(Reference: Flow Visualization VII, ed. J. P. Crowder, Begell House, NY, Sept. 1995, pp. 2-13.) For updated coverage of this subject please see Settles, G. S., Schlieren and Shadowgraph Techniques, Springer-Verlag, 2001, ISBN 3-540-66155-7, http://www.mne.psu.edu/psgdl/schlierbookwebpage.html

Full-Scale Schlieren Flow Visualization

Gary S. Settles†, Elizabeth B. Hackett†, James D. Miller†, and Leonard M. Weinstein‡ †Gas Dynamics Lab, Mechanical Engineering Dept.,

Penn State University, University Park, PA 16802 USA ‡NASA Langley Research Center, MS 493, Hampton, VA 23681 USA

Introduction The schlieren technique [1-3] holds a special place in flow visualization because it produces a natural, easily-interpretable image of refractive-index-gradient fields. It is more sensitive than its companion the shadowgraph, and better suited to qualitative visualization than its cousin the interferometer. In its three centuries of existence it has found use in an amazing number of applications across the breadth of science and technology. Perhaps more often than any other type of scientific imagery, schlieren photographs have also been regarded as works of art. However, schlieren instruments have traditionally been limited in field-of-view by the cost of large, high-quality optics. This limit occurs for lenses above a few cm in size. The parabolic mirrors of the largest conventional schlieren systems, found at major wind tunnel labs, provide a field-of-view up to about a meter in diameter, but these are seldom available for general use. Thus, to visualize most sizable phenomena by schlieren, scale-model studies have been required [4]. However, the fabrication of accurate scale models and the need to achieve the correct dynamic scaling of fluid flow and heat transfer has often interfered with this approach. Moreover, a range of important problems such as fire spreading [5], heating, ventilating and air conditioning (HVAC), and the thermal plume of the human body, to name a few, stubbornly refuse to comply with any practical scale-modeling scheme. For these reasons there has been a serious field-of-view mismatch between available schlieren optics and some key phenomena which one might wish to visualize. The present paper reviews several approaches in response to this problem. Optical Principles In some of the cases just mentioned, large conventional schlieren systems can provide a sufficient "window" with which to see significant segments of full-scale flows. One such approach using a single large mirror is described later in this paper. Unconventional schlieren arrangements with large light sources were pioneered by Schardin [1] in the 1940's. These arrangements are fundamentally different from the Toepler method [1-3], which expands the schlieren beam from a small light source in all cases. Schardin described several schlieren arrangements which have extensive light sources, small lenses, and a resulting field-of-view roughly half the size of the light source. In his schlieren method number 4, sketched in Fig. 1, a large source grid consisting of dark and bright lines is imaged by a lens upon the cutoff plane, where light is blocked by a corresponding negative grid. In a separate set of conjugate optical planes the test area is imaged upon the image plane by the same lens. The cutoff grid thus serves as a schlieren stop, preferentially passing any irregular light rays which are refracted by disturbances in the test area. The illumination is inherently non-parallel as a consequence of the extended light source. Such grid-type schlieren instruments were set up at benchtop scale by Burton [6] and by Kantrowitz and Trimpi [7] in the 1940's. Since then the use of grid-type focusing schlieren has

