HPC in Physics
(particularly astrophysics)
Reuben D. Budiardja
Scientific Computing
National Institute for Computational Sciences
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Gravitational Wave
“Einstein’s Unfinished Symphony” – Marcia Bartuciak
• Predicted by Einstein’s General Relativity (1915)
• GR  gravity is the effect of space-time curvature
due to mass, not “mysterious action-at-a-distance”
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Gravitational Wave
“Einstein’s Unfinished Symphony” – Marcia Bartuciak
• Predicted by Einstein’s General Relativity (1915)
• GR  gravity is the effect of space-time curvature
due to mass, not “mysterious action-at-a-distance”
• Accelerating massive object produces ripples of
space-time curvature  gravitational waves
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Gravitational Wave
“Einstein’s Unfinished Symphony” – Marcia Bartuciak
• GW has not been directly detected, but new
detectors are (coming) online: (A)LIGO, VIRGO,
GEO600
• GW astronomy will be a novel way to “see” nature,
complementing EW (visible lights, infrared, xray, …)
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Sources of Gravitational Wave
Gravitational wave templates (frequency, spectrum) from HPC
simulations are required to extract signals from background noise.
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Binary Neutron Star Mergers
• Neutron star: dense object
left behind by core-collapse
supernovae
• Discovered as solitary
object (pulsars) and in
binary system
• Binary NS inspiral due to
gravitational wave
emissions
• Candidate for GRB
progenitors, GW detection,
r-process nucleosynthesis
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Modeling BNS in HPC
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Fluid evolution
Magnetic fields in fluid
Nuclear Physics for equation of state
Self-gravity (Newtonian + GR correction) or Full GR
Radiation (Neutrino) transport
Multi-dimensional simulations
• Inherently 3D phenomena
• Multi-scale
• Different length-scales need to be resolved: star surface, orbital, outer
boundaries
• Adaptive Mesh Refinement (AMR) to save computational cost
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General Astrophysics Simulation System (GenASiS)
• Code base for large-scale astrophysics
simulations
• Initial target applications: Neutron star
mergers, core-collapse supernovae
• Written entirely in Modern Fortran (2003,
2008)
• Modular, Object-oriented design, Extensible
• Eulerian/grid-base, multi-dimensional
• Multi-physics solver (magnetohydrodynamics, gravity, EoS, …)
• AMR capability
Domain Decomposition Parallelization
• GenASiS uses “brick” decomposition to minimize
surface-to-volume ratio
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Test Driven Development
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Weak Scaling
How the solution time varies with increasing number of
processors given a fixed problem size per process
 add more processors to solve bigger problem
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NS Merger Simulation
• Hydrostatic polytrope as simplified model for a neutron star
• Each star is given initial velocity for circular Keplerian orbit
• Gravitational radiation removes energy causing inspiraling
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NS Merger Simulation
Visualization movie may be viewed at: http://youtu.be/lUVPrmdvDUw
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Gravitational Wave
merger
App. Point Mass
inspiral
ring down
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Core-Collapse Supernovae
• The death throes of massive star ( M > ~10 Solar M)
• Among the most powerful explosions in the universe:
• Occur about twice per century in our galaxy
• Dominant source of elements in the universe
Answers to questions relating
to our origins in the universe
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Stationary Accretion Shock Instability (SASI)
• Stalled shock-wave after
core bounces
• Unstable to non-radial
perturbation  SASI
• SASI have been
observed in many
supernova simulations
• The key ingredient to
robust supernova
explosion ?
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Magnetic Fields in Supernovae
• Weak magnetic fields in
progenitors
• Pulsars have strong
magnetic fields
• Amplification usually
relies on fast rotation
• Most progenitors are only
slowly rotating
• The role of magnetic fields in supernova dynamics is not
clear
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Can SASI amplify the magnetic
fields of the progenitor star ?
Yes, magnetic fields are amplified
exponentially
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Simulation Movie
Visualization movie may be viewed at: http://youtu.be/gmfWmIYLEzU
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Magnetic Fields
Slice on XZ-Plane
Plane’s normal is
Parallel to angular
momentum of the
flow
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Amplification Mechanism
• Stretching is the primary mechanism for amplification of magnetic
energy at the expense of the kinetic energy.
• Amplification stops due to finite resolution that cannot resolve thinner
flux rope.
2X Resolution
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Magnetic Field in Proto Neutron Star
Visualization movie may be viewed at: http://vimeo.com/27247968
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