dursi-astro-combustion

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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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Outline

Combustion inAstrophysics

The FLASH code

Testing / V&V

TowardsMultiscale/subgrid

approaches

(Rpke, Max Planck Institute for Astrophysics)


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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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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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Differences from terrestrialcombustion:

Thermonuclearreactions, not chemical

Fairly minor differencesin behaviour Arrhenius-like

Much simpler`chemistry'!

Energetics capturedwith ~10 species

(Wikipedia)


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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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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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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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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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Basiccombustionsimulations:

Cellular

detonations inwhite dwarfs

(unburnedpockets

potentially veryinteresting in

Type Ia context)



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Effect of strain/curvature on thermonuclearflame speed (`Markstein Length')



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Large-scalesimulations ofsystem

Some assumedturbulentburning model


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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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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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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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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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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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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

116

1812

2

8

7

9

14 19

202

1

15

1316

1517

3

Refinement

Level

10

1 3 4 5

2

6

7 9

11 12

13

14

1516 17

18

19 20 21

The Flash Code Hydrodynamics


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CAIMS-MITACS 2006

June 19


Hydrodynamics algorithmsinformed by highlycompressible problemstypical in astrophysics

Finite volume Godunovschemes

Dimensionally split

Extremely capable formodelling shocks,detonations



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CAIMS-MITACS 2006

June 19


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



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CAIMS-MITACS 2006

June 19


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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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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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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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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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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CAIMS-MITACS 2006

June 19

FLASH V&V

Collaboration w/experimentersessential forcomparison

Iterative process

Instabilities: canonly comparestatistically


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CAIMS-MITACS 2006

June 19

FLASH V&V


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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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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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CAIMS-MITACS 2006

June 19

Many 1d sphericalsimulations ofigniting hotspots

Determine`flammability limits

Highly nonlinear

Non-ignitinghotspots contributelittle energy to flow



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CAIMS-MITACS 2006

June 19

Large 1d, 3d

simulations ofcompressiblereactive turbulence

Extract temperature,hotspot PDF

Need largesimulations ignitionpoints arenecessarily rareevents



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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

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