Schlieren Photography

Schlieren photography maps the variations of density in fluids. This type of photography is used to visualize airflow over an aircraft traveling at supersonic speeds. Since shock waves are regions of higher pressure than normal air pressure, their density is greater than that of normal air pressure. This pressure differential created by the shock waves is what allows the shape of the shock waves to be visualized by Schlieren Photography. The photographs below show the shape of the shock cones produced by two different aircraft flying faster than the speed of sound. The darker the region on the photograph is, the greater the pressure is in that region. The airplane is shown as a solid black image since it has the greatest density being made of steel and other solid composites. The semi-circular rings in the photograph are not part of the shock wave pattern, but are caused by unsteady camera film speeds, and wind.

The photo in Figure 1 is of a F-18 traveling at Mach 1.4, an altitude of 35,000 feet. Figure 2 is a photo of a F-38 traveling at 1.1 and an altitude of 13,700 feet.

Figure 1

Stacy, Kathryn  Digital Enhancement of Schlieren Photography. [online] Available: davl-www.larc.nasa.gov /stacy/Focused_schlieren, June 1, 1997.

Figure 2

Haering, Edward A., Jr. Dryden Fact Sheet: Schlieren Photography - Ground to Air. [online]. Available: www.dfrc.nasa.gov/PAO/PAIS/HTML/FS-033-DFRC.html, June 1, 1997.

In figure 2 some shock waves originating at the mid-section of the aircraft can be seen merging together, forming two stronger shock waves towards the middle of the aircraft. The nose shock wave is represented by the thick dark band toward the left side of the photograph. The next major shock wave is to the right is produced by the jet intake ports. This shock wave is being merged by a smaller shock wave originating a little to the left of the intake ports. You can see that where the shock waves merge, the photo is darker since their pressures of both shock waves combine increasing the total pressure in that region. The third major shock wave is produced by the wings. This shock wave is also being merged by a smaller shock wave which is represented by a gray region adjoined to the right side of the dark band produced by the wings. The fourth major shock wave which is the leftmost shock wave is generated by the tail. As the shock waves move farther away from the aircraft as shown toward the top of the photograph, only four shock waves remain.

The camera used to take the photographs is setup as a streak camera. The camera is aimed at the sun, with the lens of the camera covered by a mask containing a slit. The camera is positioned so that only the edge of the sun is in the inner half of the slit, while the outer half of the slit contains the sky surrounding that edge of the sun. As the aircraft flies past the sun at a designated location, speed and altitude, the shock waves produced by the aircraft pass between the sun and the camera. The film rolls at the same speed as the aircraft image moves across the sky, in this way the entire passage of the aircraft is photographed even though at any given time only a small portion of the picture is visible to the camera. The camera maps the shock waves by the amount refraction of sun light that the change in air pressure creates. Air has different indices of refraction at different air pressures. As air pressure increases the index of refraction of air increases. Thus as a shock wave crosses the slit, the index of refraction of the air increases causing the sunlight to refract away from the film creating a darker region on the photo for that region of air pressure. As the shock wave passes the air pressure returns to normal and more light hits the film since less refraction occurs. The photo shows a sharp boundary between the front of the shock wave and the air in front of it as indicated by the sudden change from light gray or white to solid black. On the back side of each shock the boundary is less distinct. This shows the sudden onset of pressure, and the its slightly slower release of pressure. It is during this short time when this onset and release of pressure occurs that a sonic boom is generated.

Schlieren photography serves as a method of visualization of the structure of shock waves, which will be used in conjunction with computational propagation techniques, and sonic boom measurements at various altitudes to study the intensity of shock waves. This data will be used by the aerospace industry in the development of HSCT (High Speed Civil Transport). It will help determine which design for the HSCT will produce the softest sonic boom.

Now that we know what sonic booms are, click here to take a look at some measurements of sonic booms under varying conditions.

Bibliography

Haering, Edward A., Jr. Dryden Fact Sheet: Schlieren Photography-Ground to Air. [online]. Available: dfrc.nasa.gov    Path: PAO/PAIS/HTML/FS-033-DFRC.html