ORVILLE WRIGHT once remarked: "How easy it would be to improve the airplane if only we could see the splash!" Several schemes were later devised for making the flow of air visible, as the bow wave of a ship is. An early technique consisted in attaching bits of yarn to an aircraft at strategic points. The yarn served, as wind vanes do, to indicate the direction of the flow. Another useful scheme called for injecting thin trails of smoke into the airstream of a wind tunnel containing a model of the aircraft. Pressures developed on the model by the moving air could be deduced by observing the displacement of the smoke.

A similar technique involves the substitution of water for air. A sheet of water (analogous to a stream of air) flows down the upper surface of an inclined pane of glass from a trough at the upper edge of the pane. Trails of dye are injected into the water at uniform intervals across the upper edge to form a grid of straight lines. A cross section of the model to be investigated is cemented to the center of the pane and acts as an obstruction that diverts the trails of dye. Forces acting on the model can be computed from the resulting distortion of the grid by taking account of appropriate scale factors. All these stratagems, however, are useful only at rather low air speeds.

High-velocity effects are now routinely investigated by means of schlieren photography. This approach is based on an optical effect first described in the 19th century by the French physicist Jean Foucault and utilized in 1864 by the German physicist August Töpler to make differences in the density of air currents visible. In its simplest form the schlieren apparatus consists of a light source, two lenses, a pair of knife-edges and a sheet of photosensitive film.

Light from the lamp is focused by a small lens on one of the knife-edges. That edge is adjusted to the position where half of the rays are intercepted. The unobstructed rays proceed as a diverging cone to the second lens. That lens bends the rays back into a converging cone that comes to a focus on the second knife-edge, which is adjusted to the position w here half of the remaining rays are intercepted. The unobstructed rays impinge on the film, which is uniformly lighted if the air between the first knife-edge and the second lens is of uniform density.

Variations in density bend the rays by refraction, with the result that the amount of light reaching the film is altered. Some rays that initially proceeded to the film are bent in the direction of the second knife-edge, where they are intercepted. The film darkens in these regions. Conversely, other rays that were originally intercepted may be bent clear of the obstruction. The film brightens in these regions. Patterns of light and shade thus appear on the film and portray variations in density. In effect the device enables the experimenter to photograph Wright's aerodynamic "splash."

An experimenter who has specialized in schlieren photography for several years is Gary S. Settles, who is an undergraduate student at the University of Tennessee. He has used the technique to investigate the flow of air around small models in a supersonic wind tunnel. Settles describes his most recent experiments as follows:

"The schlieren system is a fascinating analytical device, but it is sensitive to the quality of its optical parts, as Foucault observed. Optical defects bend light rays just as air currents do. The defects appear on the film as patterns of light and shade. Moreover, the second schlieren lens must have a substantial diameter. The column of air that can be observed can be no larger than the diameter of this lens.

"Even small lenses of the required optical quality are costly. For this reason schlieren systems of substantial size usually employ parabolic mirrors to focus the light. Inexpensive mirrors of excellent optical quality can be made at home by the techniques described in Amateur Telescope Making: Book One, edited by Albert G. Ingalls (Scientific American, Inc., 1970). The substitution of a mirror for the second lens of the schlieren system requires that the light be directed toward the reflecting surface at an angle with respect to the optical axis of the mirror. The rays are reflected at an equal but opposite angle to the other side of the optical axis, where they can be observed.

"The angular reflection introduces a particularly undesirable form of optical distortion known as coma. A point source of light that is subjected to this form of distortion comes to focus as a small comet shape. Coma can be eliminated by the use of a twin mirror. Light from the source falls on the first mirror from one side and is reflected to the second mirror, proceeding from there to the film on the opposite side. The optical path is folded in the form of the letter Z.

"Reflection from the first mirror introduces coma. Reflection from the second mirror cancels the distortion. Light between the two mirrors is transmitted as parallel rays. This region of the optical path is sensitive to differences in the density of air currents. It serves as the test section of the apparatus [see illustration at left].

"The simple schlieren system yields a black-and-white image of air currents that depicts variations of density in only one direction: at a right angle with respect to the knife-edge. Usually the experimenter is interested in density gradients, or variations, in all directions, particularly in the investigation of supersonic flow. It is possible to provide the system with more than one knife-edge and to use slits in the form of a square or a circle. These expedients do not greatly increase the useful information recorded by the film, however, because the individual contribution of each slit to the composite image cannot be distinguished in the black-and-white pattern. Therefore the device shows all density gradients but gives no information about their direction. In practice one black-and-white schlieren photograph is made for each test condition, using a horizontal knife-edge. Additional information is then gathered by repeating the series with the knife-edge fixed in the vertical position.