been sporadic and its employment for large fields-of-view has not occurred at all until the recent work of Weinstein [8]. Some attempts in the interim may well have been thwarted by the extreme care necessary in the photographic preparation of the cutoff grid, without which the visualization tends to be disappointing. Optical formulae for a grid-type schlieren system are given by Weinstein [8], and are not repeated here. For present purposes note only that the maximum size of the test area is not limited by the lens aperture, as in conventional schlieren arrangements, but rather by the allowable size of the source grid and the axial distance available in which to set up the optical train. Thus, though its overall dimensions become quite large, it is nonetheless feasible to construct a schlieren system according to Fig. 1 with a field-of-view much larger than any conventional schlieren. A practical realization of this is discussed later in this paper. A closely-related approach for full-scale schlieren visualization, Schardin's [1] system number 5, uses a lens to image a large, distant light-dark boundary upon a single knife edge, as illustrated in Fig. 2. This is the equivalent of having only a single large gridline in the source and a single corresponding gridline in the cutoff plane of the system of Fig. 1. Of course, the approach of Fig. 2 yields schlieren sensitivity mostly along a line through the test area rather than generally across it, but that drawback can be removed by scanning the line of sensitivity across the field of view while recording an image. In fact, this schlieren effect is simple and common enough to be naturally visible when refractive disturbances pass between the eye and the horizon. The light-dark boundary of the horizon then serves as a light source at a distance, L, of some kilometers, while the remaining optical components of Fig. 2 are provided by the lens, iris, and retina of the eye. This sometimes allows one, for example, to see the turbulent flow from a truck exhaust stack while driving. If deliberately exploited, either natural or artificial light-dark boundaries can be used in the manner of Fig. 2 to realize large indoor or huge outdoor schlieren visualizations. The disadvantage of having to scan to obtain a complete schlieren image is offset by the simplicity of the light source compared to the grid-type system described earlier. Moreover, some mobile test subjects naturally lend themselves to this sort of optical traverse. An example of a schlieren system for visualizing the flow about aircraft in flight is described later in this paper. 1-Meter Double-Pass Coincident Schlieren System The most conventional of the full-scale schlieren techniques reviewed here is the double-pass coincident system shown in Fig. 3. It is based upon an f/4 parabolic mirror of one meter diameter obtained from government surplus. Since only one mirror is available, a parallel-light schlieren system is not possible. Instead, the light source is placed on the mirror centerline at its 8 m effective radius of curvature. The beam which illuminates the mirror is thus ideally returned along the same optical path, whence it is deflected by a beamsplitter to image the light source upon a schlieren cutoff followed by a focusing lens and camera. Overall, this optical arrangement is usual for single-mirror schlieren systems, and is amply described in the literature [1-3]. There is, however, one problem: for a perfectly-coincident system one wants a mirror of spherical figure rather than the parabola which was available. This problem was fortunately solved by Dall [9,10] for the Foucault testing of telescope mirrors. A simple plano-convex "corrector" lens of appropriate diameter and focal length is inserted in the schlieren beam after the light source, thus producing spherical aberration of equal magnitude but opposite sign to the difference between the actual parabolic mirror and the desired spherical figure. Uniform-field schlieren imagery is made possible by this means.

