A Review of Recent Developments in Schlieren and Shadowgraph Techniques

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A Review of Recent Developments in Schlieren and Shadowgraph Techniques Home Search Collections Journals About Contact us My IOPscience A review of recent developments in schlieren and shadowgraph techniques This content has been downloaded from IOPscience. Please scroll down to see the full text. 2017 Meas. Sci. Technol. 28 042001 (http://iopscience.iop.org/0957-0233/28/4/042001) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 132.174.254.159 This content was downloaded on 15/02/2017 at 17:21 Please note that terms and conditions apply. IOP Measurement Science and Technology Measurement Science and Technology Meas. Sci. Technol. Meas. Sci. Technol. 28 (2017) 042001 (25pp) doi:10.1088/1361-6501/aa5748 28 Topical Review 2017 A review of recent developments in © 2017 IOP Publishing Ltd schlieren and shadowgraph techniques MSTCEP Gary S Settles1,3 and Michael J Hargather2 042001 1 FloViz Inc., Port Matilda, PA 16870, United States of America 2 Mechanical Engineering Department, New Mexico Tech, Socorro NM 87801, United States of America G S Settles and M J Hargather E-mail: [email protected] and [email protected] Received 9 March 2016, revised 22 December 2016 Accepted for publication 6 January 2017 Published 15 February 2017 Printed in the UK Abstract Schlieren and shadowgraph techniques are used around the world for imaging and measuring MST phenomena in transparent media. These optical methods originated long ago in parallel with telescopes and microscopes, and although it might seem that little new could be expected 10.1088/1361-6501/aa5748 of them on the timescale of 15 years, in fact several important things have happened that are reviewed here. The digital revolution has had a transformative effect, replacing clumsy photographic film methods with excellent though expensive high-speed video 1361-6501 — — cameras, making digital correlation and processing of shadow and schlieren images routine, and providing an entirely-new synthetic schlieren technique that has attracted a lot of 4 attention: background-oriented schlieren or BOS. Several aspects of modern schlieren and shadowgraphy depend upon laptop-scale computer processing of images using an image- capable language such as MATLAB™. BOS, shock-wave tracking, schlieren velocimetry, synthetic streak-schlieren, and straightforward quantitative density measurements in 2D flows are all recent developments empowered by this digital and computational capability. Keywords: schlieren, shadowgraph, high-speed imaging, digital image processing, MATLAB, flow visualization, background-oriented schlieren (Some figures may appear in colour only in the online journal) 1. Introduction ‘white’ light for imaging, schlieren and shadowgraphy are very visual and were once thought to be mere qualitative visualiza- Schlieren and shadowgraphy were formally originated by tion methods lacking the quantitative power of laser interferom- Robert Hooke in the 17th century, well before their time [1]. etry, but that is shown no longer to be true in this review. Upon reinvention by August Toepler in the mid-19th century, The present objective is to review significant advances in schlieren optics (named for streaks or striae) became a stan- schlieren and shadowgraph techniques in the 15 years since [1], dard laboratory tool that played a key role in observing shock especially the influence of digital imaging and image process- waves, to name just one important discovery. Though simpler ing. The expected audience includes current and prospective than schlieren, shadowgraphy came along a bit later. Together, users of these instruments in fluid dynamics and heat transfer these optical tools for seeing the invisible had a great influence research, as well as in many other disciplines. Although the upon wind-tunnel testing and the development of high-speed construction and setup of schlieren and shadowgraph equip- aircraft in the mid-20th century. Today their use is widespread ment remains primarily do-it-yourself, a description is provided across scientific experimentation. Generally using non-coherent of the few commercial instruments that are currently available. Schlieren and shadowgraphy are optical techniques that rely 3 Author to whom any correspondence should be addressed. upon precise lenses, mirrors, and cameras to image refractive 1361-6501/17/042001+25$33.00 1 © 2017 IOP Publishing Ltd Printed in the UK Meas. Sci. Technol. 