"A number of schemes have been devised for making schlieren photographs in color. The first of these approaches was described some 30 years ago by Hubert Schardin in Germany. A line source of white light, such as the straight filament of an incandescent lamp, is placed in front of a small achromatic lens. The rays emerge from the lens and pass through a slit to enter a prism that disperses the light into its constituent colors. The dispersed rays are focused onto the first mirror, where they are reflected to focus on an exit slit from which they proceed to the film. The exit slit is adjusted to transmit light of a selected color, for example yellow.

"In still air the film is uniformly illuminated in yellow. Air currents in the test section bend the rays, just as in the black-and-white system, so that some yellow rays are diverted from the second slit and do not reach the screen. These rays are replaced on the screen by rays of another color, yielding a multicolored image. Increasing air densities are depicted by colors near one end of the spectrum and decreasing densities by colors at the opposite end. In effect the system color encodes differences in air density.

"A second method of making schlieren photographs in color was described in 1954 by R. J. North of the National Physical Laboratory in England. In this system the prism is omitted and a set of filters in the form of three strips of gelatin of different colors (such as red, yellow and blue) replaces the second slit. The image of the light source falls on the central strip, which may be yellow. The yellow rays proceed through the system and fall on the film. When air in the test section is undisturbed, the film is uniformly flooded with a background color of yellow. Variations in air density cause rays to fall on the red or blue sections of the filter to form an image in polychrome.

"Recently I developed a schlieren system that appears to be novel in that it displays variations of density in all directions and tags the direction of each variation with a distinctive color. Light enters the system through four slits in a rectangular array [see illustration at right]. Each of the slits is covered by a Wratten filter of a particular color. The rays proceed through the conventional mirror system to an adjustable exit aperture also assembled in the form of a square.

"The light from the source is thus dissected into four colored bands: green, yellow, blue and red. (The numeral accompanying each color in the illustration designates the number of an appropriate Kodak Wratten filter.) Other colors can be used, but I have obtained the best results with this combination. The filters could be made photographically. My filter was made by applying colored gelatin strips directly to a glass cover slide and masking the opaque areas with black electrical tape.

"The light source should be white or nearly so and large enough to flood the slit assembly. The slits should be at least 1/4 inch long and are typically about .01 inch wide. The narrow slit width imparts high sensitivity to the system and the 1/4-inch slit length keeps the exit aperture large enough to avoid undesirable diffraction effects, which spoil the sharpness of the image.

"The light source must be relatively intense to provide adequate illumination for making reasonable exposures. A 750 watt slide-projector lamp with a filament grid about 1/2 inch square serves as an adequate source. A small condensing lens can be placed between the source and the slit assembly to increase the illumination. The source can be further intensified by putting a reflector behind the lamp. If the source does not illuminate the slit assembly uniformly, insert a piece of flashed glass, which is a white, translucent material, between the source and the slit assembly. Ground glass can be used, but it is inefficient compared with white flashed glass, which is readily available from dealers in sheet glass.

"Light from the colored slits mixes to form a single color in the test section but is refocused into an image of the colored slits at the exit aperture. The exit-aperture assembly consists of four knife-edges cut from springy sheet metal and fastened to a metal collar with adjustment screws [see illustration at left]. The dimensions of the exit aperture should match those of the companion square at the entrance slit.

"When the instrument is assembled the entrance slit and exit aperture are placed in the focal planes of the respective parabolic mirrors. The focal length of the mirrors should be at least 11 times their diameter (a focal ratio of f/11). Faster focal ratios, such as f/8, encourage coma. Slower focal ratios needlessly increase the length of the instrument. Moreover, at f/11 the curvature of the mirrors approaches that of a sphere, which is somewhat easier to make than a parabola.

"The light source, entrance-slit assembly and parabolic mirrors should be mounted on a hat, solid base with rigid fixtures. The instrument is sensitive to vibration. The entrance-slit and exit aperture assemblies are located exactly in the focal plane of the mirrors. The position of all four components must be established experimentally as you assemble them.

"The exit-aperture assembly is mounted on a fixture that provides for screw adjustment in three dimensions [see illustration at right]. Although this fixture may appear formidable at first glance, it is in fact rather easy to make with ordinary hand tools. The sliding ways consist of drill-rod stock that can be ordered by dealers in hardware. The flat stock can be aluminum plate. The ways and riders can be fastened with epoxy cement instead of screws.

"To align the instrument during assembly first place the source slit at a distance from the first parabolic mirror equal to the focal length of the mirror. Flood the slit assembly with light. The second parabolic mirror can be positioned at an arbitrary distance from the first. Adjust the angle of the first mirror in the vertical and the horizontal plane so that the second mirror is flooded with light.