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As a consequence of its long focal length and the doubling of sensitivity caused by double passage of the light beam through the test area, this is inherently a high-sensitivity schlieren apparatus. An estimate of the minimum detectable schlieren deflection ε at 50% knife-edge cutoff is found from Weinstein's [8] Eqn. 1 to be 0.6 arcsec. As the knife-edge is advanced, weak air currents along the entire optical path become visible. Many color schlieren images [11] and videotapes have been taken with this equipment over the last decade. From these visualizations, five examples are now given which emphasize the use of the apparatus to reveal full-scale flows. The thermal plume of the human body has been a fascinating subject for full-scale schlieren visualization since the work of Lewis, et al. [12]. Fig. 4 is an image of the plume of a girl standing in profile, taken using the apparatus of Fig. 3 [13]. Fig. 5, an instantaneous image of a human cough in progress, has appeared in scores of publications worldwide as an illustration of airborne disease spread. There is untapped potential here to better understand indoor air quality issues involving human motion and pollutant transport by the human thermal plume [14]. Fig. 6 shows a human subject wearing cleanroom garments in a typical ½ m/sec cleanroom ventilation downflow [15]. The downflow suppressed the body plume, but had the undesirable effect of creating a recirculation region between the upper body and the work table, where particulate contamination becomes trapped. In Fig. 7, J. D. Miller is shown operating a combustion-driven "HVOF" thermal spray torch used to apply metallic coatings to surfaces. The hot supersonic jet produced by this equipment spreads rapidly and vigorously entrains the surrounding air. This visualization was critical to understand the process [16], since direct visual observation belies its true character. Similar observations have been made for sandblasting nozzle flows [17]. Finally, Fig. 8 shows the instantaneous muzzle blast, wake, and bow wave of a bullet being fired from a 22-caliber rifle [18]. Such photos are taken using the apparatus of Fig. 3 with the laboratory darkened, the camera shutter open, and the microsecond stroboscopic light source triggered by a microphone (visible near the bottom of Fig. 8). 2x3-Meter Grid-Type Schlieren System Despite the full-scale studies done with the large conventional schlieren system just described, there were many more which we wished to do but lacked sufficient field-of-view. It first became clear during a discussion among the authors in August, 1993, that it was feasible, given a building of sufficient size, to construct an apparatus according to Fig. 1 which would be by far the largest indoor schlieren system in the world. The initial challenge, finding suitable housing, was eventually met by obtaining the use of a 12.2 x 13.7 m (40'x45') Penn State building formerly used for fruit storage. However, the limited size of this building required some optical design compromises. As shown by the top view in Fig. 9, the schlieren system we designed uses a large front-lit retroreflective source grid mounted against one wall of the building. A beamsplitter is used to fold the optical axis parallel to the opposite wall, thus gaining several meters of effective length. A 10 cm diameter f/6 flat-field aerial camera lens produces a cutoff grid size of 20x25 cm (8"x10") and an image plane of about the same size (dictated by the available size of photographic film). The maximum test area of this apparatus is a rectangle 2.1 m high by 2.67 m wide, or about 7x9 feet, which is more than seven times larger than the field-of-view of the mirror schlieren system described earlier.

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A key element of this new schlieren system, and its second major challenge, was the front-illuminated source grid. It is made of white 3M retroreflective highway-sign material Type 3970-G. This material is mounted to 12 1.2 x 2.4 m x 2.5 mm aluminum sign panels which are assembled on a frame. Upon the retroreflective material are silkscreened 5.08 mm-wide vertical black gridlines spaced 5.08 mm apart. The desired accuracy went beyond normal highway-sign silkscreening practice, and was eventually met using a special direct-emulsion photographic silkscreen process based on a precise 1.2 x 2.4 m gridline template on photographic film. The 2.1 m high by 2.7 m wide test area (See Fig. 9) lies midway between the source grid and lens, is centered 2.4 m above the concrete floor, and has roughly a 1.2 m depth-of-field. Two other test areas are also available, 2 x 2.5 m and 1.8 x 2.3 m in size at locations nearer the lens. The next challenge we faced was the beamsplitter, which has a high optical flatness requirement. A 35 x 51 x 2.5 cm glass beamsplitter, polished to one wavelength per 2.5 cm and coated for 50% reflectance on its front face and anti-reflection on its back face, was fabricated for us by Sydor Optics Inc. and coated by Evaporated Metal Films Corp. It was made necessary by the limited size of the building, requiring that the optical axis be folded. Source grid lighting is had from a Calumet PS4N studio flash unit having a a 3 kW power supply and a Series 2 flash-head with a 24 cm diameter reflector. A 70 deg. conical diffuser was made of frosted Mylar and fitted over the reflector for more even source grid illumination. 60-125 J of flash energy was typically required for the photographic exposures described below. Oblique frontlighting of solid objects in the test area was done using two additional flash-heads driven by the same power supply, but required ten times more flash energy. Obtaining a high-quality cutoff grid is the final and most important challenge to be met in a grid-type schlieren system. Our cutoff grid is held rigidly between glass plates in a plate holder with vernier adjustments perpendicular to the gridlines and along the optical axis. The grid image is brought into sharp focus in the cutoff plane under continuous illumination from the incandescent "modeling light" of the flash-head. A 20x25 cm sheet of Kodalith film is then exposed in the cutoff plane by a flash discharge. This yields a photographic negative grid with 21 gridlines/cm. Solid, uniform black gridlines and unclouded clear gridlines are an absolute necessity for good schlieren sensitivity. All grid-type schlieren systems, regardless of size, require attention, persistence, and patience at this stage. The image plane is either observed on a ground-glass screen with continuous lighting or flash-photographed using a Polaroid 20 x 25 cm film back. Videography is also done from the ground-glass with the aid of a Fresnel lens. Polaroid photographs have been made using Type 809 color film (ISO 80) or Type 803 black-and-white film (ISO 800). The image resolution is estimated to be 1.5 mm in the test area, yielding razor-sharp large-format photographs. Per Weinstein's [8] Eqn. 2, an estimate of the minimum detectable schlieren deflection ε at 50% knife-edge cutoff is found to be 7.9 arcsec, which is not spectacular sensitivity. More cutoff can be used, but sensitivity is ultimately limited by the "extinction ratio" of image-plane luminance at 0% cutoff to that at 100% cutoff. Current values range from 10:1 at the center of the image to 4:1 at the edges, leaving considerable room for improvement in uniformity of source grid illumination and cutoff grid quality. (The grid-type schlieren system of Alvi et al. [19] initially had an inadequate 6:1 extinction ratio, later improved to 15:1 [20].) At present the full-scale schlieren reveals the human thermal plume to visual observation and in the original photos, but it does not reproduce well here. The present images, which result from digital scanning and image processing of the original photos, should thus be regarded as intermediate results rather than illustrations of the best results obtainable with this system.

