MASTER'S THESIS An Assessment of Colour Schlieren Photography for Supersonic Flows Jens Kanje Nordberg 2015 Master of Science in Engineering Technology Space Engineering Luleå University of Technology Department of Computer Science, Electrical and Space Engineering An Assessment of Colour Schlieren Photography for Supersonic Flow Visualization Jens Kanje Nordberg [email protected] 2015-10-12 1 October 12, 2015 Jens Kanje Nordberg Abstract Research in aerodynamics and flow physics often require visual experiments in order to understand the phenomenon at work. These experiments can be very sensitive, and in order to capture and image these events a non-intrusive imaging technique is very useful. Schlieren photography is such a technique, able to capture changes in density in a fluid, by recording light waves refracted along the light ray path through the medium. This is all done without injecting anything into the flow field, such as dye or small objects, which is common in other techniques. For supersonic research purposes schlieren has been used for a long time, however colour schlieren may be able to capture additional information about the flow in the experiment. Colour schlieren can be achieved in many different ways, however this thesis focuses on the use of two main techniques. For supersonic flow imaging, a density gradient directiona detection schlieren system and a density gradient magnitude detection system. The magnitude indicating system has been designed, however not tested (due to practical complications). The gradient direction system was tested and analysed in order to assess the use of this technique in supersonic flow research. This thesis concludes that this is indeed a very good method, capable of distinguishing different directions in the density gradient (coded by colour) as well as providing a clear image of the flow in the medium. These results may also be used quantitatively in order to calculate more precise gradient directions. 2 October 12, 2015 Jens Kanje Nordberg Preface The aim of this thesis is to investigate and assess the use of colour schlieren photography for supersonic flows, using techniques in this area to produce colour schlieren images. Schlieren photography is an old, non-intrusive technique used to look at changes in density in a fluid, mainly for scientific experiments in aerodynamics and flow physics. The use of colour as an additional way to convey information in schlieren images has been around since the middle of the 20th century, and many different techniques has been tried before. This can be an important technique in flow studies, since it is inherently non-intrusive, and doesn’t disturb the flow in any way, which other methods can not always guarantee. Implementation of this can be done in several ways, which is also investigated in this thesis report. 3 October 12, 2015 Jens Kanje Nordberg Contents 1 Introduction 7 2 Schlieren Photography 8 2.1 History ............................................ 8 2.2 DifferenttypesofSchlierensystems. .............. 9 2.2.1 LensSchlierenSystems ............................. 9 2.2.2 MirrorSchlierenSystems . 9 2.3 Schlierenproblems ................................. 11 2.3.1 Coma ............................................. 11 2.3.2 Astigmatism . 11 2.3.3 Chromatic aberration . 13 3 Schlieren Theory 14 3.1 SchlierenObject .................................. 16 3.2 LightSource ......................................... 17 3.3 Introducingcolourintoaschlierensystem . 18 3.3.1 Directionindicatingsystem . 18 3.3.2 Magnitude indicating system . 18 3.4 Design of colour masks . 19 3.4.1 Design of round source mask . 19 3.4.2 Design of Bulls Eye Mask . 20 4 Objectives 22 5 Experiment setup and equipment 23 5.1 Software programs . 23 5.2 MirrorsandLenses.................................... 23 5.2.1 Cylindricallens.................................. 23 5.2.2 Achromatic doublet . 23 5.3 Camera ............................................ 24 5.3.1 Camera software . 24 5.4 Irisdiaphragm ....................................... 24 5.5 LightSource ......................................... 24 5.6 JetRig........................................... 24 5.6.1 JetNozzles ........................................ 26 5.7 Colour Calibration . 27 6 Procedures 28 6.1 EquipmentSetupAlignment. 28 6.2 SourceMaskProduction............................... 29 6.2.1 Source Mask Layering . 29 6.3 Astigmatism Correction . 30 6.3.1 Measuring the Astigmatic Difference . 30 6.4 Taking a Colour Schlieren Image . 30 6.4.1 Camera Software Settings . 30 7 Results 32 7.1 Astigmatism Correction . 32 7.2 NozzleEffects....................................... 33 7.3 Monochromatic Schlieren Comparisons . 33 7.4 CameraEffects ...................................... 35 7.5 FinalImages ........................................ 