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NASA - NASA Dryden Fact Sheets - Schlieren Photography: Ground to Air
November 25, 2004
 
 

Schlieren Photography - Ground to Air

Project Summary

Schlieren photography allows the visualization of density changes, and therefore shock waves, in fluid flow. Schlieren techniques have been used for decades in laboratory wind tunnels to visualize supersonic flow about model aircraft, but not full scale aircraft until recently.
F-18 at Mach = 1.4, altitude = 35,000 ft, NASA Dryden, pilot: Ed Schneider
Figure 1. F-18 at Mach = 1.4, altitude = 35,000 ft, NASA Dryden, pilot: Ed Schneider.

Dr. Leonard Weinstein of NASA Langley Research Center developed the first Schlieren camera, which he calls SAF (Schlieren for Aircraft in Flight), that can photograph the shock waves of a full sized aircraft in flight. He successfully took a picture which clearly shows the shock waves about a T-38 aircraft on December 13, 1993 at Wallops Island, Md. The camera was then brought to the NASA Dryden Flight Research Center because of the high number of supersonic flights there. Two Schlieren photographs from this camera are shown in Figures 1 and 2. The shock waves emanate from the vehicle, and then begin to coalesce with each other. The banding in these photos is not part of the flowfield and is an artifact of the camera as described in the section How the Camera Works.

Why Are Shock Waves Worth Studying?

This work is geared toward developing a High-Speed Civil Transport (HSCT) aircraft for operational use early in the next century. At supersonic speeds, an aircraft generates numerous shock waves that emanate from major components, such as the nose, canopy, inlets, wings, and vertical tails. These multiple shock waves tend to merge into a strong bow shock and a strong tail shock as they propagate through the atmosphere. Presently, supercomputers and wind tunnels are used to predict the structure of these shock waves, but only within a few body lengths of the aircraft. Other computational techniques are then used to determine the propagation and merging of these shock waves down to ground level. Shock merging can be seen in the T-38 Schlieren photograph above. To verify and enhance the quality of these computational propagation techniques, a database of sonic boom measurements at various altitudes is being gathered for use by the aerospace industry, universities, and NASA research centers. These organizations will use the enhanced computational techniques in the design of the HSCT. Varying the design of a HSCT could help soften the intensity of sonic booms at ground level.
T-38 at Mach = 1.1, altitude = 13,700 ft, NASA Wallops
Figure 2. T-38 at Mach = 1.1, altitude = 13,700 ft, NASA Wallops.

How the Camera Works

The camera is set up at a predetermined location, and is aimed at the sun. By means of a curved slit only one edge, or limb, of the sun is imaged. The inner half of the curved slit is illuminated by the sun, and the outer edge is the sky. The camera tracks the sun as the sun appears to move across the sky. The aircraft is flown at a predetermined speed, altitude, and location so that its shock waves pass between the sun and the camera. A 16mm movie film camera is used to take the picture. Only the small slit is illuminated at a given instant, so the film rolls in such a way that the film speed is matched to the speed of the aircraft image across the camera. In this way, the system is used as a streak camera. As the shock wave crosses the slit, it refracts the light, causing more or less light to illuminate the film. The banding pattern that is seen in Figures 1 and 2 is caused by unsteady film speed and wind effects on the camera. The bands are the same shape as the slit.
The illustration demonstrates how the astronomical telescope captures the shock waves using a mask placed inside the telescope.
Figure 3. The illustration above demonstrates how the astronomical telescope captures the shock waves using a mask placed inside the telescope.

Dr. Weinstein at NASA Langley recently developed a new version of the camera that replaces the movie film portion with a Time Delay Integration (TDI) electronic camera and computer. The Electronic Schlieren camera removes most of the banding effect seen on the Film Schlieren camera. This Electronic Schlieren camera was successfully demonstrated on June 22, 1995, taking a photograph of shock waves of an F-18. The Film Schlieren camera was also used simultaneously to compare results from the two cameras. The setup from this test is shown below.

Flight Test Technique

As with most of life, timing and positioning are the most difficult aspects for Schieren photography. The aircraft (or the shock waves of interest) must eclipse the sun relative to the camera. The sun is constantly moving in the sky, so the pilot must fly through a location possibly only a few hundred feet large within 30 seconds of the prescribed time while flying supersonically.

The sun position and geodetics of the earth are used in flight planning. The sun ephemeris is taken from the 1995 Astronomical Almanac and automated into a FORTRAN program. This program calculates the elevation and azimuth of the sun as a function of time of day and camera location, and calculates where the plane should fly to achieve the desired photograph. The camera location is selected so that the aircraft can fly in an established supersonic corridor, and near a road so the camera doesn't have to be carried too far. The camera crew uses Global Positioning System (GPS) receivers to navigate to and locate the precise camera location. The aircraft is navigated by Inerital Navigation System (INS), Astro Navigation System (ANS), or GPS.
Film Schlieren camera
Figure 4. Left to Right: Film Schlieren camera, Electronic Schlieren camera, Dan Baize, Elise Gravance, Dr. Leonard Weinstein. Photo by Edward A. Haering, Jr.

Future Activities

  • Photograph SR-71 shocks to compare to pressure measurements taken of shock waves
  • Photograph Space Shuttle during landing at NASA Dryden
  • Refine development of Electronic Schlieren camera
  • Develop Air-to-Air Schlieren photography capability

Personnel

  • Dr. Leonard Weinstein, NASA Langley, Schlieren camera inventor, camera operator
  • Edward A. Haering, Jr., NASA Dryden, Flight Planning, Geodetics, former Principal Investigator, camera operator
  • Albion Bowers, NASA Dryden, current Principal Investigator, camera operator
  • Daniel Baize, NASA Langley, Electronic Schlieren camera electronics development
  • Elise Gravance, NASA Dryden (PRC), camera operator
  • David Richwine, NASA Dryden (PRC), air-to-air Schlieren development
  • Edward Schneider, NASA Dryden, F-18 pilot
  • Rogers Smith, NASA Dryden, F-18 pilot
  • Steve Ishmael, NASA Dryden, F-18 pilot

Bibliography

"An Optical Technique for Examining Aircraft Shock Wave Structures in Flight", Leonard M. Weinstein, , NASA CP-3279, October, 1994.

"An Electronic Schlieren Camera for Aircraft Shock Wave Visualization", Leonard M. Weinstein, , NASA CP-3335, October, 1995.
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Page Last Updated: July 28th, 2013
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