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Shock tube studies bring astronomical understanding light years closer

Late-time interaction (725 ms) of a helium bubble in nitrogen gas
                        with an M=2.95 shock wave

Late-time interaction (725 ms) of a helium bubble in nitrogen gas with an M=2.95 shock wave: The left side is an experimental image taken from the shock tube; the image on the right is a computational simulation with the code Raptor. It depicts the late-time interaction, showing volume fraction and vorticity magnitudes. (large image)

Because of their massive size, often-fleeting nature, and distance from Earth, celestial bodies and events such as nebulae, galaxies, stars and supernovas are difficult, at best, to understand. Scientists’ knowledge of these astrophysical phenomena is limited primarily to observations they glean from images like those sent back from the Hubble Space Telescope.

At the University of Wisconsin-Madison, however, engineering physics researchers are conducting unique, centimeter-scale experiments that may shed light on astronomical events and processes that shape the universe. The January 2008 Journal of Fluid Mechanics features their research on its cover.

The researchers’ tools include a 10-meter-tall shock tube, some high-speed imaging technologies, a couple of gases, and a 4-centimeter soap bubble. “It turns out that the physics that happens at this scale is also the same physics that happens at astrophysics scales,” says Mark Anderson, a senior scientist in the UW-Madison Department of Engineering Physics.

In the universe, a stellar explosion, or supernova, triggers a shock wave that propagates through the gas, dust and other media that compose the matter that exists between stars in a galaxy. Stars both form in and “feed” this interstellar medium, a cycle that is crucial to a galaxy’s ability to support active star formation.

Upper two-thirds of the shock tube

Upper two-thirds of the shock tube (large image)

To study the physics of gas mixing that occurs during a stellar explosion, the UW-Madison researchers re-create the hydrodynamics of a simple supernova in the shock tube. Using a retractable stainless-steel “straw,” they blow and float a soap bubble filled, for example, with helium, inside the lower third of tube, which contains a gas such as nitrogen. Then, they send a shock wave barreling down the tube. As the wave hits the bubble, the two gases mix and a high-resolution camera captures two-dimensional images of the event (illuminated by a laser pulse), which lasts just thousandths of a second. A single experiment may yield between one and three images; the researchers combine images from several shock-tube runs to paint a more complete picture.

Filled with such geometric features as vortices and trailing plumes, the shock-tube images mirror images of actual astronomical events. “Using the shock tube, with its very simple geometry of a spherical bubble with a planar shock-wave interaction, we can help explain the physics of some of those geometric features,” says Jason Oakley, a lecturer and associate scientist in the Department of Engineering Physics.

Because they experiment with known, well-characterized quantities, the UW-Madison researchers can extract meaningful data from the shock-wave images, such as how vortex cores grow as a function of time or how the vortex draws in surrounding gas. Recent diagnostic and technological advances have enabled them to capture more detailed photos of the gases mixing and thus, to better understand the physics underlying their experiments. “We’re able to experimentally identify features that have never been seen before,” says Oakley.

This image, rendered from a 3-D multifluid compressible Euler
                        Scheme (Raptor), shows the interaction of a M=2.88 shock wave with an
                        Argon bubble.

Former graduate student John Niederhaus and colleagues at Lawrence Livermore National Laboratory created this image, rendered from a 3-D multifluid compressible Euler Scheme (Raptor). It shows the interaction of an M=2.88 shock wave with an Argon bubble. (large image)

Supercomputing advances also have enabled scientists to develop and run complex, three-dimensional computer simulations of the same features. In fact, University of Chicago computational scientists are working with Oakley, Anderson and Engineering Physics Professor Riccardo Bonazza to experimentally validate simulations of how different gases react after they’ve been hit with a shock wave. As part of that group, current graduate student Devesh Ranjan and former graduate student John Niederhaus, now a scientist at Sandia National Laboratories, conducted experiments and developed simulations that accurately predict how two gases in the shock tube mix after the wave passes.

Although the physics of the researchers’ shock-tube experiments translate well to astrophysics proportions, they also scale down just as easily, says Anderson. Laboratories around the country are applying the concept to learn more about how energy is released during inertial-confinement fusion, a process in which lasers bombard and explode a pea-sized fuel pellet. “Our work really complements that research because it simplifies the interaction and is at a much larger scale,” says Oakley. “So, we can look at much finer features and understand the hydrodynamic mixing.”