A $22 million contract from the U.S. Department of Energy
will help the University of Chicago’s Center for Astrophysical
Thermonuclear Flashes model the turbulent mix and flow of gases that
trigger exploding stars over the next five years.

Just the seemingly simple act of stirring cream into a cup of
coffee presents a scientifically complicated problem in fluid
dynamics and mixing. Unfortunately for scientists in the Flash
Center, checking the accuracy of a numerical model that involves
pouring a cup of coffee is much simpler than running the same test
for an X-ray burst, a classical nova or a type Ia supernova. This
checking process, which demonstrates that models and simulations
accurately describe nature, is called “validation.”

“If you aren’t reasonably sure that your simulations can
meaningfully describe nature, then you’re really wasting your time,”
said Alan Calder, a Research Scientist in the Flash Center. “It’s not
necessarily that the codes have a bug. It’s just that how the codes
do the physics and the physics that are included in the codes don’t
necessarily mesh with what is really needed.”

Validation, along with building a computer code that
simulates exploding stars, has been a significant part of the Flash
Center’s effort since it was established in 1997. Theorists and
experimentalists working with the center also carefully examine the
issues involved with modeling scientifically important physical
processes. With the new DOE contract, the center’s researchers will
be able to devote more effort to the problem over the next five
years. As a result of this increased effort, Calder and 17 of his
colleagues have written a validation paper, which appears in the
November issue of the Astrophysical Journal Supplement.

“Validation is almost a new scientific subject in
astrophysics,” said Robert Rosner, the William Wrather Distinguished
Service Professor in Astronomy & Astrophysics at the University of
Chicago, and a co-author of the paper. “If you’re trying to look for
papers and books, it is amazing how little there is. In fact, Alan is
the chief author of probably the first thorough-going validation
paper on astrophysics.”

The validation of the Flash Center’s simulation code has
implications for institutions across the country and around the
globe, for national security and for a variety of fundamental
scientific questions.

The Flash Center has received nearly 200 requests for its
simulation code from various institutions. Outside the United States,
the heaviest users reside in Germany, Italy, Japan, Poland and
Norway. The heaviest users domestically include various national
laboratories; schools in the University of California system; the
University of Texas, Austin; the University of Illinois,
Urbana-Champaign; and the State University of New York, Stony Brook.

“The Flash code is a tool that can be used for all sorts of
problems, ranging from astrophysical calculations to simulations of
laboratory experiments,” Rosner said. “We want this to be a community
code. It’s used by lots of people, not just here at Chicago.” Because
the DOE must ensure the reliability of the nation’s stockpile of
nuclear weapons, it has a vested interest in the simulations.
“Nuclear weapons are very fragile objects,” Rosner said. “They’re
known to age in destructive ways, hopefully in predicted ways.”

In the scientific realm, astrophysicists use type Ia
supernovae as astronomical measuring devices, which led them to
conclude that the universe will expand forever. X-ray bursts provide
information about the basic characteristics of neutron stars, which
are supernovae remnants. Classical novae help scientists calculate
the abundance of certain elements in the universe and understand the
dynamics of aging stars that closely orbit each other in binary
systems.

The journal paper explains two tests that compared the
simulation code with experimental data. In one case, the code agreed
well with the experimental results, while in the other case, it did
not. These results highlight both the need for and difficulty of
conducting validation tests, which must assess error and uncertainty
in theory, experiment and computation, wrote Calder and his
colleagues.

The simulation codes tested well against results produced in
laser-driven shock experiments conducted at the University of
Rochester’s Omega laser facility. These experiments, designed to
replicate the collapsing core of a supernova, involve laser blasting
a tiny capsule that consists of three layers of decreasing density.
The capsule becomes compressed as vaporized material jets from its
surface.

The Flash team found that its simulations did not agree with
the findings of the Rayleigh-Taylor experiments conducted at Lawrence
Livermore National Laboratory’s Linear Electric Motor. These
experiments demonstrate how, under gravitational pressure, a heavier
fluid interacts with an underlying light fluid. The experiments are
named for the Rayleigh-Taylor instability, a process that leads to
the turbulent mixing of fluids in supernovae and on Earth. The
disagreement indicates significant uncertainty in the initial
conditions or other aspects of either the experiments or the models
and is the subject of ongoing research by both theorists and
experimentalists.

“These tests served to increase our understanding of the
physics relevant to the problems of interest, to improve our
simulation techniques and to build confidence in our results,” wrote
Calder and his co-authors.