July 3, 2014

3D Simulations Raise Bar for Astrophysics

Tiffany Trader
core-collapse supernovae in 3D by SXS

For those outside the HPC/science realm who question why there need to be ever-more powerful supercomputers, one need only look at the amazing breakthroughs that the petascale age has facilitated. Astrophysics research out of Caltech is the latest example. Because of leadership-class systems like Stampede and Blue Waters and their experienced support staff, researchers from Caltech were able to perform fully 3D model simulations of supernova explosions.

The scientists are studying a somewhat rare phenomenon called extreme core-collapse supernovae. While these events comprise only one percent of all observed supernova, they are “extreme” in the amount of energy that’s emitted into space.

Up until recently, simulations in this field were mainly relegated to two dimensions (2D), and due to computational limitations codes could not incorporate all of the relevant physics, for example general relativistic effects were intentionally excluded. This study marks the first time that scientists are running fully general relativistic three dimensional (3D) simulations.

Because of the added realism, the research team, led by Philipp Mösta, postdoctoral scholar at Caltech, and Christian D. Ott, professor of astrophysics at Caltech, is discovering that previously held theories about how these explosions work might not be accurate.

The heart of the new finding is that the explosion is a highly dynamic process.

“What we’ve shown is that the jets that appear stable in 2D are actually unstable in 3D,” explained Mösta in an article by Liz Murray at the XSEDE website. “They twist, rotate and become unstable due to a phenomenon that is called the magneto-hydrodynamic kink instability. This instability of the magnetic field itself is the same that is also seen in fusion reactors that are using magnetic fields to confine the plasma.”

Supercomputing has been instrumental to the the project since it started in 2013 with an Extreme Science Engineering Discovery Environments (XSEDE) allocation on the Stampede supercomputer, installed at the Texas Advanced Computing Center (TACC) at The University of Texas at Austin.

The early work centered on code optimization, tweaking the code to take advantage of modern computing architectures. This crucial step enabled the team to run larger simulations without using up their alloted CPU hours too quickly.

“We were able to perform the first fully general relativistic 3D simulations without any symmetries and the difference in comparison to 2D was drastic,” stated Ott. “We now know if we want to predict what the signature of these extreme supernova explosions might look like, we need to do it in full 3D.”

After performing the initial general-relativistic magnetohydrodynamics (GRMHD) simulations on Stampede, the team hit a wall when trying to computationally reproduce the shockwave the extends out from the core of a massive star as it collapses to a proto-neutron star. To simulate this part of the process in 3D, they moved over to the Blue Waters supercomputer, a larger resource managed by the National Center for Supercomputing Applications (NCSA) at the University of Illinois at Urbana-Champaign.

This short video below shows the time-evolution of the shock wave. The 2D simulation is depicted on the left, while the corresponding meridional slice from a 3D simulation is shown on the right.

The scientists credit both of these supercomputers with their foray into 3D simulation. After further refining their code and running additional simulations, their goal is to create full 3D kinetic models of these extreme supernova explosions. The project involves connecting the simulations to actual observations collected from one of the NASA satellite telescopes.

Aside from being a valuable breakthrough for human understanding of supernovas, the emergence of 3D simulation has greater implications according to Mösta. “It will probably indicate to other groups who, so far, have focused on performing simulations with symmetries imposed, that they will have to move to full 3D simulations as well, which will ultimately strengthen our community,” he stated.