Focusing-Schlieren PIV Measurements of a Supersonic Turbulent Boundary Layer
05 Jan 2009-
TL;DR: A focusing-schlieren optical system has been developed for performing velocity measurements in refractive turbulent flows using commercial particle image velocimetry (PIV) algorithms.
Abstract: A focusing-schlieren optical system has been developed for performing velocity measurements in refractive turbulent flows using commercial particle image velocimetry (PIV) algorithms. Focusing-schlieren optics allows the visualization of refractive disturbances within a limited depth-of-focus, resulting in quasi-planar schlieren images. The schlieren “PIV” technique makes use of naturally-occurring refractive-turbulent eddies in a flow as PIV “particles” upon which velocimetry is performed. Current experiments are performed in a small supersonic wind tunnel to measure the Mach 3 turbulent boundary layer mean-velocity profile. Results from both focusing-schlieren PIV and shadowgraph PIV are compared to the velocity profile from a standard pitot-pressure survey. The natural intermittency of the outer part of the turbulent boundary layer plays a role in the schlieren PIV results, but useful measurements of the velocity profile can still be made. We also introduce an important improvement in schlieren “PIV”, the use of a pulsed LED light source in place of the twin pulsed lasers typically required for traditional PIV measurements. This comparatively-inexpensive white-light source eliminates the traditional problems of laser illumination in schlieren optical systems and improves the overall results.
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TL;DR: In this article, the authors investigated the use of high-power light-emitting diode (LED) illumination for tomographic particle image velocimetry (PIV) as an alternative to traditional laser-based illumination.
Abstract: This paper investigates the use of high-power light-emitting diode (LED) illumination for tomographic particle image velocimetry (PIV) as an alternative to traditional laser-based illumination. Modern solid-state LED devices can provide averaged radiant power in excess of 10 W and by operating the LED with short high current pulses theoretical pulse energies up to several tens of mJ can be achieved. In the present work, a custom-built drive circuit is used to drive a Luminus PT-120 high-power LED at pulsed currents of up to 150 A and 1 μs duration. Volumetric illumination is achieved by directly projecting the LED into the flow to produce a measurement volume of ≈3–4 times the size of the LED die. The feasibility of the volumetric LED illumination is assessed by performing tomographic PIV of homogenous, grid-generated turbulence. Two types of LEDs are investigated, and the results are compared with measurements of the same flow using pulsed Nd:YAG laser illumination and DNS data of homogeneous isotropic turbulence. The quality of the results is similar for both investigated LEDs with no significant difference between the LED and Nd:YAG illumination. Compared with the DNS, some differences are observed in the power spectra and the probability distributions of the fluctuating velocity and velocity gradients. These differences are attributed to the limited spatial resolution of the experiments and noise introduced during the tomographic reconstruction (i.e. ghost particles). The uncertainty in the velocity measurements associated with the LED illumination is estimated to approximately 0.2–0.3 pixel for both LEDs, which compares favourably with similar tomographic PIV measurements of turbulent flows. In conclusion, the proposed high-power, pulsed LED volume illumination provides accurate and reliable tomographic PIV measurements in water and presents a promising technique for flow diagnostics and velocimetry.
46 citations
TL;DR: In this article, a wind tunnel is constructed to provide stable supersonic freestream for fuel injection into supersonically cross flow, and a dimensionless model is deduced and analyzed.
24 citations
28 Jun 2010
TL;DR: Schlieren and shadowgraph techniques are hundreds of years old, yet several important developments have occurred in the last decade, as summarized in this paper as discussed by the authors, which can be useful, for example, in cases where particles cannot be seeded in a turbulent flow under study.
Abstract: Schlieren and shadowgraph techniques are hundreds of years old, yet several important developments have occurred in the last decade, as summarized in this paper. Progress has been made in using the turbulent eddies of a refractive flow as “particle” tracers for seedless velocimetry. This approach will never supplant standard PIV, but “schlieren PIV” can be useful, for example, in cases where particles cannot be seeded in a turbulent flow under study. Background-oriented schlieren (BOS) has become very popular in just a few years. Given modern image-processing software for PIV or digital image correlation, a digital camera and a proper background, schlieren-like images of all sorts are easy to make without parabolic mirrors or even a knife-edge. “Rainbow schlieren” is the name for a quantitative schlieren instrument that uses color to make density, temperature, or species measurements in steady and unsteady planar or axisymmetric flows. Data acquisition and reduction are highly automated and the range of applications is very broad. Finally, shadowgraphy has not been forgotten: it is the simplest of all the optical flow visualization methods, but is often the best choice for imaging shock waves and turbulence. The addition of a retroreflective screen and a high-speed camera makes direct shadowgraphy a robust tool for the study of large-scale events in harsh environments.