"At a distance equal to the focal length set up a screen of white cardboard that faces the second mirror. Adjust the second mirror to project an image of the entrance-slit assembly on the screen and move the screen toward or away from the mirror as necessary to create the sharpest image of the entrance slit. Center the adjustment screws of the fixture that supports the exit-aperture assembly, place the fixture in the position occupied by the screen and with the adjustment screws move the exit aperture into exact register with the image. When you have completed these steps, adjust the jaws of the exit aperture to intercept about half of each color band of the entrance slit image.-

"Disturbances that alter the density of air between the mirrors cause weal: images of the color bands to shift at the exit aperture. The direction in which they shift determines which colors pass through the aperture to illuminate the schlieren image. The appearance in the image of one of the pure colors, such as red, yellow, blue or green, necessarily corresponds to an entirely horizontal or an entirely vertical deflection of the light rays, depending on the orientation of the entrance-slit assembly.

"Similarly, deflection in any other radial direction causes two adjacent color to move into the exit aperture. Part of the schlieren image then appears on the film as a mixture of the two colors. The resulting pattern of color identifies the direction in which light is deflected by corresponding portions of the test region. This effect, together with the knowledge that light is deflected toward regions o higher air density, enables the experimenter to use the instrument as a powerful tool for the analysis of airflow.

"In effect the system displays in one color photograph information that would otherwise require at least two black-and-white schlieren photographs made in sequence. The performance of the instrument is illustrated by three of the accompanying photographs that show density gradients in air flowing over a blunt object at three times the speed of sound. The model in this case is the head of a carriage bolt. One black-and-white photograph [left] was made with the knife-edge in the vertical position. Note the absence of details in the wake of the body. Boundary layers that are actually present on the walls of the wind tunnel and on the model do not appear.

"The second black-and-white photograph [right] was made with the knife-edge placed horizontally. In this photograph the boundary layer and wake region are clearly visible, but the illumination changes sign across the center line of the test section, so that half of the image is white and half is dark. The dark region near the silhouette of the model tends to confuse the shape of the model and to obscure the details of lesser gradients in the region.

"The missing details are apparent in the accompanying color photograph of the same model, which was made under the same conditions. Features of the wake and boundary layer appear simultaneously, as do the shock waves and expansions. The black silhouette of the model appears in sharp contrast to the colored stream of flowing air.

"The instrument can be used for investigating effects other than those that appear in supersonic wind tunnels. For example, one of the accompanying photographs depicts a jet of acetylene gas impinging on a flat plate. Before striking the plate the vertical jet appears primarily in yellow and blue, indicating increasing density toward the center of the jet. After striking the plate the jet spreads horizontally, changing to red (and green, which is not evident in the photograph) as the density decreases toward the center.

"In its present state of development the system is not useful for measuring quantitatively the density of the medium involved. Such determinations are routinely made with an interferometer. The four-color schlieren system is capable of a quantitative determination of the radial directions of the density gradients in test object.

"One possible source of error must be considered in applying the four-color schlieren technique, particularly if the color scheme differs from the one that has been described. Assume that the basic colors red, yellow, green and blue are selected for a certain application. The additive mixture of blue and red, blue and green, yellow and red or yellow and green causes no problems, because these combinations yield the expected colors magenta, cyan, orange and yellow-green. If the red and green or yellow and blue should mix, however, an ambiguous result will be observed in the schlieren image because red and green combine to form yellow, and the complementary colors blue and yellow combine to form white.

"The obvious solution is to place red and green filters on opposite sides of the slit assembly and to place the yellow and blue filters similarly in opposition. This arrangement takes advantage of the fact that adjacent colors mix whereas opposing colors can merely replace each other in the schlieren image. When the filters are positioned as suggested, colors in the image can consist only of the four basic hues plus various shades of magenta, cyan, orange and yellow-green, depending on the direction of the refracted light.

"Another possible source of error is the astigmatism that often plagues schlieren systems. It causes the horizontal and vertical bands of the entrance-slit image to come into sharp focus at different points along the optical axis. The solution involves displacing the jaws of the exit aperture correspondingly along the optical axis.

"The concept of a direction-indicating color schlieren system is not new; Hans Wolter of Germany did it with a color-filter exit slit some 20 years ago. It is the idea and the quality of this four-color method that are new. The results of the technique, as shown by the accompanying photographs, are quite an improvement over other color schlieren methods."

Bibliography

SCHLIEREN PHOTOGRAPHY. Kodak Pamphlet P-ll. Eastman Kodak Company, Rochester, N. Y.

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