Synthetic schlieren imaging helps Sandia researchers see blast-tube shock waves

Sept. 10, 2018
The tests demonstrate how well nuclear weapons could survive the shock wave of a blast from an enemy weapon.
Sandia National Laboratories researchers use wavefront imaging taken at 35,000 frames per second to analyze blast-wave dynamics invisible to the eye and determine how well nuclear weapons could survive a shock wave. (Photo courtesy of Sandia National Laboratories)

Researchers at Sandia National Laboratories (Albuquerque, NM) are using a 6-ft-diameter blast tube configurable to 120 feet in length to demonstrate how well nuclear weapons could survive the shock wave of a blast from an enemy weapon and to help validate the modeling. To do this, they are coupling blast experiments with computer modeling and a high-speed imaging technique known as synthetic schlieren.

Sandia recently completed a two-year series of blast-tube tests for one nuclear-weapon program and started tests for another. Each series requires instrumentation, explosives, high-speed cameras, and computer modeling.

High-speed imaging that measures pressure changes (along with pressure sensors) helps assess a shock wave's impact on an object. In the past, researchers used streak cameras that viewed images through a quarter-inch by 6-inch slit. Streak cameras image a column of pixels and generate an image as the object moves rapidly past the scan.

Now, synthetic schlieren, implemented for harsh environments by optical engineer Anthony Tanbakuchi, enables a much larger view. Synthetic schlieren detects changes in the refractive index of air induced by changes in pressure, temperature, and density. The schlieren effect is comparable to seeing ripples from heat on a road.

Regular schlieren (a German word that means streak in the singular) techniques require large optics, special lighting and other complex, sensitive optical configurations that aren't practical for large-scale tests, Tanbakuchi says. Synthetic schlieren doesn't require any special setup other than an optional background and has no size limit because it looks for subpixel shifts in the background to detect optical index changes.

The team combines synthetic-imaging algorithms with image-stabilization codes Tanbakuchi developed to image a blast wavefront. Sandia's 50-year history of extreme testing means it has a huge code base to solve these problems.

Synthetic schlieren can be used for everything from pressure to temperature imaging. “But the most value comes when we also combine it with the data fusion techniques we’ve developed so you can see the pressure wavefronts with instrumentation data and model data," Tanbakuchi says. "That's when the full picture really emerges."

For more info on the complete technique, see:

Source: Sandia National Laboratories

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