Hand-held Schlieren Photography with Light Field probes

We introduce a new approach to capturing refraction in transparent media, which we call Light Field Background Oriented Schlieren Photography (LFBOS). By optically coding the locations and directions of light rays emerging from a light field probe, we can capture changes of the refractive index field between the probe and a camera or an observer. Rather than using complicated and expensive optical setups as in traditional Schlieren photography we employ commodity hardware; our prototype consists of a camera and a lenslet array. By carefully encoding the color and intensity variations of a 4D probe instead of a diffuse 2D background, we avoid expensive computational processing of the captured data, which is necessary for Background Oriented Schlieren imaging (BOS). We analyze the benefits and limitations of our approach and discuss application scenarios.GRANT NC

Computational Schlieren Photography with Light Field Probes

We introduce a new approach to capturing refraction in transparent media, which we call light field background oriented Schlieren photography. By optically coding the locations and directions of light rays emerging from a light field probe, we can capture changes of the refractive index field between the probe and a camera or an observer. Our prototype capture setup consists of inexpensive off-the-shelf hardware, including inkjet-printed transparencies, lenslet arrays, and a conventional camera. By carefully encoding the color and intensity variations of 4D light field probes, we show how to code both spatial and angular information of refractive phenomena. Such coding schemes are demonstrated to allow for a new, single image approach to reconstructing transparent surfaces, such as thin solids or surfaces of fluids. The captured visual information is used to reconstruct refractive surface normals and a sparse set of control points independently from a single photograph.Natural Sciences and Engineering Research Council of CanadaAlfred P. Sloan FoundationUnited States. Defense Advanced Research Projects Agency. Young Faculty Awar

Initial operation and calibration of the UMR supersonic axisymmetric wind tunnel

Initial testing and a preliminary calibration were conducted in the UMR supersonic variable Mach number axisymmetric blowdown wind tunnel facility. During initial operations problems were encountered with control valve seat failure, control valve response and stability, and automatic operation. After having brittle fracture failures with seats made of nylon, Teflon and Telfon-copper composite, a copper seat was found to perform satisfactorily with minimal valve leakage. Control valve response and stability were greatly influenced by the setting of the needle valve located between the total pressure probe and the controller. The needle valve setting was observed to depend upon the stagnation pressure, the supply pressure, and the rate at which the valve was stroked. The result of each of the various methods of automatic operation tried was a pressure overshoot, bursting bypass diaphragms. Through manual operation excellent repeatability of stagnation pressure was obtained. The tunnel calibration vats performed at a stagnation pressure of 180 psig. By means of schlieren flow visualization the free jet was found to be completely expanded at this pressure. From cone-shock angle measurements the test section Mach number was approximately 2.8. A more discreetly defined nozzle contour and improved machining and polishing techniques are needed to rid the flow field of Mach waves emanating from the nozzle. An estimate of the run time by an empirical quasi-steady isentropic analysis was found to be in good agreement with the experimentally determined value --Abstract, page ii