16 citations
TL;DR: A novel structured light-field focusing system is developed for flowfield measurements and visualization that provides true planar, two-dimensional, refractive measurements of flow structures and the limits of the system have been demonstrated as a method for micro-scale visualizations and dense medium imaging.
7 citations
TL;DR: In this article , the supersonic wake of a circular cylinder in Mach 3 flow was studied through high-speed, focussing schlieren photography, and the mean and unsteady behaviour of the separated shear layers, the reattachment process, the recompression wave and the early wake were analysed, and discussed in detail.
Abstract: Abstract The supersonic wake of a circular cylinder in Mach 3 flow was studied through high-speed, focussing schlieren photography. The mean and unsteady behaviour of the separated shear layers, the reattachment process, the recompression wave and the early wake are analysed, and discussed in detail. The fluctuations in the wake are stronger and more coherent than those within the approaching shear layers and the recirculation region. The recompression of the shear layers energises the finer scales in the flow which leads to a departure from a $-$1 spectral roll-off observed in the schlieren spectra further upstream. The recompression wave exhibits low-frequency unsteadiness and a ripple-type motion which occurs as it is perturbed by shocklets radiating from the coherent structures in the wake. The wake consists of coherent disturbances with the same characteristic frequency as that for an incompressible flow over a cylinder; however, this instability is suppressed as the wake accelerates, presumably due to increasing compressibility. The primary instability of the wake flow has a characteristic frequency nearly twice that of its incompressible counterpart and it is shown to be driven by the presence of aeroacoustic resonance in the wake. It is also shown that the resonance, which leads to the formation of broadband standing waves in the wake, is the result of an interaction between the wake instabilities and upstream propagating acoustic waves in the wake. The acoustic waves originate upstream of the reattachment region and are believed to be generated by the unsteady separation on the cylinder surface.
5 citations
References
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Book•
11 Jun 2002TL;DR: In this paper, the authors present a practical guide for the planning, performance and understanding of experiments employing the PIV technique, which is primarily intended for engineers, scientists and students, who already have some basic knowledge of fluid mechanics and nonintrusive optical measurement techniques.
Abstract: This practical guide intends to provide comprehensive information on the PIV technique that in the past decade has gained significant popularity
throughout engineering and scientific fields involving fluid mechanics. Relevant theoretical background information directly support the practical
aspects associated with the planning, performance and understanding of experiments employing the PIV technique. The second edition includes
extensive revisions taking into account significant progress on the technique as well as the continuously broadening range of possible
applications which are illustrated by a multitude of examples. Among the new topics covered are high-speed imaging, three-component methods,
advanced evaluation and post-processing techniques as well as microscopic PIV, the latter made possible by extending the group of authors by
an internationally recognized expert. This book is primarily intended for engineers, scientists and students, who already have some basic
knowledge of fluid mechanics and non-intrusive optical measurement techniques. It shall guide researchers and engineers to design and perform
their experiment successfully without requiring them to first become specialists in the field. Nonetheless many of the basic properties of PIV are
provided as they must be well understood before a correct interpretation of the results is possible.
4,811 citations
01 Jan 1989
TL;DR: In this paper, the authors use hot-wire (HW) or laser velocimetry (LV) to estimate the velocity, vorticity, and pressure fields of wake flows.
Abstract: One of the most challenging and time-consuming problems in experimental fluid mechanics is the measurement of the overall flow field properties, such as the velocity, vorticity, and pressure fields. Local measurements of the velocity field (i.e., at individual points) are now done routinely in many experiments using hot-wire (HW) or laser velocimetry (LV). However, many of the flow fields of current interest, such as coherent structures in shear flows or wake flows, are highly unsteady. HW or LV data of such flows are difficult to interpret, as both spatial and temporal information of the entire flow field are required and these methods are commonly limited to simultaneous measurements at only a few spatial locations.