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We endeavored to photograph some examples with which to explore the capabilities of the system and vaunt its potential. The first of these is Fig. 10, which shows a gas barbecue grill producing a strong thermal plume. Future research will examine thermal plumes of commercial cooking equipment and the ventilation hood design required to capture them efficiently. Fig. 11 shows a schlieren image of a full-scale trash fire in a barrel. This exemplifies possible studies of fire spread and fire safety. In addition to the strong thermal plume and fine-scale turbulence, large-scale structures and light absorption by smoke are also apparent. In Fig. 12 volatile fumes upon removing the lid of a chemical drum are visualized. This exemplifies scenarios where fume spreading and hazardous materials are involved and refractive index gradients are produced, but the fumes are invisible to the unaided eye. The release of gasoline vapor while filling an automobile fuel tank could similarly be visualized by full-scale schlieren, as can the exhaust stream of an automobile or the chainsaw shown in Fig. 13. The convection from a hot motorcycle, shown in Fig. 14, illustrates the potential for full-scale schlieren studies of product ergonomics, such as the interaction of convective heat transfer from equipment with the people using that equipment. Fig. 15 shows authors Miller and Hackett seated at a table with candles and cups of hot coffee. Such full-scale schlieren images of people, especially when videotaped during normal movement, are useful to gain a better understanding of how airflow, turbulence, and heat transfer affect the safety and quality of our indoor environment [21]. Finally, Fig. 16 shows HVAC airflows in a full-scale domestic setting. The portable electric heater at the lower left produces a thermal plume which is typical of those from forced-air heating systems. However, except for a few cases when smoke was added as a tracer [22], such plumes have never before been observed at full scale. Ventilation texts illustrate the "throw, spread, and rise" of such plumes by sketches rather than actual images. Scale-modeling of such situations has not generally been possible when thermal gradients are present [23]. A seminal visualization study of full-scale ventilation flows was done by Daws [24], but his tracer particles more often revealed where the plumes were not than where they were. We believe full-scale schlieren has a key role to play in visualizing and understanding all types of ventilation flows. Schlieren for Aircraft in Flight Weinstein [25] first realized the potential of Schardin's schlieren method number 5 using a specially-fitted telescope. Comparing Fig. 17 with Fig. 2, the edge of the sun against the sky is used as a light source, the telescope optics serve as the lens, and a 16 mm streak camera records an image and provides the necessary scanning, discussed earlier, of the line of schlieren sensitivity across the image. The schlieren stop is a mask, shown in the inset of Fig. 17, mounted inside the telescope at the position where the sun is sharply focused. It contains a curved slit which matches the edge of the sun. The amount of cutoff is determined by adjustment of this slit with respect to the sun's image. The telescope drive maintains the sun's image fixed with respect to the mask while an aircraft is flown across the sun as seen by the telescope. The distance from telescope to aircraft is pre-arranged so that the latter will be sharply focused on the film plane. The motion of the film is in the direction of flight of the aircraft, and is adjusted to maintain the image of the aircraft fixed while scanning the masked edge of the sun across it to record the schlieren field. The practicality of this approach was demonstrated by Weinstein in Dec., 1993, when he photographed a NASA T-38 aircraft flying across the sun at Mach 1.1 and 9.75 km range from his specially-fitted 20 cm f/10 telescope. The resulting image shown here in Fig. 18, with a

