Fig. 1 Sandia National Laboratories
Sandia National Laboratories researchers use a blast tube to demonstrate how well nuclear weapons could survive the shock wave of a blast from an enemy weapon and to help validate the computer modeling.

Simulating Shock Waves for Testing Nuclear Weapons

Blast-tube tests at Sandia simulate shock wave conditions nuclear weapons could face. The researchers use synthetic schlieren photography to capture images of those shockwaves for analysis.

Sandia National Laboratories researchers are using a 120-ft long blast tube to see how nuclear weapons would react to the shock wave of a blast from an enemy weapon and to help validate the modeling of that test. Sandia recently completed a two-year series of blast-tube tests for a nuclear weapon program and started tests for another.

Each series starts with calibration shots that let team members verify blast wave parameters and validate the computer model of the process. The team hangs an explosive charge at one end of the 6-ft diameter tube and mounts pressure transducers along its length that detect the strength of the blast pressure moving through the tube—higher pressure closer to the charge, falling off farther away.

In each test, the pressure drives how big a charge is needed and how the test sample is placed in the tube, and that determines the loading (or amount of force) applied to the sample. In turn, the loading determines the sample’s structural response. The team does end-to-end calculations to simulate the explosive going off, the shock wave through the tube, the shock propagation over the sample, and then the structural response of the sample to the shock wave. All that data determines the right orientation and shock level to validate the models.

One software program simulates the explosive going off and the shock wave moving through the tube. A second calculates the shock moving over the sample. A third computes the sample’s response to shock and vibration. The fourth simulates how the sample will fly from the tube so the researchers can estimate where it’s going, how fast it’s moving, and how they’re going to catch it safely. Each software package helps determine the response of the overall test process to validate the models and helps design the test.

Sandia National Laboratories researchers use synthetic schlieren photography to capture images of a wave front taken at 35,000 frames per second and analyze blast wave dynamics (pressure, temperature, and density) invisible to the eye. The purpose is to determine how well nuclear weapons could survive a shock wave. (Photo courtesy of Sandia National Laboratories)

Software that simulates the explosive going off, for example, tells engineers the size of the charge. They do several shots in the tube to calibrate that—conducting enough to build a calibration curve that reveals how much explosive is needed to get a specific target pressure.

More than a hundred channels of data might be collected on pressures, strains, and acceleration responses from the tests. Analysts process experimental data using embedded information, apply identical signal processing methods to the experimental and analysis data, and compare responses to assess the credibility of the model.

The objective is to develop validated analytic models for predicting responses to blast loads with a high degree of confidence. Researchers then use the validated model to help qualify a weapon to withstand harsh conditions, such as a nuclear blast, that cannot be directly simulated with ground-level blast tube tests.

Instrumentation is critical. Tests that last mere milliseconds require months of planning, and there’s only one chance at getting data from the extreme environment of a blast. To streamline testing, Sandi build a mobile instrumentation unit, a large-data acquisition device that mounts inside a hardened trailer that will be placed closet to the blast test and-checks the accuracy and “health” of connections before and after testing. It stores up to 16 million samples per channel and records about 1 gigabyte per second at the maximum sample rate. For comparison, this equates to more than 70 hours of digital music or about 1,100 songs.

The engineers also rely on high-speed imaging that measures pressure changes, which also helps assess a shock wave’s impact. In the past, researchers used streak cameras that viewed images through a quarter-inch by 6-in. slit. Streak cameras are like document scanners, imaging a column of pixels and generating an image by the object moving rapidly past the scan. They now use a photographic technique called synthetic schlieren which has been upgraded to withstand  harsh environments and to take much larger views.

Synthetic schlieren detects changes in the atmosphere’s optical index 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 optics that aren’t practical for large-scale tests. Synthetic schlieren requires no 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 developed to image a blast wave front. Sandia’s 50-year history of extreme testing has given it 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 it undergoes data fusion techniques Sandia developed to show pressure wave fronts with instrumentation and model data. That’s when researchers say the full picture really emerges.

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