1,798 citations
01 Jan 1955
TL;DR: In this article, the results of an experimental investigation of a turbulent boundary layer with zero pressure gradient are presented and the importance of the region near the wall and the inadequacy of the concept of local isotropy are demonstrated.
Abstract: The results of an experimental investigation of a turbulent boundary layer with zero pressure gradient are presented. Measurements with the hot-wire anemometer were made of turbulent energy and turbulent shear stress, probability density and flattening factor of u-fluctuation (fluctuation in x-direction), spectra of turbulent energy and shear stress, and turbulent dissipation. The importance of the region near the wall and the inadequacy of the concept of local isotropy are demonstrated. Attention is given to the energy balance and the intermittent character of the outer region of the boundary layer. Also several interesting features of the spectral distribution of the turbulent motions are discussed.
1,122 citations
Book•
19 Oct 2012TL;DR: In this article, the Schlieren approach is used to estimate the sensitivity and range of the Schlieser image. But, the sensitivity of the image is not a function of the light source, but of the background.
Abstract: 1 Historical Background.- 1.1 The 17th Century.- 1.2 The 18th Century.- 1.3 The 19th Century.- 1.4 The 20th Century.- 2 Basic Concepts.- 2.1 Light Propagation Through Inhomogeneous Media.- 2.2 Definition of a Schliere.- 2.3 Distinction Between Schlieren and Shadowgraph Methods.- 2.4 Direct Shadowgraphy.- 2.5 Simple Lens-Type Schlieren System.- 2.5.1 Point Light Source.- 2.5.2 Extended Light Source.- 2.6 On the Aspect of a Schlieren Image.- 3 Toepler's Schlieren Technique.- 3.1 Lens- and Mirror-Type Systems.- 3.1.1 Lens Systems.- 3.1.2 Mirror Systems.- 3.2 Sensitivity.- 3.2.1 Definition and Geometrical Theory.- 3.2.2 Sensitivity Examples.- 3.2.3 The Limits of Sensitivity.- 3.2.4 Sensitivity Enhancement by Post-Processing.- 3.3 Measuring Range.- 3.3.1 Definition of Measuring Range.- 3.3.2 Adjustment of Measuring Range.- 3.4 Estimating the Sensitivity and Range Required.- 3.5 Resolving Power.- 3.6 Diffraction Effects.- 3.6.1 Diffraction Halos Due to Opaque Edges in the Test Area.- 3.6.2 Diffraction at the Knife-Edge.- 3.7 Magnification and Depth of Field.- 3.7.1 Image Magnification and the Focusing Lens.- 3.7.2 Depth of Field.- 4 Large-Field and Focusing Schlieren Methods.- 4.1 Large Single- and Double-Mirror Systems.- 4.1.1 Availability of Large Schlieren Mirrors.- 4.1.2 Examples of Large-Mirror Systems.- 4.1.3 Perm State's 1-Meter Coincident Schlieren System.- 4.2 Traditional Schlieren Systems with Large Light Sources.- 4.3 Lens-and-Grid Techniques.- 4.3.1 Simple Background Distortion.- 4.3.2 Background Grid Distortion.- 4.3.3 Large Colored Grid Background.- 4.3.4 The Modern Focusing/Large-Field Schlieren System.- 4.3.5 Penn State's Full-Scale Schlieren System.- 4.4 Large-Field Scanning Schlieren Systems.- 4.4.1 Scanning Schlieren Systems for Moving Objects.- 4.4.2 Schlieren Systems with Scanning Light Source and Cutoff.- 4.5 Moire-Fringe Methods.- 4.6 Holographic and Tomographic Schlieren.- 5 Specialized Schlieren Techniques.- 5.1 Special Schlieren CutoffsIll.- 5.1.1 Graded Filters.- 5.1.2 Exponential Cutoffs and Source Filters.- 5.1.3 Matched Spatial Filters at Source and Cutoff.- 5.1.4 Phase Contrast.- 5.1.5 Photochromic and Photorefractive Cutoffs.