dursi-astro-combustion
TRANSCRIPT
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CITA|ICATJonathan Dursi CAIMS-MITACS 2006June 19
Simulating Astrophysical Combustionwith the FLASH code
Jonathan Dursi (and many, many others)Canadian Institute for Theoretical Astrophysics
University of Toronto
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CITA|ICATJonathan Dursi CAIMS-MITACS 2006June 19
Outline
Combustion inAstrophysics
The FLASH code
Testing / V&V
TowardsMultiscale/subgrid
approaches
(Rpke, Max Planck Institute for Astrophysics)
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CITA|ICATJonathan Dursi CAIMS-MITACS 2006June 19
Combustion in Astrophysics
Almost all astrophysical
systems are fluid Hot, dense
Many interestingphenomenon involve
energetic phasetransitions -- `burning'
Some very exotic
phase transitions inearly universe
quark-matterdeconfinement (Rpke, Max Planck
Institute for Astrophysics)
Vladimirova, FLASH Center
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CITA|ICATJonathan Dursi CAIMS-MITACS 2006June 19
Combustion in Astrophysics Most systems:
Burning =thermonuclear reactions
Stable burning(simmering/smouldering):
Stars like the sun (well-mixed reactor)
Explosive burning(thermonuclear flashes):
Novae
X-Ray Bursts
Supernovae: SOHO - EIT Consortium, ESA, NASA
http://sohowww.nascom.nasa.gov/http://umbra.nascom.nasa.gov/eit/http://www.esrin.esa.it/export/esaCP/index.htmlhttp://www.nasa.gov/http://www.nasa.gov/http://www.esrin.esa.it/export/esaCP/index.htmlhttp://umbra.nascom.nasa.gov/eit/http://sohowww.nascom.nasa.gov/ -
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CITA|ICATJonathan Dursi CAIMS-MITACS 2006June 19
Combustion in Astrophysics
Differences from terrestrialcombustion:
Thermonuclearreactions, not chemical
Fairly minor differencesin behaviour Arrhenius-like
Much simpler`chemistry'!
Energetics capturedwith ~10 species
(Wikipedia)
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CITA|ICATJonathan Dursi CAIMS-MITACS 2006June 19
Combustion in Astrophysics
Differences from terrestrialcombustion:
Equation of State
(Partially) degenerate
material Supported by
degenerate electronpressure
Pressure insensitive totemperature at highdensities
Explosive burning
Andrew Truscott &Randall Hulet (Rice U.)
http://atomcool.rice.edu/http://atomcool.rice.edu/http://physics.rice.edu/http://physics.rice.edu/http://atomcool.rice.edu/ -
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CITA|ICATJonathan Dursi CAIMS-MITACS 2006June 19
Combustion in Astrophysics: Novae
Burning on surface of whitedwarf
Accretes matter (hydrogen,helium) from neighborfaster then can stably burn
Burst of convectiveburning, lifts accretedenvelope, sends burnedmaterial into surroundings
Important source of heavyelements for new stars,planets
Courtesy Hubble STScI
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CITA|ICATJonathan Dursi
CAIMS-MITACS 2006June 19
Combustion in Astrophysics: X-Ray Bursts
Burning on surface ofneutron star
Higher gravity: higherdensities
Burning proceeds asdeflagration (detonation?)
Visible in X-rays from greatdistances. Gravity too
strong for importantamounts of ejectaCourtesy Chandra X-Ray Observatory
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CITA|ICATJonathan Dursi
CAIMS-MITACS 2006June 19
Combustion in Astrophysics: X-Ray Bursts
Surface of a large (~30km) body: can considerlocal piece
Burning propagatesalong layer of fuel as
flame or detonation
Heats, roils atmosphere
Simulations of large
scale behaviour, smallscale flame/detonationphysics
Zingale, SUNY Stony Brook
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CITA|ICATJonathan Dursi
CAIMS-MITACS 2006June 19
Combustion in Astrophysics: Supernovae Ia
White dwarf accretes material
slowly Centre gets hotter, denser
Simmering, rotating -- highlyturbulent
Burning begins in centre of staras flame
Transition to detonation?
