Schlieren and Shadowgraph Techniques: Visualizing Phenomena in Transport Media

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.