- 5.2 Color Schlieren Methods.- 5.2.1 Reasons for Introducing Color.- 5.2.2 Conversion from Monochrome to Color Schlieren.- 5.2.3 Classification of Color Schlieren Techniques.- 5.2.4 Recent Developments.- 5.3 Stereoscopic Schlieren.- 5.4 Schlieren Interferometry.- 5.4.1 The Wollaston-Prism Shearing (Differential) Interferometer.- 5.4.2 Diffraction-Based Schlieren Interferometers.- 5.5 Computer-Simulated Schlieren.- 5.6 Various Specialized Techniques.- 5.6.1 Resonant Refractivity and the Visualization of Sound.- 5.6.2 Anamorphic Schlieren Systems.- 5.6.3 Schlieren Observation of Tracers.- 5.6.4 Two-View Schlieren.- 5.6.5 Immersion Methods.- 5.6.6 Infrared Schlieren.- 6 Shadowgraph Techniques.- 6.1 Background.- 6.1.1 Historical Development.- 6.1.2 The Role of Shadowgraphy.- 6.1.3 Advantages and Limitations.- 6.2 Direct Shadowgraphy.- 6.2.1 Direct Shadowgraphy in Diverging Light.- 6.2.2 Direct Shadowgraphy in Parallel Light.- 6.3 "Focused" Shadowgraphy.- 6.3.1 Principle of Operation.- 6.3.2 History and Terminology.- 6.3.3 Advantages and Limitations.- 6.3.4 Magnification, Illuminance, and the Virtual Shadow Effect.- 6.3.5 "Focused" Shadowgraphy in Ballistic Ranges.- 6.4 Specialized Shadowgraph Techniques.- 6.4.1 Large-Scale Shadowgraphy.- 6.4.2 Microscopic, Stereoscopic, and Holographic Shadowgraphy.- 6.4.3 Computed Shadowgraphy.- 6.4.4 Conical Shadowgraphy.- 7 Practical Issues.- 7.1 Optical Components.- 7.1.1 Light Sources.- 7.1.2 Mirrors.- 7.1.3 Schlieren Cutoffs and Source Filters.- 7.1.4 Condensers and Source Slits.- 7.1.5 The Required Optical Quality.- 7.2 Equipment Fabrication, Alignment, and Operation.- 7.2.1 Schlieren System Design Using Ray Tracing Codes.- 7.2.2 Fabrication of Apparatus.- 7.2.3 Setup, Alignment, and Adjustment.- 7.2.4 Vibration and Mechanical Stability.- 7.2.5 Stray Light, Self-Luminous Events, and Secondary Images.- 7.2.6 Interference from Ambient Airflows.- 7.3 Capturing Schlieren Images and Shadowgrams.- 7.3.1 Photography and Cinematography.- 7.3.2 Videography.- 7.3.3 High-Speed imaging.- 7.3.4 Front-Lighting.- 7.4 Commercial and Portable Schlieren Instruments.- 7.4.1 Soviet Instruments.- 7.4.2 Western Instruments.- 7.4.3 Portable Schlieren Apparatus.- 8 Setting Up Your Own Simple Schlieren and Shadowgraph System.- 8.1 Designing the Schlieren System.- 8.2 Determining the Cost.- 8.3 Choosing a Setup Location.- 8.4 Aligning the Optics.- 8.5 Troubleshooting.- 8.6 Recording the Schlieren Image or Shadowgram.- 8.7 Conclusion.- 9 Applications.- 9.1 Phenomena in Solids.- 9.1.1 Glass Technology.- 9.1.2 Polymer-Film Characterization.- 9.1.3 Fracture Mechanics and Terminal Ballistics.- 9.1.4 Specular Reflection from Surfaces.- 9.2 Phenomena in Liquids.- 9.2.1 Convective Heat and Mass Transfer.- 9.2.2 Liquid Surface Waves.- 9.2.3 Liquid Atomization and Sprays.- 9.2.4 Ultrasonics.- 9.2.5 Water Tunnel Testing and Terminal Ballistics.- 9.3 Phenomena in Gases.- 9.3.1 Agricultural Airflows.- 9.3.2 Aero-Optics.- 9.3.3 Architectural Acoustics.- 9.3.4 Boundary Layers.- 9.3.5 Convective Heat and Mass Transfer.- 9.3.6 Heating, Ventilation, and Air-Conditioning.- 9.3.7 Gas Leak Detection.- 9.3.8 Electrical Breakdown and Discharge.- 9.3.9 Explosions, Blasts, Shock Waves, and Shock Tubes.- 9.3.10 Ballistics.- 9.3.11 Gas Dynamics and High-Speed Wind Tunnel Testing.- 9.3.12 Supersonic Jets and Jet Noise.- 9.3.13 Turbomachinery and Rotorcraft.- 9.4 Other Applications.- 9.4.1 Art and music.