Total incineration of whitedwarf
One of largest explosions inuniverse
Courtesy Hubble STScI
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CITA|ICATJonathan Dursi
CAIMS-MITACS 2006June 19
Basiccombustionsimulations:
Cellular
detonations inwhite dwarfs
(unburnedpockets
potentially veryinteresting in
Type Ia context)
Combustion in Astrophysics: Supernovae Ia
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CITA|ICATJonathan Dursi
CAIMS-MITACS 2006June 19
Effect of strain/curvature on thermonuclearflame speed (`Markstein Length')
Combustion in Astrophysics: Supernovae Ia
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CITA|ICATJonathan Dursi
CAIMS-MITACS 2006June 19
Combustion in Astrophysics: Supernovae Ia
Large-scalesimulations ofsystem
Some assumedturbulentburning model
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CITA|ICATJonathan Dursi
CAIMS-MITACS 2006June 19
Modeling Combustion in Astrophysics
Large, small scale simulations
Turbulent burning, flames,detonations
Complex EOS, highlycompressible
Want code that is
Robust
Well-tested methods
Could scale to masssivelyparallel systems
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CITA|ICATJonathan Dursi
CAIMS-MITACS 2006June 19
The Flash Code
Cellular detonation
Compressed turbulence
Helium burning on neutron stars
Richtmyer-Meshkov instability
Laser-driven shock instabilities
Nova outbursts on white dwarfs Rayleigh-Taylor instability
Flame-vortex interactions
Gravitational collapse/Jeans instability
Wave breaking on white dwarfs
Shortly: Relativistic accretion onto NS
Orzag/Tang MHDvortex
Type Ia Supernova
Intracluster interactions
MagneticRayleigh-Taylor
Th Fl h C d
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CITA|ICATJonathan Dursi
CAIMS-MITACS 2006June 19
Cellular detonation
Compressed turbulence
Helium burning on neutron stars
Richtmyer-Meshkov instability
Laser-driven shock instabilities
Nova outbursts on white dwarfs Rayleigh-Taylor instability
Flame-vortex interactions
Gravitational collapse/Jeans instability
Wave breaking on white dwarfs
Shortly: Relativistic accretion onto NS
Orzag/Tang MHDvortex
Type Ia Supernova
Intracluster interactions
MagneticRayleigh-Taylor
FLASH code: Explicit reactive hydrodynamics code AMR, massively parallel (65536 procs+) Scales very well Highly portable Used, tested on wide variety of problems Rigorously tested Modular (easy to add/change physics modules) Widely available (http://flash.uchicago.edu)
The Flash Code
Th Fl h C d AMR
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CITA|ICATJonathan Dursi
CAIMS-MITACS 2006June 19
The Flash Code AMR
Required because of verylarge dynamic range of
scales. Permitted by locality of
problems Can do bigger problems But hard because: Frequent redistribution Load balancing Irregular, unpredictable
memory/message
patterns; hard toprecompute things
Refinement/derefinementa black art.
Th Fl h C d AMR
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CITA|ICATJonathan Dursi
CAIMS-MITACS 2006
June 19
The Flash Code AMR
PARAMESH librarydeveloped at NASA/GSFC
Local physics occurs on ablock as if isolated.
Number of guardcellsdepends on stencil size.
Number of interior points : More cells - more efficient
(until block too big forcache)
Fewer cells - can refinemore quickly in smallerarea.
Guard Cells Interior Cells
Th Fl h C d AMR
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CITA|ICATJonathan Dursi
CAIMS-MITACS 2006
June 19
The Flash Code AMR
Blocks and refinement arein an oct-tree structure.
Refining block -> 2dchildren created, each with2x resolution of parent
Neighbor blocks must differby at most one level of
refinement. Drawback: resolution can
only fall of linearly indistance.