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height scale of 28 m, is easily the largest-scale schlieren image ever captured on film. The coalescence of shock waves in the near field of the aircraft is clearly shown. This full-scale approach to schlieren flow visualization is important to High-Speed Civil Transport research, since sonic boom propagation is yet another problem which does not scale down easily. Conclusions This paper has surveyed a variety of approaches to the visualization of full-scale phenomena by schlieren imaging. Special emphasis was given to a large new grid-type schlieren system which is reported here for the first time. Images obtained with this system reveal thermal plumes and other flow and heat transfer phenomena never seen before, or seen only poorly using tracer visualization. Many potential applications of full-scale schlieren visualization remain to be explored. In addition to those already mentioned, others include full-scale basic heat transfer experiments, automobile interior and exterior heat transfer, solvent evaporation from automobiles after painting, welding fume removal, airflow and convective heat transfer about animals, paint spraying, thermal cutting, flow patterns associated with full-scale boilers, chillers, refrigerators, and air-handling equipment, leakage from valves, air curtains, fume hoods, and full-scale studies of explosive events. The exploration of these topics is expected to go on for some years. Acknowledgments We thank Lori J. Dodson and Sanjay Garg for their contributions to this work. Additional thanks are due to the 3M Corp. for the donation of retroreflective material, and to Jeff Lamens of D. W. Miller Industries, Huntingdon, PA, for his contribution to the grid silkscreening technique. References 1) Schardin, H."Schlieren Methods and Their Applications," Ergebnisse der Exakten Natur-wissenschaften, Vol. 20, pp. 303-439, 1942 (English translation: NASA TT F-12,732). 2) Holder, D. W., and North, R. J., "Schlieren Methods," National Physical Laboratory Notes on Applied Science No. 31, Her Majesty's Stationery Office, London, 1963. 3) Settles, G. S., Schlieren and Shadowgraphy, book in press. 4) Taylor, H. G., and Waldram, J. M., "Improvements in the Schlieren Method," J. Scientific Instruments, Vol. 10, p. 378, 1933. 5) Weinberg, F. J., and W-Y. Wong, "Optical Studies in Fire Research," Proc. 16th Intl. Symposium on Combustion, 1977, pp. 799-807. 6) Burton, R. A., "A Modified Schlieren Apparatus for Large Areas of Field," J. Optical Soc. America, Vol. 39, pp. 907-908, Nov. 1949 7) Kantrowitz, A. and Trimpi, R. L., "A Sharp-Focusing Schlieren System," J. Aeronautical Sciences, Vol. 17, pp. 311-314, May 1950. 8) Weinstein, L. M., "Large-Field High-Brightness Focusing Schlieren System," AIAA J., Vol. 31, pp. 1250-1255, July 1993. 9) Dall, H. E., "A Null Test for Paraboloids," J. British Astronomical Assoc., Vol. 57, No. 5, Nov. 1947. 10) Malacara, D., ed., Optical Shop Testing, John Wiley and Sons, 1991. 11) Settles, G. S., "Colour-Coding Schlieren Techniques for the Optical Study of Heat and Fluid Flow," International Journal of Heat and Fluid Flow, Vol. 6, No. 1, March 1985, pp. 3-15.