- 9.4.2 Biomedical Applications.- 9.4.3 Combustion.- 9.4.4 Geophysics.- 9.4.5 Industrial Applications.- 9.4.6 Materials Processing.- 9.4.7 Microscopy.- 9.4.8 Optical Processing.- 9.4.9 Optical Shop Testing.- 9.4.10 Outdoor Schlieren and Shadowgraphy.- 9.4.11 Plasma Dynamics.- 9.4.12 Television Light Valve Projection.- 9.4.13 Turbulence.- 10 Quantitative Evaluation.- 10.1 Quantitative Schlieren Evaluation by Photometry.- 10.1.1 Absolute Photometric Methods.- 10.1.2 Standard Photometric Methods.- 10.2 Grid-Cutoff Methods.- 10.2.1 Focal Grids.- 10.2.2 Defocused Grids.- 10.2.3 Defocused Filament Cutoff.- 10.3 Quantitative Image Velocimetry.- 10.3.1 Background.- 10.3.2 Multiple-Exposure Eddy and Shock Velocimetry.- 10.3.3 Schlieren Image Correlation Velocimetry.- 10.3.4 Focusing Schlieren Deflectometry.- 10.3.5 The Background-Oriented Schlieren System.- 10.4 Quantitative Shadowgraphy.- 10.4.1 Double Integration of d2n/ dy2.- 10.4.2 Turbulence Research.- 10.4.3 Shock-Wave Strength Quantitation.- 10.4.4 Grid Shadowgraphy Methods.- 11 Summary and Outlook.- 11.1 Summary.- 11.1.1 Perceptions Outside the Scientific Community.- 11.1.2 Other Lessons Learned.- 11.1.3 Further Comments on Historical Development.- 11.1.4 Further Comments on Images and Visualization.- 11.1.5 Renewed Vitality.- 11.2 Outlook: Issues for the Future.- 11.2.1 Predictions.- 11.2.2 Opportunities.- 11.2.3 Recommendations.- 11.3 Closing Remarks.- References.- Appendix A Optical Fundamentals.- A. 1 Radiometry and Photometry.- A.2 Refraction Angle 8.- A.2.1 Small Optical Angles and Paraxial Space.- A.2.2 Huygens' Principle and Refraction.- A.3 Optical Components and Devices.- A.3.1 Conjugate Optical Planes.- A.3.2 Lensf/number.- A.3.3 The Thin-Lens Approximation.- A.3.4 Viewing Screens and Ground Glass.- A.3.5 Optical Density.- A.4 Optical Aberrations.- A.5 Light and the Human Eye.- A.6 Geometric Theory of Light Refraction by a Schliere.- Appendix B The Schlieren System as a Fourier Optical Processor.- B. 1 The Basic Fourier Processor with no Schlieren Present.- B.2 The Addition of a Schlieren Test Object.- B.3 The Schlieren Cutoff.- B.4 Other Spatial Filters.- B.5 Partially-Coherent and Polychromatic Illumination.- Appendix C Parts List for a Simple Schlieren/ Shadowgraph System.- C.l Optics.- C.2 Illumination.- C.3 Miscellaneous Components.- C.4 Optical Mounts.- Appendix D Suppliers of Schlieren Systems and Components.- D.l Complete Schlieren Systems.- D.2 Schlieren Field Mirrors.- D.3 Light Sources.- D.4 Components.- D.5 Focusing Schlieren Lenses.- D.6 Miscellaneous.- Color Plates.
935 citations
TL;DR: In this paper, the velocity field of human cough was measured using particle image velocimetry (PIV) and the average width of all coughs ranged between 35 to 45 mm.
Abstract: Cough generated infectious aerosols are of interest while developing strategies for the mitigation of disease risks ranging from the common cold to SARS. In this work, the velocity field of human cough was measured using particle image velocimetry (PIV). The project subjects (total 29) coughed into an enclosure seeded with stage fog. Cough flow velocity profiles, average widths of the cough jet, and maximum cough velocities were measured. Maximum cough velocities ranged from 1.5 m/s to 28.8 m/s. The average width of all coughs ranged between 35 to 45 mm. Wide variability in the data suggests that future cough simulations consider a range of conditions.
433 citations