. A laser-light source was used with two systems to provide high monochromatic purity. The interferometric method of Nisida and Saito is correlated with the absolute-retardation method. The opportunity to obtain information for a complete stress solution from one photograph makes the method attractive for both static and dynamic problems.     

提高PIV片光源质量的研究

根据高斯光束的性质,本文设计了一种利用普通连续激光器产生较高质量PIV片光源的光路系统。整个光路分为两部分,第一部分为原始光束优化光路,第二部分为片光分光光路。原始光束优化光路通过一系列凸凹透镜有序布置,将原来直径大于2mm的光斑在PIV实验区间控制到1mm以下。优化光路能有效集中激光能量,提高片光亮度。片光分光光路使用鲍威尔棱镜将激光光束分为扇形片光,再用平凸柱面镜将扇形片光汇聚为矩形片光。鲍威尔棱镜分得片光的能量在宽度方向分布较常规双凹柱面镜均匀。平凸柱面镜将扇形光源中分散在极宽区域的能量集中在固定宽度里,使得激光能量的有效利用率提高,有利于PIV实验。     

Recent high-resolution resonant refractivity studies of a sodium-seeded flame

A new experimental technique, namely, the combination of a Fabry-Perot etalon with a Mach Zehnder interferometer (MZI) makes it possible to deduce the index of refraction resonant dispersion curve near the center of an atomic line from the superimposed fringe system. A tunable pulsed dye laser as light source and a sodium seeded flame as test flow enabled us to verify the very large enhancement of sensitivity for refractive flow-visualization methods such as shadowgraph, schlieren and interferometry. The specific refractivity is a direct measure of the sensitivity, and that quantity increases from a non-resonant value of 4 × 10^?3 m'/kg to a peak value of 189 m3/kg at the half-power points of the sodium D₂-line.     

Microwave drying of various shape particles suspended in an air stream

Fluidization is an efficient way to dry granular materials. Incorporating microwave heating into the fluidization makes the overall drying process shorter, and the quality of the final products can be improved. However, in order to understand the mechanisms of water removal, an exact knowledge of changes inside the dried material is necessary. The temperature and moisture distribution pattern within the heated material should be identified and analyzed. Unfortunately, the microwave environment makes the measurements very difficult. This paper gives new information on the temperature distribution inside small particles of various shapes dried with microwaves. The tests were carried out in a laboratory-scale, fluid-bed dryer equipped with a microwave source. Five different shapes were examined: sphere, cylinder, half-cylinder, rectangular prism, and prism with triangle base. All particles tested were suspended in an air stream and heated with microwaves. The internal temperature distribution has been analyzed in each case. The rate of drying is also presented and discussed for every case tested.     

High-magnification moiré interferometer for crack tip analysis of steels

A high-magnification moiré interferometer, particularly suitable for near-tip field analysis in cracked materials, is described. It has a submillimeter field of view, a high-resolution image sensor (1.4 million pixels), X-Y-Z translation stage and an optical fiber light delivery system. These features enable the microscope head to observe the crack tip while the specimen is loaded in a standard tensile test machine. Automated fringe pattern analysis, using temporal phase shifting and spatial phase unwrapping, enables thex ory displacement component to be measured and the corresponding in-plane strain component computed. The displacement placement accuracy is better than 40 nm, and the effective strain gage dimension is ∼ 25 μm. Furthermore, the interferometer has a built-in white light microscope that allows the observation of the specimen granular microstructure in exact registration with the displacement field. The interferometer has hence been employed to investigate the near-tip fields of a precracked stainless steel specimen under load. The influence of the grain boundaries on the measured displacement fields was relatively minor. The near-tip strain field shows a significant asymmetrical behavior despite pure mode lloading conditions.     

Absolute phase measurement in extrinsic Fabry-Perot optical fiber sensors using multiple path-match conditions

This paper describes the use of the first and second optical return paths in a moderate-to-high finesse Fabry-Perot sensor to measure the absolute phase in extrinsic Fabry-Perot interferometry (EFPI) sensors. A path-matched differential interferometry (PMDI) using a highfinesse EFPI sensors, a low-finesse Fabry-Perot readout interferometer and a broadband light source consisting of amplified spontaneous emission (ASE) from an erbium-doped fiber amplifier (EDFA) is used to illustrate the idea. The first and second multiple paths in the Fabry-Perot readout sensor are used to provide two distinct path-match conditions from the same scanning Fabry-Perot readout interferometer. The difference in fringe numbers between the centers of two orders of interference fringe packets formed by the distinct path-match conditions makes possible a simple method of measuring the cavity length of EFPI sensors, which in turn can be used to measure absolute phase and the corresponding strain. Sensor cavity length measurement using the multiple return paths in the high-finesse Fabry-Perot sensor is correlated to that measurement using the modulation transfer function found using an optical spectrum analyzer; the multiple return path technique is then used to make strain measurements on a cantilever beam. Comparisons with resistance strain gage measurements are favorable. Characterization tests indicate that the proposed technique has a cavity length measurement resolution on the order of 1.1 μm, which translates to a strain resolution of 28 με for a 4-cm gage length sensor.     