Feature: simplifies, speedsup accurate calculation of`boundary conditions'(guardcells)
10
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1812
2
8
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14 19
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15
1316
1517
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Refinement
Level
10
1 3 4 5
2
6
7 9
11 12
13
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1516 17
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19 20 21
The Flash Code Hydrodynamics
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CITA|ICATJonathan Dursi
CAIMS-MITACS 2006
June 19
The Flash Code Hydrodynamics
Hydrodynamics algorithmsinformed by highlycompressible problemstypical in astrophysics
Finite volume Godunovschemes
Dimensionally split
Extremely capable formodelling shocks,detonations
The Flash Code Hydrodynamics
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CITA|ICATJonathan Dursi
CAIMS-MITACS 2006
June 19
The Flash Code Hydrodynamics
Piecewise ParabolicMethod
Defines an upwindedparabola at each pointwith correct cell average
Very aggressive`flattening' to enforce a
very strict measure ofmonotonicity
Also flattens at contactdiscontinuities
Long history incompressibleastrophysical flows
The Flash Code Hydrodynamics
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CITA|ICATJonathan Dursi
CAIMS-MITACS 2006
June 19
The Flash Code Hydrodynamics
Parabolas improve spaceaccuracy
To improve time accuracy,must modify how left,rightstates are chosen forRiemann solve
Estimate characteristicspeeds in cell and find regionwhich is connected tointerface in timestep
Average over reconstructionin that region
Those are left, right states forRiemann solve
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CITA|ICATJonathan Dursi
CAIMS-MITACS 2006
June 19
Simulating situations
unavailable toexperiment
Testing code results
particularly important
Testing must takesuch forms as it can
FLASH V&V
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CITA|ICATJonathan Dursi
CAIMS-MITACS 2006
June 19
Simplest: check for
bugs from multipledevelopers,compilers...
Test suite run nightly
on multiple platforms Includes each physics
module, integration
Differences (tomachine precision)are flagged, alongwith code changes
FLASH V&V
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CITA|ICATJonathan Dursi
CAIMS-MITACS 2006
June 19
Test suite includes
standard test casesfor physics moduleswith known solutions
More complicated test
cases with`benchmark' solutions
FLASH V&V
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CITA|ICATJonathan Dursi
CAIMS-MITACS 2006
June 19
Validation: Cannot compare to astrophysicalproblems directly
Compare to experiments of relevant fluidinstabilities
Very challenging tests
FLASH V&V
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CITA|ICATJonathan Dursi
CAIMS-MITACS 2006
June 19
FLASH V&V
Collaboration w/experimentersessential forcomparison
Iterative process
Instabilities: canonly comparestatistically
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CITA|ICATJonathan Dursi
CAIMS-MITACS 2006
June 19
FLASH V&V
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CITA|ICATJonathan Dursi
CAIMS-MITACS 2006
June 19
Propagation of turbulent
flame Total burning needed for
large-scale models
Simulations of buoyantlyturbulent flames in low-speed code
Development of modelsfor inclusion into large-scale models
Turbulent burning ischallenging!
Development of subgrid models: flames
Zingale, SUNY Stony Brook
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CITA|ICATJonathan Dursi
CAIMS-MITACS 2006
June 19
Ignition insupernovae likelyhappen at `turbulenthotspots'
Large-scale reactiveturbulence
For given turbulence
intensity, how doesignition happen?
Development of subgrid models: ignition
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CITA|ICATJonathan Dursi
CAIMS-MITACS 2006
June 19
Many 1d sphericalsimulations ofigniting hotspots
Determine`flammability limits
Highly nonlinear
Non-ignitinghotspots contributelittle energy to flow
Development of subgrid models: ignition
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CITA|ICATJonathan Dursi
CAIMS-MITACS 2006
June 19
Large 1d, 3d
simulations ofcompressiblereactive turbulence
Extract temperature,hotspot PDF
Need largesimulations ignitionpoints arenecessarily rareevents
Development of subgrid models: ignition
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CITA|ICATJonathan Dursi
CAIMS-MITACS 2006
June 19
Development/Integration of all-speedsolvers essential for modeling ignitionthrough explosion
Development of meaningful subgridmodels must continue
Continuing testing methods againstinstability experiments: often interesting
research problems in their own right.
Future Work