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12) Lewis, H. E., A. R. Foster, B. J. Mullan, R. N. Cox, and R. P. Clark, "Aerodynamics of the Human Microenvironment," The Lancet, June 28, 1969, pp 1273-1277. 13) Settles, G. S., and Kuhns, J. W., "Visualization of Airflow and Convection Phenomena About the Human Body," Bulletin of the American Physical Soc., Vol. 29, No. 9, 1984, p. 1515. 14) Wallace, L. A., "Hot Showers Produced Elevated Levels of Chloroform," EPA Journal, Indoor Air Issue, Fall 1993. 15) Settles, G. S., Huitema, B. C., McIntyre, S. S., and Via, G. G., "Visualization of Clean Room Flows for Contamination Control in Microelectronics Manufacturing," Flow Visualization IV, ed. C. Veret, Hemisphere Press, 1987, pp. 833-838. 16) Hackett, C. M., Settles, G. S., and Miller, J. D., "On the Gas Dynamics of HVOF Thermal Sprays," Journal of Thermal Spray Technology, Vol. 3, No. 3, September 1994, pp. 299-304. 17) Settles, G. S., and Garg, S.,"A Scientific View of the Productivity of Abrasive Blasting Nozzles," Journal of Protective Coatings and Linings, Vol. 12, April 1995, pp. 28-41, 101, 102. 18) Settles, G. S., "Large-Field Color Schlieren Visualization of Transient Fluid Phenomena," Bulletin of the American Physical Soc., Vol. 28, No. 9, 1983, p. 1404. 19) Alvi, F. S., Settles, G. S., and Weinstein, L. M., "A Sharp-Focusing Schlieren Optical Deflectometer," AIAA Paper 93-0629, Jan. 1993. 20) Garg, S., and Settles, G. S.,"Turbulence Measurements in a Supersonic Boundary Layer by Focusing Schlieren," Bulletin of the American Physical Soc., Vol. 39, No. 9, 1994, p. 1890. 21) Settles, G. S., "Indoor Environments," Chapter 37 of Handbook of Flow Visualization, ed. W.-J. Yang, Hemisphere Press, Washington, 1989, (pp. 619-626). 22) Lang, V. P., Principles of Air Conditioning, Delmar Publishers, 1972. 23) Awbi, H. B., and Nemri, M. M., "Scale Effect in Room Airflow Studies," Energy and Buildings, Vol. 14, pp. 207-210, 1990. 24) Daws, L. F., "Movement of Airstreams Indoors," in Airborne Microbes: Proc. 17th Symp. of the Soc. for Gen. Microbiology, Cambridge Univ. Press, 1967, pp. 31-59. 25) Weinstein, L. M., "An Optical Technique for Examining Aircraft Shock Wave Structures in Flight," in High-Speed Research: 1994 Sonic Boom Workshop, NASA CP-3279, 1994.

Fig. 1 - Diagram of Grid-Type Schlieren System

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Fig. 2 - Diagram of Schardin's Schlieren Method Number 5

Fig. 3 - Diagram of 1-Meter Double-Pass Coincident Schlieren System

Fig. 4 - Human Thermal Plume Fig.5 - Human Cough

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Fig. 6 - HVOF Thermal Spray Fig.7 - Subject in Cleanroom Downflow

Fig. 8 - Instantaneous Image of Bullet and Muzzle Blast from 22-Caliber Rifle

Fig. 9 - Top View of 2x3m Grid-Type Full-Scale Schlieren System Layout

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Fig. 10 - Gas Barbecue Grill Fig.11 - Trash-Barrel Fire

Fig.12 - Volatile Chemical Fumes Fig. 13 - Chainsaw Exhaust

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Fig.14 - Plume from Hot Motorcycle Fig. 15 - Dinner Table, Candles, and Coffee

Fig. 16 - Full-Scale Schlieren Image of Domestic HVAC Airflows

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Fig. 17 - Diagram of Optics of Schlieren for Aircraft in Flight

Fig. 18 - Full-Scale Schlieren Image of T-38 Aircraft at Mach 1.1

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