角色散FP干涉测速技术应用与分析

发展了固定腔结构的角色散FP干涉测速系统,干涉仪结构紧凑,采用固定腔标准具,实现了干涉 条纹永久免调,并且条纹常数的标定非常简单。该系统可用于靶面反射光强动态变化很大的场合,在电炮驱 动Mylar膜飞片实验中,光强变化达100倍时仍然获得了很好的结果。分析了标准具厚度误差、标准具端面 不平行、干涉条纹动态展宽和扫描图像畸变等因素对系统测量精度的影响以及系统的速度和时间分辨能力。     

Dual hologram shearing interference technique for wind tunnel flow fields testing

 A novel optical diagnostic technique, dual hologram shearing interferometry, for measuring density gradients of different phase objects is proposed and demonstrated. The lateral shearing is achieved by using a phase grating. A holographic interferometer has been developed and designed on the base of a single pass Z type conventional schlieren device. The interferometer’s scheme is insensitive to acoustical disturbances, similarly to the conventional schlieren layout, and is capable of recording holograms with a continuous wave laser during the wind tunnel run. The features of the technique make it tolerant to both the temporal coherence of the laser light source and to the relatively low, schlieren quality optical windows of the wind tunnel’s test section. The obtained reconstructed lateral shearing interferograms with a large region of overlap have high contrast and may have an arbitrary orientation and/or spacing of the background interference fringes. It is believed that the proposed approach will become a useful tool for visualization and accurate mapping of the density gradients of gas dynamic flow fields, in wind and shock tunnels, where acoustic noise problems may dramatically affect reference beam holographic schemes. Received: 9 January 1997 / Accepted: 12 April 1997     

Experimental study on the flow regimes past a confined prism undergoing self-sustained oscillations

An experimental study based on Particle Image Velocimetry (PIV) is presented with the objective of studying the flow regimes that appear in the flow past a confined prism undergoing self-sustained oscillations at low Reynolds numbers (Re). The square-section prism, placed inside a 3D square cross-section vertical channel with a confinement ratio of 1/2.5, was tethered to the channel walls and, therefore, it was allowed to move freely transverse to the incoming flow. Re (based on the prism cross-section height) was varied in the range from 100 to 700. Three different prism to fluid density ratios (m¹) were considered: 0.56, 0.70, and 0.91. These two parameters, Re and m¹, were used to map the results obtained. In particular, it was found that five different regimes appear: (1) steady prism with steady recirculation bubble, (2) steady prism with unsteady vortex shedding wake, (3) large amplitude low frequency oscillating prism with unsteady vortex shedding wake, (4) small amplitude high frequency oscillating prism with unsteady vortex shedding wake, and (5) irregular/chaotic motion of both the prism and the wake. The PIV results and associated numerical simulations were used to analyze the different prism and wake states.     

SUPPRESSION OF FLUID FORCES ACTING ON TWO SQUARE PRISMS IN A TANDEM ARRANGEMENT BY PASSIVE CONTROL OF FLOW

The suppression of fluid forces acting on two square prisms in a tandem arrangement in which a flow approaching the upstream prism was controlled by a thin flat plate was examined, with variation in spacing between the plate and the upstream prism. The width of the plate was one-ninth of the prism width. The position of the control plate was varied from the front surface of the upstream prism to 2·25 times the prism width in the upstream direction, and the position of the downstream prism was varied from the rear surface of the upstream prism to 10 times the prism width in the downstream direction. A dramatic decrease in fluid forces acting on both prisms was observed for a certain range of control plate positions. For such optimum positions of the control plate, the shear layers that separated from the control plate attached near the edges of the front surface of the upstream prism and each shear layer bifurcated into two layers, one part of the shear layers making a quasi-steady recirculating region between the control plate and the upstream prism, and the other part separating from the leading edges and attaching again to the side-surfaces of the upstream prism. When the control plate was placed with spacings S/W=1·501·90 (S: spacing between the control plate and the upstream prism, W: width of the prism), the above-mentioned flow pattern appeared, and the fluid forces and vortex shedding of the upstream prism were almost completely suppressed. Also, the upstream prism was found to be insensitive to the existence of the downstream prism when the latter was located downstream, approximately six or more times the prism width.     