36 4 October 12, 2015 Jens Kanje Nordberg 8 Conclusion 38 5 October 12, 2015 Jens Kanje Nordberg Nomenclature Symbol Units Description L m Length Ma − Mach number p Pa Pressure ρ kg/m3 Density n − Refraction index T K Temperature ε ◦ Refraction angle 6 October 12, 2015 Jens Kanje Nordberg 1 Introduction In many areas of fluid flow research, visualization of the flows that are studied are important, and can be achieved in a number of ways, most of which involves adding small particles into the fluid, which then can be observed. Many of these methods are quite intrusive, and the particles introduced into the flow have a direct effect on the flow phenomenon that are studied within the fluid flow field. Pure optical methods does not introduce any particles or tracers into the fluid, and is often the only means to study the inner processes of flow fields and phenomenon occurring in fluids. For shockwave and Mach-number flow studies, relies mostly on optical methods, since the addition of particles or tracers in the fluid might both disturb the fluid flow, and not follow the flow characteristics, thereby giving a false representation of the flow. Schlieren photography is a collective term used for many different (but similar) optical flow visualization techniques, where the common factor is the detection of changes in density in the fluid flow. These changes in the transparent media can be visualized in several ways, but relies on the principle that light rays are refracted at an angle when passing through a region of changing fluid density. Ever since the days of Robert Hooke[2] in 1665 and later August Toepler[3] in the 1860s, schlieren photography has been under investigation, development and practice, being used in many different fields, but primarily in fluid flow research and aerodynamics. One of the true pioneers in schlieren photography in the early 1900s was Hurbert Schardin, who further developed and improved the state of flow visualization with his illustratrative techniques, for example in his work with spherical shock wave research using various schlieren techniques [12] in 1954. Schlieren photography can with modern computer techniques and numerical methods give quantitative results, and not only qualitative, which has been the limitation with many optical techniques for a long time. With image processing and computer algorithms these techniques can give more complex and useful information about the flow fields and fluid phenomenon that are studied. Colour schlieren has been in use for some time, with one of the first uses often credited to Paul Chords in 1967[8]. The same method (dissection method) has then been improved and used for a variety of purposes, for example in 1991 by Harald Kleine [4] in shock wave research. Various other forms of colour schlieren has also been used earlier, such as rainbow schlieren, a quantitative method used to measure the magnitude of a density gradient in the flow, notably in 3D - schlieren tomography by Agrawal, Butuk, Gollahalli and Griffin [11] in 1998, and with jet research by Nakajima, Oka, Konishi, Ono and Miyazato Y [5] in 2014. The Laboratory for Turbulence Research in Aerospace and Combustion (LTRAC) at Monash University has used monochromatic high-speed schlieren photography very successfully in visualizing various flows, for example impinging jets (Mitchell, Honnery and Soria [9]) and high-velocity jet for research in jet screech and noise (see Mitchell, Oberleithner, Honnery and Soria [10]). A colour schlieren setup has not been done, however, which is then a suitable task for this thesis project. The existing monochromatic schlieren system is to be configured into a setup capable of producing high-quality colour schlieren images, with the intention to use two-dimensional schlieren images to represent flow field density gradients. This system offers several advantages to a monochromatic system, such as the ability to detect gradients in all directions with a single image. 7 October 12, 2015 Jens Kanje Nordberg 2 Schlieren Photography The following sections gives a brief introduction to schlieren photography, its history, use and development, as well as its theoretical background and some of its problems. 2.1 History The schlieren method were first experimented with by Robert Hooke (1635-1703)[1], when he was working with microscopy, telescopy, glass and optical testing, which are all related to the schlieren method. His research in these areas, along with his fascination with refraction in the atmosphere, led to his establishment of the scientific field of optics in inhomogeneous mediums. Hooke’s primary schlieren system consisted of a candle, a lens, and his eye. With the candle as a light source, the lens projected the image of the candle onto the pupil of his eye, and he was able