28 (2017) 042001 Topical Review Figure 1. Diagrams of five types of schlieren and shadowgraph instruments discussed in this paper: (a) Toepler’s lens-type schlieren system, (b) z-type schlieren system using twin parabolic mirrors, (c) direct shadowgraphy, (d) background-oriented schlieren (BOS), (e) focusing (lens-and-grid) schlieren system. disturbances in transparent media. Detailed descriptions of the knife-edge and proceeds to the camera, while another the various optical arrangements, along with the support- refracted downward (solid line) is blocked by the knife-edge. ing theory, are found in [1] and many other references cited Generalizing this to every ray in the light beam, we have an therein. Here, for brevity, five simplified optical systems are instrument that captures a pure phase disturbance S as a gray- diagramed in figure 1. scale amplitude image in the camera. This is the essence of Figure 1(a) shows a classical schlieren setup first used by what schlieren instruments do. Readers familiar with Fourier Toepler [2]. Light from a small bright source LS is collimated optics [3] will also recognize figure 1(a) as a Fourier optical by lens L1 and brought to a focus by lens L2, beyond which processor, the knife-edge acting as a half-plane mask of the the light beam is captured by a camera C. A sharp knife-edge spatial frequency spectrum. KE is inserted at the focus, blocking part of the light. A refrac- The lens diameter in figure 1(a) determines the field-of- tive disturbance in the air between the two lenses, labeled S view of the instrument. For diameters greater than 15 cm or so, for Schlieren, is represented by squiggles. In this simple parabolic telescope mirrors PM1 and PM2 are used instead, diagram a light ray refracted upward (dashed line) avoids figure 1(b), thus folding the optical path and introducing 2 Meas. Sci. Technol. 28 (2017) 042001 Topical Review usually-manageable off-axis aberrations. These mirrors range Against that background, this review considers how new from a few cm up to more than 1 m in diameter, and they rep- cameras, light sources and laptop-scale computer image pro- resent by far the most common class of traditional schlieren cessing have recently changed schlieren and shadowgraphy. instruments. A simple and effective single-mirror, double-pass It begins with the shift from photographic film to digital instrument is also useful [1], but is not shown here. imaging. Note that the schlieren arrangements in figures 1(a) and (b) can also produce shadowgrams upon removal of the knife- edge and defocusing the camera lens. (Shadowgraphy cannot 2. Digital versus analog imaging occur when the optics are sharply focused on the schlieren object [1].) A great deal has been said about the transition from photo- Turning to figure 1(c), a very simple and robust optical graphic film to digital imaging. Konica-Minolta stopped mak- arrangement called direct shadowgraphy requires only a small ing their respected line of film cameras in 2006 and Kodak bright light source LS (typically 1 mm or less in diameter), a itself went bankrupt in 2012. Now digital cellphone cameras camera C, and a projection screen SC. Light refraction by S allow everyone to be a photographer, with results published casts, upon the distant screen, a telling shadow of itself that immediately on social media. Some worry that the nuance of we call a shadowgram. More detail on this is found later in film has been lost [7] while others ask how many image mega- section 9 on retroreflective shadowgraphy. pixels are enough. All the darkrooms are now gone, replaced Next comes a new synthetic schlieren technique, back- by image-processing software that makes it too easy, some ground-oriented schlieren or BOS, figure 1(d). As in shadow- believe, to alter the evidence. graphy, a camera images a distant screen, but in this case the For scientific imaging, though, and especially for schlieren screen is a patterned background. Images of schlieren object S and shadowgraphy, there is a strong case in favor of the digital in front of this background show subtle shifts due to refraction revolution and little reason to mourn the demise of film. when compared with an undistorted ‘tare’ background image. Digital image processing is able to measure these shifts 2.1. Digital high-speed video cameras and construct from them a pseudo-schlieren image without the usual optics of figures 1(a) and (b). This background- Traditional film-based high-speed drum, rotating-mirror, distortion effect was known to schlieren pioneer H Schardin rotating-prism, and streak cameras have largely been rendered 80 years ago [4], but modern digital computers and cameras obsolete by a new generation of easy-to-use digital high-speed were necessary to make it a useful schlieren technique, dis- cameras with CMOS sensors, megapixel resolution up to cussed in detail in section 4. 20 000 frames s−1 (fps), and peak frame rates extending to one Finally, figure 1(e) diagrams a ‘lens-and-grid’ or ‘focus- million fps and beyond. Commercial examples of such cam- ing’ schlieren system of the type proposed by Weinstein [5]. eras include the Photron Fastcam SA-Z (photron.com) and the In this case the effective light source is a large source grid SG, Vision Research Phantom V2512 (phantomhighspeed.com). backlit by several lamps whose light is directed by a Fresnel Triggering these cameras for events like shock tube tests, lens FL toward a ‘schlieren lens’ SL, which can be a large- gunshots and explosions is greatly simplified by a rolling format camera lens (a large, precise, expensive lens used in memory buffer that continuously records several real-time sec- studio photography).
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