光纤位移干涉仪在爆轰加载飞片速度测量中的应用

利用光学多普勒效应和外差方法搭建了一台光纤位移干涉仪。装置采用光通信行业中已经发展成熟的器件,主要由带尾纤的半导体激光器、三端口环形器、光纤探头、宽带探测器以及宽带高采样率示波器等构成。整个装置结构简单,价格便宜,采用了信号光和参考光同轴结构,实现了任意反射面的速度测量,克服了偏振模式色散的影响,能够实现长量程测量,量程达到20 mm。利用该装置进行了爆轰加载下飞片速度测量,测量最高速度达到1 300 m/s,工作距离达到20 mm,同时利用VISAR对飞片速度进行了对比测量,结果表明用两种不同方法所测得的速度曲线吻合很好。     

Quantitative flow visualization using the hydraulic analogy

The current work describes the development of a non-intrusive optical method for the quantitative determination of water heights along a hydraulic jump in shooting water flows on a water table. The technique involves optically superimposing a series of alternating dark and clear fringes on the water flow. It is proposed that the fringe deviations seen under a hydraulic jump can be simulated using a series of optical prisms oriented along the direction of the hydraulic jump. The height of each prism gives the local maximum water height at the fringe location. Three types of theoretical prism configurations (isosceles flat-topped prism, scalene flat-topped prism and rounded-topped prism models) have been studied for two flow systems: shooting flow around a wedge and around a cylinder. Equations relating the physical characteristics of the deviated fringes to the height of the theoretical prism and hence the local water height are presented. The variation in water height along a hydraulic jump for flow around a wedge obtained using the optical technique has been compared with heights obtained using a depth gauge. The results were in good agreement for the range of Froude numbers studied (Fr=1.9−3.6). The rounded-topped prism model led to the best agreement with the physical measurements, within 11% throughout the range of conditions studied. The uncertainty associated with the water height determination using the optical technique is ±10%. Received: 15 September 1998/Accepted: 16 April 1999     

压剪炮的激光测速系统的研究

压剪炮试验是获取材料超高应变率下本构关系的最常用手段,其中普遍采用的测试方法是利用激光干涉技术同时测量靶板背面一点的纵向和横向速度。但传统的横向位移干涉仪抵抗靶板撞击过程中的运动而引起的光路偏移能力很有限,因而压剪试验成功率很低。本文讨论了撞击过程中靶板运动对横向位移干涉仪试验信号的影响,为提高横向位移干涉仪特别是非理想撞靶情况下的测量能力,对几种不同干涉光路进行详细分析。通过改进传统横向位移干涉光路,研制了一种新干涉光路,并通过试验证明:相对于传统光路,新光路更简单并更可靠。     

Time-average measurement of velocity, density, temperature, and turbulence velocity fluctuations using Rayleigh and Mie scattering

Measurement of time-averaged velocity, density, temperature, and turbulence velocity fluctuations in sparsely seeded gas flows using a non-intrusive, point-wise technique based on Rayleigh and Mie scattering is discussed. A Fabry-Perot interferometer (FPI) is used to spectrally resolve laser light scattered by molecules and particulates in gas flows. The spectral content of the scattered light provides information about velocity, density, and temperature of the gas. A CCD camera is used to record images of the fringes formed by scattered light passing through the interferometer. Models of the spectral components are used in a least squares fitting routine to estimate the parameters from fringe images. Flow measurements are presented for subsonic and supersonic jet flows. The application range for this technique is mostly for high velocity situations (>25 m/s). Velocity, density, temperature, and turbulence velocity fluctuations were determined with accuracies within 5 m/s, 4%, 2%, and 5 m/s, respectively.     

Schlieren interferometry applied to a gravity wave in a density-stratified liquid

A stably density-stratified liquid is produced in a rectangular glass tank by variation of the concentration of salt in water as a function of height. The glass tank is placed into the parallel beam of a schlieren interferometer with Wollaston prism adjusted to produce straight vertical fringes. A gravity wave of the cross wave type is excited and the resulting periodically deforming fringes are recorded by taking photographs. A method is developed to obtain from the fringe patterns results about propagation of the wave in space and time and about amplitude attenuation. The results are compared with the linear theory of Thomas and Stevenson (1972) and excellent agreement is found within the limits of the linear approach.     

有限长正方形棱柱绕流的双稳态现象

运用大涡模拟(large eddy simulation, LES) 方法对有限长正方形棱柱绕流进行了研究. 棱柱高宽比为5,一端固定于平板上,另一端为自由端. 平板表面边界层厚度可忽略不计. 基于自由流速度和棱柱宽度的雷诺数为3 900. 由计算结果发现,有限长棱柱的尾流在自由端后下扫流的作用下具有显著的三维性. 尾流中会交替出现两种典型的流动状态:一种是柱体两侧所脱落的涡在尾流中形成类似卡门涡街的交错排列状态;另一种是柱体尾流中涡团对称分布且无明显的周期性. 两种典型流动状态的交替出现,对有限长棱柱气动力特性有直接的影响.