流动显示技术及其现代发展概况

本文回顾了流动显示发展的历史,以及它在流体力学研究中所起的作用。根据获取流动信息的方式不同,流动显示技术可分为两大类型。作为近代技术发展的例子,三维非对称流干涉图的定量计算、片光技术、散斑照相术、激光诱导萤光法、多普勒干涉图象显示速度场、光电信号转换激光干涉仪、以及图象处理和计算机产生数字流谱技术均作了简要的介绍。     

The optical properties of the laser beam passed through a turbulent shear layer

The related fluid dynamic theory, experimental device and some preliminary results of the study of the optical properties of a laser beam passed through a turbulent shear layer are presented. The method of image processing for the determination of the optical density from a interferogram by means of a Microneye Bullet and a Microcomputer is also described.     

The background oriented schlieren technique: sensitivity, accuracy, resolution and application to a three-dimensional density field

Three-dimensional density information of a double free air jet was acquired using optical tomography. The projections of the density field were measured using the background oriented schlieren method (BOS). Preceding the free jet measurements, the sensitivity, accuracy and resolution of the BOS method were investigated. The sensitivity depends mostly on the focal length of the lens used, the relative position of the object between camera and background and the smallest detectable shift in the image plane. The accuracy was found to be sufficiently high to apply a tomographic reconstruction process. The resolution is determined by the transfer function of the BOS-method. It is not constant and depends on the size of the interrogation windows used for the cross-correlation-algorithm. The reconstruction of the free jet was computed, using filtered back projection. The reconstructed 3D density field shows with good resolution the typical diamond structure of the density distribution in under-expanded free jets.     

测量温度场分布的激光散斑照相计量术

本文讨论了激光散斑照相术在测量密度(或温度)扬中的应用。提出了激光散斑——纹影干涉仪的光学系统,可在同一瞬时得到一张散斑图和一张差分干涉图。对沿竖直加热平板自然对流的温度边界层进行了定量地测量。从散斑图和从干涉图中获得的实验结果符合极好。为了说明本方法的应用范围,对一个非稳定的波动甚大的本申灯火焰进行了测量。实验结果表示,激光散斑照相术更适于研究湍流场,因为干涉术在这种情况下已失去其定量计量的作用。     

Computer aided evaluation of interferograms

A semi-automatic technique is presented for obtaining the density field from interferograms provided by a Mach-Zehnder setup in infinite fringe width adjustment using short exposure times. The algorithm relies on extracting the fringe edges from the original image and then numbering the resulting polygons according to their order of interference using a graphical editor. The density field is then interpolated between the polygons by calculating its value from the intensity in the original interferogram. This results in highly accurate, high resolution scalar data fields. The results may then be presented in a quasi three-dimensional form using a shading model.     

Application of particle image velocimetry and the background-oriented schlieren technique in the high-enthalpy shock tunnel G?ttingen

The applicability of the particle image velocimetry (PIV) and the background-oriented schlieren (BOS) techniques in the high-enthalpy shock tunnel G?ttingen of the German Aerospace Center, DLR is demonstrated. As a part of this feasibility study two different experiments are performed. The velocity field past a wedge in a Mach 6 flow at a total specific enthalpy of 1.5 MJ/kg is determined by means of PIV and the results are compared to numerical predictions. The BOS technique is applied to investigate the density field in the shock layer of a sphere at 12 and 22 MJ/kg total specific enthalpies. Using a ray tracer method, the BOS results are compared to the data obtained by corresponding numerical computations.     

从流场干涉图定量计算冲击波传播速度和压力

研究了含冲击波流场干涉图的图像处理方法。首先利用快速条纹细化算法细化干涉条纹,并利用条纹追踪算法提取冲击波波阵面,然后根据配准算法把序列冲击波波阵面配准在同一幅画面上。根据上述算法,在PC-VISION100图像处理机上开发了应用软件。实验结果表明,该方法不但可用于从流场干涉图中提取冲击波波阵面,更重要的是可用于从含冲击波流场干涉图中定量计算冲击波的传播速度和压力分布。     

Digital interferometry for flow visualization

Digital holographic interferometry is a hybrid optical-digital technique for determining the phase of an interferogram. This technique improves the accuracy of interferometric measurement of fluid properties and enhances the utility of interferometric flow visualization. Displays of the interferometric phase produce excellent images of weakly refracting two-dimensional flows and can be used to produce integral projection images of three dimensional flows which differ from and complement schlieren and shadowgraph images. The technique is explained herein and examples of its use in both continuous wave and pulsed interferometry are presented.This work was presented in part at the 1985 Optical Society of America Annual Meeting     

Whole-field density measurements by ‘synthetic schlieren’

