Scientists use the Roadrunner supercomputer to model a fundamental process in physics that could help explain how stars begin to explode into supernovae

Despite decades of research, understanding turbulence, the seemingly random motion of fluid flows, remains one of the major unsolved problems in physics.

“With the Roadrunner supercomputer, we can now look in detail at previously inaccessible flows,” said Daniel Livescu, of Laboratory’s Computational Physics and Methods group.  Involving a technique known as Direct Numerical Simulations (DNS), researchers use the exact equations of fluid flow to calculate pressures, densities, and velocities, at very high resolution for both time and space, high enough to resolve the smallest eddies in the turbulent flow. This makes the DNS results as “real” as experimental data but requires immense computer power.

In many instances, these simulations are the only way turbulence properties such as those found in cosmic explosions like supernovae can be accurately probed.  In these cases, turbulence is accompanied by additional phenomena such as exothermic reactions, shock waves, and radiation, which drastically increase the computational requirements.

Livescu and colleague Jamaludin Mohd-Yusof of the Laboratory’s Computational Physics and Methods group are using Roadrunner and a high performance Computational Fluid Dynamics code to perform the largest turbulent reacting flow simulations to date. The simulations consider the conditions encountered in the early stages of what is known as a “type Ia” supernova, which results from the explosion of a white dwarf star.

Type Ia supernovae have become a standard in cosmology due to their role in measuring the distances in the universe. Yet, how the explosion occurs is not fully understood. For example, the debate around the models that describe burn rate and explosion mechanics is still not settled. In addition, the flame speed — that is the rate of expansion of a flame front in a combustion reaction — is one of the biggest unknowns in current models.

“Solving the flow problem in a whole supernova is still very far in the future,” said Livescu, “but accurately solving the turbulent flow in a small domain around a single flame, characterizing the early stages of the supernova, has become possible. The very high resolution reacting turbulence simulations enabled by Roadrunner can probe parameter values close to the detonation regime, where the flame becomes supersonic, and explore for the first time the turbulence properties under such complex conditions.”