Optical density and velocity measurements in cryogenic gas flows

This paper presents the application of optical measurement techniques in dense-gas flows in a heavy-gas channel to determine planar two-component (2C) velocity profiles and two-dimensional (2D) temperature profiles. The experimental approach is rather new in this area, and represents progress compared with the traditional techniques based on thermocouple measurements. The dense-gas flows are generated by the evaporation of liquid nitrogen. The optical measurement of both the velocity and density profiles is accomplished by the implementation of particle image velocimetry (PIV) and background-oriented schlieren (BOS) systems. Supplemental thermocouple measurements are used as independent calibrations to derive temperatures from the density data measured with the BOS system. The results obtained with both systems are used to quantify the dilution behavior of the propagating cloud through a global entrainment parameter . Its value agrees well with the results obtained by earlier studies.     

Visualization of compressible vortex rings using the background-oriented schlieren method

In this article, we attempt to validate flow visualization using the high-speed background-oriented schlieren (HiBOS) method, which is the BOS technique combined with a high-speed video camera as the recording device in the experiment. The method has been applied to shock-induced flow near the open end of a shock tube. Three incident shock Mach numbers were examined so that the BOS measurements could be compared with results given in the literature of particle-image velocimetry (PIV) measurements. Using the HiBOS technique, we were able to clearly view developing, compressible vortex rings and diffracted shock waves discharged from the open end of the shock tube. From the BOS images, we extracted the history of the propagation velocity, the diameter of the vortex ring, and the diameter of the vortex core, all of which agree with the corresponding PIV values reported in the literature.     

Numerical and experimental study of shock waves emanating from an open-ended rectangular tube

We examine the dynamics of a high-speed shock-induced flow near the open end of a shock tube using the particle image velocimetry (PIV) and the background oriented schlieren (BOS) methods along with two- and three-dimensional numerical simulations. In experiments, planar shock waves (\(M=1.3\)–1.6) are discharged from a rectangular (\(24\,\hbox {mm} \times 48\,\hbox {mm}\)) low-pressure section of a shock tube open to the atmosphere. Due to the rectangular exit geometry, the resulting flow is highly three-dimensional and, thus, more complicated, compared to well-studied circular/axisymmetric geometries. The study focuses on the spatio-temporal flow structure up to 1 ms after the shock wave diffraction. PIV and BOS visualization techniques share the same post-processing principle, and the iterative multi-step cross-correlation algorithm applied in the PIV software is adapted here for the calculation of background pattern displacement on the BOS images. Particular attention is given to the resolution of flow regions where sharp gradients are present, such as a diffracted shock front or embedded shocks. Computational fluid dynamic simulations of the problem are also conducted to validate the experimental results and methods and to gain more insight into the three-dimensional flow dynamics. PIV and BOS images are found to be consistent with the corresponding numerical flow visualizations.     

Background-oriented schlieren with natural background for quantitative visualization of open-air explosions

This study describes an attempt of quantitative visualization of open-air explosions via the background-oriented schlieren method (BOS). The shock wave propagation curve and overpressure distribution were extracted from the obtained images and compared with the results of the numerical analysis. The potential of extracting the density distribution behind the shock front is also demonstrated. Two open-air explosions were conducted; one with a $36$ -kg emulsion explosive and the other with a $7.89$ -kg composition C4 explosive. A high-speed digital video camera was used with a frame rate of $10{,}000\,\mathrm{Hz}$ and a pixel size of $800 \times 600$ . A natural background, including trees and grass, was used for BOS measurements instead of the random dots used in a laboratory. The overpressure distribution given by the passing shock was estimated from the visualized images. The estimated overpressures agreed with the values recorded by pressure transducers in the test field. The background displacement caused by light diffraction inside the spherical shock waves was in good agreement, except at the shock front. The results shown here suggest that the BOS method for open-air experiments could provide increasingly better quantitative and conventional visualization results with increasing spatial resolution of high-speed cameras.     

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.     

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.     

An evaluation of optical flow algorithms for background oriented schlieren imaging

The background oriented schlieren method (BOS) allows for accurate flow measurements with a simple experimental configuration. To estimate per-pixel displacement vectors between two images, BOS systems traditionally borrow window-based algorithms from particle image velocimetry. In this paper, we evaluate the performance of more recent optical flow methods in BOS settings. We also analyze the impact of different background patterns, suggesting the use of a pattern with detail at many scales. Experiments with both synthetic and real datasets show that the performance of BOS systems can be significantly improved through a combination of optical flow algorithms and multiscale background.     

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 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.     

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.     

A four colours line grid schlieren method for quantitative flow measurement

This article describes the schlieren apparatus developed at the Institut für Luftfahrttechnik und Leichtbau (ILT, Institute of Aeronautics and Structures) for the Transonic Wind Tunnel of the Munich University of the Federal Armed Forces (TWM). Special features are the application of four separate, variously coloured beams of rays to identify the direction of density gradients and the line grids which are attached to the knife edges, in order to permit a quantitative evaluation of the amount of ray deflection and of the thermodynamical quantities.List of symbols c sensitivity - h light deflection - M Mach number - s distance - O order of schlieren lines - T spacing of the line grids - x horizontal direction - z vertical direction - angle of attack - angle of light deflection - density     

The calculation of the density field from axisymmetric schlieren interferograms by the image processing techniques

The schlieren interferograms used to be analyzed in a qualitative way. In this paper, by means of the powerful computational ability and the large memory of computer; the image processing method is investigated for the digitalization of an axisymmetric schlieren interferogram and the determination of the density field. This method includes the 2-D low-pass filtering, the thinning of interferometric fringes, the extraction of physical information and the numerical integration of the density field. The image processing results show that the accuracy of the quantitative analysis of the schlieren interferogram can be improved and a lot of time can be saved in dealing with optical experimental results. Therefore, the algorithm used here is useful and efficient.     

