New Work

• Performed and analyzed new run 125 that evolves binary for 3 times longer than old run 116 (60 days as opposed to 20 days), made possible by forcing ambient medium to have lower resolution.
• Compared run 125 with run 116 to check to what degree forcing the ambient medium to be low res affects the region of interest in the center.
• Compared run 125 with results of Ohlman+16a.
• Peformed new relaxation run 129 with resolution doubled for RG, made possible by making resolution of ambient medium much lower.
• Set up new high res binary run 132 that uses 129 and also the trick of run 125 which forces low resolution in the ambient medium [pending on stampede].
• Set up new run 133 that uses standard resolution (like 125) but now makes max resoln refinement zone a function of inter-particle separation (so evolves with time) [pending on bluehive].

Summary of New Results

• Run 125, which removes resolution in the surrounding medium, is reasonably consistent with run 116 in the highly resolved region of interest for the first 20 days (first few orbits). This suggests that it is okay to greatly reduce the resolution outside of the region of interest.
• Run 125 is qualitatively similar to the results of Ohlmann+16a for the first 60 days, but the amplitude and frequency of variation with time of the separation are smaller than in O+16a. This suggests that the gravitational force is still not being fully resolved in our sims (but a higher res version is pending).

Detailed Results

Binary run 116 (see also previous blog post) with half the softening length of old run binary run 088 (and relaxation run 062)
Relaxation run: 096
First frame: 75 (5 RG freefall times, when velocity damping ended)
Last frame: 168
Total simulation time: 3.36e6 s or ~4.2 RG sound-crossing times (93 frames)
Machine and partition: Stampede 1 normal
Number of cores: 1024
Total wall time: 96 hours
Hydro BCs: extrapolated
Poisson BCs: multipole expansion
Box size: L=4e13 cm
Base resolution: 9.0 Rsun (64³ cells)
Highest resolution: 0.29 Rsun (2048³ cells, 5 levels AMR)
AMR implementation: set automatically by AstroBear
Softening length: 2.4 Rsun
Ambient density: 6.7e-9 g/cc
Ambient pressure: 10⁶ dyne/cm²

Comments

• See last blog post for movies and discussion.
• Evidence of a disk around secondary.
• Problem: too computationally demanding.
• Solution: reduce resolution in outer regions that are not directly interesting. But will this affect region of interest?

Binary run 125 with longer simulation time and lower resolution in ambient medium
Relaxation run: 096
First frame: 75 (5 RG freefall times, when velocity damping ended)
Last frame: 335
Total simulation time: 5.2e6 s or 60 days ~6.5 RG sound-crossing times (260 frames)
Machine and partition: Bluehive standard
Number of cores: 120
Total wall time: about 8.5 days
Hydro BCs: extrapolated
Poisson BCs: multipole expansion
Box size: L=4e13 cm
Base resolution: 9.0 Rsun (64³ cells)
Highest resolution: 0.29 Rsun (2048³ cells, 5 levels AMR)
AMR implementation: set by hand to have max level around point particles
Max resolution zone: within 50 Rsun of primary and within a cylinder of radius 50 Rsun and height 50 Rsun around secondary
Buffer zones: 0 cells (no buffer zones)
Softening length: 2.4 Rsun
Ambient density: 6.7e-9 g/cc
Ambient pressure: 10⁶ dyne/cm²

Movies
Density sliced at half-way point of box:
​face-on full box
​face-on zoom-in
​edge-on zoom-in
​face-on slice through secondary, extra zoom-in
​face-on slice through secondary, extra zoom-in with velocity vectors

Edge-on slice through secondary as viewed from primary point particle:
​viewed from P1 with P2 at center

Edge-on slice through secondary AND primary:
​slice through P1 (left side) and P2 (center)

Mach number:
​face-on full box

Particle mass:
​face-on zoom-in

Comparison of runs 116 and 125:
Comparison of density (116 on left, 125 on right):
​face-on slice through secondary, extra zoom-in

Comparison of particle mass (116 on left, 125 on right):
​face-on zoom-in

Comparison of separation-time graph and orbits (116 on left, 125 on right):

Comparison of density at 10 and 20 days (116 on left, 125 on right):

Density at 40 days (left) and 60 days (right) for run 125:

Ohlmann+16a Fig 3:

Comments

• The results compare well qualitatively to those of run 116. However, there are small differences:
  • The "disk" that forms arond the secondary is still slightly less circular and less extended than in run 116.
  • The separation variation AMPLITUDE is reduced by about 20% by t=20 days compared to run 116.
  • The mass accreted onto the secondary is increased by about 1% at t=21 days compared to run 116.
• The results compare well qualitatively with those of Ohlmann+16a. However, the differences are significant:
  • The separation-time plot has a smaller amplitude of oscillations.
  • The separation-time plot has a smaller frequency of oscillations.
  • The mean separation is larger by about 25% by t=60 days.