 This paper outlines novel techniques for producing qualitative visualisations of density fluctuations and for obtaining quantitative whole-field density measurements in two-dimensional density-stratified flows. These techniques, which utilise image processing technology, are much simpler to set up than the classical schlieren and interferometry methods, and provide useful information in situations where shadowgraph is of little or no value. Moreover, they may be set-up to analyse much larger domains than is feasible with the classical approaches, and do not require high quality optical windows in the experimental apparatus. Ultimately the greatest strength of these techniques is the ability to extract accurate, quantitative measurements of the density field. Application of these techniques is illustrated by an internal wave field produced by an oscillating cylinder. Recent theoretical advances for this classical problem make it the ideal test bed. Results are presented for both a circular and a square cylinder oscillating vertically in a linear stratification. Further aspects of the techniques are illustrated by considering thermal convection from a hand and flow over an obstacle towed through a density stratified fluid. Received: 27 October 1998 /Accepted: 24 May 1999     

Natural-background-oriented schlieren imaging

The background-oriented schlieren (BOS) flow visualization method has the potential for large-scale flow imaging outside the laboratory by using natural backgrounds instead of the artificial patterns normally used indoors. The natural surroundings of an outdoor test site can sometimes be used as such a background, subject to criteria of fine scale, randomness and contrast that are developed here. Some natural backgrounds are more appropriate than others for a given application. Backgrounds used here to visualize both high- and low-speed schlieren disturbances include a sunlit cornfield and a backlit grove of trees. A range of image post-processing methods is considered for qualitative BOS. It is found that high sensitivity and a broad measuring range are in conflict here, much as they are in traditional schlieren instruments. Applications of natural-BOS include explosive characterization, firearms and artillery testing, chemical and natural-gas leak detection, and related phenomena.     

Application of rainbow schlieren deflectometry for concentration measurements in an axisymmetric helium jet

 The rainbow schlieren deflectometry technique was used to measure oxygen concentrations in a laminar, isothermal helium jet discharged vertically into ambient air. The concentration distributions were inferred from the color schlieren image by taking into consideration the sampling interval and noise in measurements, especially near the jet center. Excellent quantitative agreement was reached between measurements from schlieren and a continuous sampling probe. This work demonstrates the capability of the schlieren technique for providing accurate, spatially-resolved, nonintrusive, full-field of view measurements of species concentration in an isothermal binary system. Because the basic quantity measured is the refractive index, the present schlieren technique can be extended for quantitative measurements of other scalar flow properties related to the refractive index. Received: 21 April 1997 / Accepted: 14 November 1997     

Associated disturbances around a vortex ring in a stratified fluid

A motion of a vortex ring in a stratified fluid is accompanied by associated disturbances which, in the schlieren visualization in the field of a horizontal density gradient, have the shape of a symmetric four-petal configuration. The criterion of the existence of the disturbances is the Froude number Fr based on the motion velocity and the vertical vortex size. On the range Fr > 1, the disturbances are stable with respect to the variation of themotion regime and the distortion of the vortex shape. For Fr < 1 the disturbances disappear. Computer processing of the schlieren photographs showed that the experimental spatial dependences of the disturbance amplitude are close to the functions describing the distribution of the vertical velocity component in the inviscid flow past a sphere.     

Background-oriented schlieren diagnostics for large-scale explosive testing

Experimental measurements of shock wave propagation from explosions of C4 are presented. Each test is recorded with a high-speed digital video camera and the shock wave is visualized using background-oriented schlieren (BOS). Two different processing techniques for BOS analysis are presented: image subtraction and image correlation. The image subtraction technique is found to provide higher resolution for identifying the location of a shock wave propagating into still air. The image correlation technique is more appropriate for identifying shock reflections and multiple shock impacts in a region with complex flow patterns. The optical shock propagation measurements are used to predict the peak overpressure and overpressure duration at different locations and are compared to experimental pressure gage measurements. The overpressure predictions agree well with the pressure gage measurements and the overpressure duration prediction is within an order of magnitude of the experimental measurements. The BOS technique is shown to be an important tool for explosive research which can be simply incorporated into typical large-scale outdoor tests.     

Assessment and application of quantitative schlieren methods: Calibrated color schlieren and background oriented schlieren

Two quantitative schlieren methods are assessed and compared: calibrated color schlieren (CCS) and background oriented schlieren (BOS). Both methods are capable of measuring the light deflection angle in two spatial directions, and hence the projected density gradient vector field. Spatial integration using the conjugate gradient method returns the projected density field. To assess the performance of CCS and BOS, density measurements of a two-dimensional benchmark flow (a Prandtl-Meyer expansion fan) are compared with the theoretical density field and with the density inferred from PIV velocity measurements. The methods performance is also evaluated a priori from an experiment ray-tracing simulation. The density measurements show good agreement with theory. Moreover, CCS and BOS return comparable results with respect to each other and with respect to the PIV measurements. BOS proves to be very sensitive to displacements of the wind tunnel during the experiment and requires a correction for it, making it necessary to apply extra boundary conditions in the integration procedure. Furthermore, spatial resolution can be a limiting factor for accurate measurements using BOS. CCS suffers from relatively high noise in the density gradient measurement due to camera noise and has a smaller dynamic range when compared to BOS. Finally the application of the two schlieren methods to a separated wake flow is demonstrated. Flow features such as shear layers and expansion and recompression waves are measured with both methods.     