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     

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     

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     

Evaluation of aero-optical distortion effects in PIV

Aero-optical distortion effects on the accuracy of particle image velocimetry (PIV) are investigated. When the illuminated particles are observed through a medium that is optically inhomogeneous due to flow compressibility, the resulting particle image pattern is subjected to deformation and blur. In relation to PIV two forms of error can be identified: position error and velocity error. In this paper a model is presented that describes these errors and particle image blur in relation to the refractive index field of the flow. In the case of 2D flows the model equations can be simplified and, furthermore, the background oriented schlieren technique (BOS) can be applied as a means to assess and correct for the optical error in PIV. The model describing the optical distortion is validated by both computer simulation and real experiments of 2D flows. Two flow features are considered: one with optical distortion normal to the velocity (shear layer) and one with optical distortion in the direction of the flow (expansion fan). Both simulation and experiments demonstrate that the major source for the velocity error is the second derivative of the refractive index in the direction of the velocity vector. The aero-optical distortion effect is less critical for shearing interfaces in comparison with compression/expansion fronts, the most critical case being represented by shock waves. Based on the results from the simulated experiments, it is concluded that for the 2D flow case the BOS method allows a measurement of the mean velocity error in PIV and can reduce it to a large extent.     

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     

Impact of density stratification on the global mode in a swirling jet: Stochastic modelling and Lagrangian coherent structures

In an experimental investigation, the stochastic dynamics of the global mode in a turbulent swirling jet are considered. From the application of the swirling jet in gas turbine combustors, it was observed that a specific density gradient in the flow leads to a suppression of the global mode. This phenomenon was replicated in a generic swirling jet using an electrical heating coil placed inside the breakdown bubble. In the present investigation, the dynamics of the global mode obtained from PIV and pressure measurements are analysed using a stochastic reduced-order model to describe the instability. The stochastic model is necessary to explain the interaction between the deterministic dynamics of the global mode and the perturbations by the background turbulence. The calibration of the stochastic model provides the amplification rate of the global mode that defines the transition of the flow, dependent on the swirling strength and the density difference. The spatial structure of the global mode is further investigated from Lagrangian coherent structures of the flow field which are computed from the 3D time-resolved velocity field reconstruction based on planar PIV measurements. The Lagrangian visualisations and schlieren visualisations are used to explain the absence of the density effects on the global mode at larger Reynolds numbers. The analysis gives a detailed view of the stochastic dynamics of a hydrodynamic instability in a turbulent flow.     

Flash lighting instrumentation for color schlieren shock wave photography

Single-shot color schlieren photography of supersonic flow phenomena requires a technically complex flash light source. In particular, it should be of ultrashort duration to minimize motion blur, of high intensity to expose regular color film with only one flash, and should emit an almost white spectrum to provide the full scale of possible hues. Furthermore, matching Settles' (1970) direction indicating color dissection method the source should be of an areal geometry to uniformly illuminate a tricolor filter in the source plane. Two different types of light sources have been constructed and compared to each other: an array of five Xe-filled linear flash lamps and a module of eight closely stacked U-shaped loops of micro gliding sparks. The spark module consists of a total of 280 individual micro gliding discharges which are all arranged uniformly within an area of 40×40 mm. Connected in a series and pulsed by a modified eight-stage 15 kV Marx-surge generator they are operated in a nitrogen gas atmosphere doped with xenon by about 15% of volume at a total pressure of 1.5 bar. The emitted flash has a peak light intensity of 29 Mcd and a flash duration of 292 ns FWHM. Advantageously in comparison to flash lamps, the flash of a gliding spark module is shorter and free of oscillatory afterglow, thus providing clear and sharp schlieren photos of shock waves which, for example, are purged from preceding phantom shock fronts.     

Dual-plane stereo particle image velocimetry (DSPIV) for measuring velocity gradient fields at intermediate and small scales of turbulent flows

A two-frequency dual-plane stereo particle image velocimetry (DSPIV) technique is described for highly resolved measurements of the complete nine-component velocity gradient tensor field u_i/x_j on the quasi-universal intermediate and small scales of turbulent flows. The method is based on two simultaneous, independent stereo particle image velocimetry (PIV) measurements in two differentially spaced light sheet planes, with light sheet characterization measurements demonstrating the required sheet thicknesses, separation, and two-axis parallelism that determine the measurement resolution and accuracy. The present approach uses an asymmetric forward–forward scatter configuration with two different laser frequencies in conjunction with filters to separate the scattered light onto the individual stereo camera pairs, allowing solid metal oxide particles to be used as seed particles to permit measurements in nonreacting as well as exothermic reacting turbulent flows.