Relaxation run 129 with double max resolution, lower resolution in ambient medium, 10x smaller ambient pressure than run 096
First frame: 0
Last frame: 138 (2.76e6 s or 9.2 RG freefall times of primary star, with damping implemented up to frame 75)
Total simulation time: 2.76e6 s or 3.45 RG sound-crossing times
Machine and partition: Stampde 1 normal
Number of cores: 1024
Total wall time: 92.5 hours (about 49 hours to reach frame 75)
Hydro BCs: extrapolated
Poisson BCs: multipole expansion
Box size: L=4e13 cm
Base resolution: 2.25 Rsun (256³ cells)
Highest resolution: 0.14 Rsun (4096³ cells, 3 levels AMR)
AMR implementation: set by hand to have max level around point particles
Max resolution zone: within 5e12 cm (71.87 Rsun) of primary point particle
Buffer zones: 2 cells
Softening length: 2.4 Rsun
Ambient density: 6.7e-9 g/cc
Ambient pressure: 10⁵ dyne/cm²

Movies
Density sliced at half-way point of box:
​zoom-in

Mach number sliced at half-way point of box:
​zoom-in

Comments

• We have run the relaxation run up to frame 138 but present the results up to frame 75 here.
• Frame 75 or 1.5e6s is the point at which damping has been reduced to zero (5 free-fall times).
• We see that the density is reasonably stable, though it is still evolving at t=1.5e6s.
• This suggets that the new refinement criteria used here are reasonable.
• The plot of Mach number shows that grid effects are present.
• In the future we may want to:
  • Increase the resolution, enabling higher resolution in the binary runs.
  • Increase the relaxation time from 5 freefall times to 5 sound-crossing times with damping + 5 sound-crossing times without damping, as in O+16a.
  • Increase the box size.
• Note that the relaxation run is quite costly (but several binary runs can be executed from a single relaxation run). This run up to frame 75 took 49 hours on stampede with 1024 cores.
• One way to speed it up would be to resolve the core and a shell that includes the surface, rather than the whole star. But then the resolution in between would still have to be reasonably high, so would need to use large buffer zones.
• Note that with the higher resolution, we were able to decrease the ambient pressure by 1 order of magnitude. This also helps to reduce the ambient temperature and sound speed, as the density is kept the same as for run 125. This may help to relax the CFL constraint.
• Maybe it would be worth trying a binary run that DOES NOT USE ANY RELAXATION RUN (i.e. no damping) and see how much difference there is.

Binary run 132 with double max resolution, lower resolution in ambient medium, 10x smaller ambient pressure than run 116
Relaxation run: 129
First frame: 75 (5 RG freefall times, when velocity damping ended)
Last frame:
Total simulation time:
Machine and partition: Stampde 1 normal (running)
Number of cores: 1024
Total wall time:
Hydro BCs: extrapolated
Poisson BCs: multipole expansion
Box size: L=4e13 cm
Base resolution: 2.25 Rsun (256³ cells)
Highest resolution: 0.14 Rsun (4096³ cells, 4 levels AMR)
AMR implementation: set by hand to have max level around point particles
Max resolution zone: within 5e12 cm (71.87 Rsun) of primary center and within a cylinder of radius 20 Rsun and height 20 Rsun around secondary center
Buffer zones: 0 cells (no buffer zones)
Softening length: 2.4 Rsun
Ambient density: 6.7e-9 g/cc
Ambient pressure: 10⁵ dyne/cm²

Binary run 133 similar to run 125 but now the refinement zone changes with time
Relaxation run: 096
First frame: 75 (5 RG freefall times, when velocity damping ended)
Last frame:
Total simulation time:
Machine and partition: Bluehive standard (pending)/Bluestreak standard (running)
Number of cores: 120(bh)/8192(bs)
Total wall time: about
Hydro BCs: extrapolated
Poisson BCs: multipole expansion
Box size: L=4e13 cm
Base resolution: 9.0 Rsun (64³ cells)
Highest resolution: 0.29 Rsun (2048³ cells, 5 levels AMR)
AMR implementation: set by hand to have max level around point particles
Max resolution zone: within min(5e12cm, 2.5*particle_separation)
Buffer zones: 2 cells
Softening length: 2.4 Rsun
Ambient density: 6.7e-9 g/cc
Ambient pressure: 10⁶ dyne/cm²

Discussion

• Refinement of the central region only works reasonably well and is the way forward.
• Should work toward making the refinement a function of the particle separation as attmpted in ongoing run 133.
• Comparison of separation vs time and orbit plots with those of O+16a suggests we still lack enough resolution.

Next Steps

• Analyze ongoing runs.
• Do run that combines merits of 132 and 133.
• Output diagnostics including accretion rate onto secondary, drag force on secondary
• Experiment with variations to "fiducial parameter values": initial separation, primary spin, accretion onto primary.