Performance analysis of the heterodyne schlieren system

The performance of the electronic heterodyne schlieren system is investigated. The sensitivity, spatial resolution, and dynamic range of the system are calculated. It is shown that these features are improved compared with the conventional intensity schlieren readout technique. Without changing the system setup, the sensitivity and accuracy of the system can be tripled by using the third harmonic of the heterodyne signal. In this case, the spatial resolution and the measurement range are reduced by a factor of three. The system is self-calibrated. For whole field measurements, the method is limited by the camera to milliseconds time scale. However, for selected points measurements fast variations, the order of tens of microseconds, can be measured. The system was evaluated experimentally by measuring the focal length of a weak lens and a multiscale phase plate.     

Quantitative schlieren measurements of coherent structures in a cavity shear layer

Quantitative flow-field data were obtained in a planar shear layer spanning an open cavity with an extension of the schlieren method. The technique is based on the measurement of light-intensity fluctuations in a real-time schlieren image. Data were collected using a fiber-optic sensor embedded in the imaging screen coupled to a photodetector. Time-resolved measurements of the instantaneous density gradient at a point in the two dimensional flow cross section were thus obtained. Detailed surveys were carried out with both the optical instrument as well as a hot wire at a Mach number of 0.25 and with the optical instrument alone at a Mach number of 0.6. A comparison of the results shows that the non-intrusive technique can accurately measure the growth rates of instability waves in the initial “linear” region of the shear layer. The density-gradient fluctuations measured at different locations (and times) were synchronized by using a microphone inside the cavity as a reference and integrated to yield profiles of the density fluctuations associated with the dominant large-scale structures in the shear layer. Such quantitative visualization is expected to clarify the mechanism of sound generation by shear-layer impingement at the cavity trailing edge and elucidate the nature of this sound source. Received: 28 December 1999/Accepted: 15 March 2000     

Interferometric measurement and tomography of the density field of supersonic jets

 The density field of cylindrical supersonic jets is investigated by Mach-Zehnder interferometry. The optical phase shift is extracted from the interferograms by digital image processing. Disturbing turbulence effects in the free shear layer are eliminated by sequential averaging of the phase shifts of several experimental images. The resulting steady state phase shift is used to calculate time-averaged interferograms and to reconstruct the density field by computerized tomography. The obtained results are compared with theoretical predictions and good quantitative agreement is found. Received: 3 March 1997/Accepted: 3 October 1997     

Measurements of a supersonic turbulent boundary layer by focusing schlieren deflectometry

 Some novel, non-intrusive, high-frequency, localized optical measurements of turbulence in compressible flows are described. The technique is based upon focusing schlieren optics coupled with high-speed quantitative measurement of light intensity fluctuations in the schlieren image. Measurements of density gradient fluctuations confined to a thin slice of the flowfield are thus obtained. The new instrument was used to investigate the structure of a two-dimensional, adiabatic, wind tunnel wall boundary layer at a Mach number of 3. The measurements were compared to data obtained using hot-wire anemometry and good agreement was found between the two. Distributions of broadband convection velocity of large-scale structures through the boundary later were also measured. In marked contrast to earlier results, it is shown here that the convection velocity is essentially identical to the local mean velocity. Further, results obtained using the VITA conditional sampling technique shed new light on the turbulent boundary layer structure. Overall, the data presented herein serve to validate the new measurement technique. Received: 12 February 1997/Accepted: 31 January 1998     

Background oriented schlieren for flow visualisation in hypersonic impulse facilities

Experiments to demonstrate the use of the background-oriented schlieren (BOS) technique in hypersonic impulse facilities are reported. BOS uses a simple optical set-up consisting of a structured background pattern, an electronic camera with a high shutter speed and a high intensity light source. The visualization technique is demonstrated in a small reflected shock tunnel with a Mach 4 conical nozzle, nozzle supply pressure of 2.2 MPa and nozzle supply enthalpy of 1.8 MJ/kg. A 20° sharp circular cone and a model of the MUSES-C re-entry body were tested. Images captured were processed using PIV-style image analysis to visualize variations in the density field. The shock angle on the cone measured from the BOS images agreed with theoretical calculations to within 0.5°. Shock standoff distances could be measured from the BOS image for the re-entry body. Preliminary experiments are also reported in higher enthalpy facilities where flow luminosity can interfere with imaging of the background pattern. A version of this paper was presented at the 25th International Symposium on Shock Waves in Bangalore in July 2005.