슐리렌 기법 및 레일리 산란을 이용한 초음속 제트의 고주파 밀도 변화 계측

학위논문(석사)--서울대학교 대학원 :공과대학 기계항공공학부,2019. 8. 황원태.In high speed transportation, supersonic jet flows can appear in the propulsion system. Vehicle power efficiency is correlated with the jet flow structure and its physical characteristics. Thus, it is important to understand supersonic jet flow structure, but these flows generally have high fluctuation components which are difficult to measure with traditional diagnostic techniques. Using non-intrusive optical diagnostics can solve this problem. In this paper, Schlieren photography and Rayleigh scattering were utilized to measure density fluctuations of a supersonic jet. The shockwave structure of an over-expanded supersonic jet at Mach 1.5 was first observed using Schlieren photography. Next, high frequency density fluctuations were measured using laser Rayleigh scattering with a sample rate of 250 million per second. The experiments were carried out for four different nozzle pressure ratio (NPR) cases. The unique shockwave structure of each case was quantitatively and qualitatively analyzed.고속 수송체에서는 초음속 제트 유동이 추진 장치에 나타날 수 있다.수송체 동력 효율은 제트 유동 구조 및 그 물리적 특정과 상관관계가 있다. 따라서 초음속 제트 유동 구조를 이해하는 것은 중요하다. 그러나 일반적으로 이러한 유동은 전통적인 진단 기술로는 측정하기 어려운 높은 변동 요소를 가지고 있다. 이러한 문제는 비침입성 광학 진단을 이용하면 해결할 수 있으며 이 논문에서는 슐리렌 기법과 레일리 산란을 이용하여 초음속 제트의 밀도 변화를 측정하였다. 먼저 마하1.5 에서 과도하게 팽창된 초음속 제트의 충격파 구조를 슐리렌 기법으로 관찰하였다. 다음 초당 2 억 5 천만개 샘플 속도로 레이저 레일리 산란을 이용하여 고주파 밀도 변화를 측정하였다. 실험은 네 가지 다른 노즐 압력비 ( 의 경우에 대해 수행되었으며 각 경우의 고유한 충격파 구조에 대해 정량적으로 , 정성적으로 분석하였다.Abstract Contents List of Figures List of Tables Nomenclature Chapter 1. Introduction 1 Chapter 2. Experimental method 5 2.1 Experimental set up 5 2.2 Measurement techniques 6 2.2.1 Schlieren photography 6 2.2.2 Rayleigh scattering 7 Chapter 3. Experimental results 12 3.1 Schlieren photography 12 3.1.1 Shockwave structure 12 3.1.2 Oblique shock wave angle 13 3.2 Rayleigh scattering 14 3.2.1 Photon counting method 14 3.2.2 Shockwave structure and density fluctuation correlation 17 Chapter 4. Discussion 32 4.1 Comparison of average and fluctuation density 32 Chapter 5. Conclusion 33 Bibliography 34 Abstract in Korean 36Maste

Principles and Techniques of Schlieren Imaging Systems

This paper presents a review of modern-day schlieren optics system and its application. Schlieren imaging systems provide a powerful technique to visualize changes or nonuniformities in refractive index of air or other transparent media. With the popularization of computational imaging techniques and widespread availability of digital imaging systems, schlieren systems provide novel methods of viewing transparent fluid dynamics. This paper presents a historical background of the technique, describes the methodology behind the system, presents a mathematical proof of schlieren fundamentals, and lists various recent applications and advancements in schlieren studies

Refractive shape from light field distortion

Acquiring transparent, refractive objects is challenging as these kinds of objects can only be observed by analyzing the distortion of reference background patterns. We present a new, single image approach to reconstructing thin transparent surfaces, such as thin solids or surfaces of fluids. Our method is based on observing the distortion of light field background illumination. Light field probes have the potential to encode up to four dimensions in varying colors and intensities: spatial and angular variation on the probe surface; commonly employed reference patterns are only two-dimensional by coding either position or angle on the probe. We show that the additional information can be used to reconstruct refractive surface normals and a sparse set of control points from a single photograph

A study of two phase detonation as it relates to rocket motor combustion instability

Two-phase detonation in rocket motor combustion instability - production of monodisperse spray

An experimental study of mixing in the wake of arcs

Imperial Users onl

Refraction Wiggles for Measuring Fluid Depth and Velocity from Video

We present principled algorithms for measuring the velocity and 3D location of refractive fluids, such as hot air or gas, from natural videos with textured backgrounds. Our main observation is that intensity variations related to movements of refractive fluid elements, as observed by one or more video cameras, are consistent over small space-time volumes. We call these intensity variations “refraction wiggles”, and use them as features for tracking and stereo fusion to recover the fluid motion and depth from video sequences. We give algorithms for 1) measuring the (2D, projected) motion of refractive fluids in monocular videos, and 2) recovering the 3D position of points on the fluid from stereo cameras. Unlike pixel intensities, wiggles can be extremely subtle and cannot be known with the same level of confidence for all pixels, depending on factors such as background texture and physical properties of the fluid. We thus carefully model uncertainty in our algorithms for robust estimation of fluid motion and depth. We show results on controlled sequences, synthetic simulations, and natural videos. Different from previous approaches for measuring refractive flow, our methods operate directly on videos captured with ordinary cameras, do not require auxiliary sensors, light sources or designed backgrounds, and can correctly detect the motion and location of refractive fluids even when they are invisible to the naked eye.Shell ResearchMotion Sensing Wi-Fi Sensor Networks Co. (Grant 6925133)National Science Foundation (U.S.). Graduate Research Fellowship (Grant 1122374)Microsoft Research (PhD Fellowship