Update on common envelope simulations

Recap of Last Post

  • I had performed a run up to a simulation time of 20 days which had the same parameters as the Ohlmann+16a simulation, except that:
    • the softening length is larger by a factor of 2
    • the smallest resolution element is larger by a factor
    • the RG primary is not rotating initially
    • the ambient density is equal to that at the surface of the RG, which is orders of magnitude larger than in Ohlmann+16a
    • the binary run starts just after velocity damping has completely turned off (rather than waiting 5 more dynamical times) and the dynamical time used is the free-fall time, which is about 3/8 of the sound-crossing time used by them
  • In the binary run that we had done, the closest separation was already down to about 2.5 smoothing radii. In Ohlmann+16a they set a floor that the softening radius should not exceed 1/5 of the separation. Their softening radius thus changed dynamically.

New Work

  • I attended the "Evolved Stars: Role of Binarity" conference in Nice and presented this work.
  • I did a relaxation run for the RG star with the softening length reduced by a factor of two but the resolution the same.
  • I tried several runs with the same parameters as for the original relaxation run but with lower ambient density.

Summary of New Results

  • The talk went well. The slides are available ​here.
  • With limited computing resources, it doesn't make much sense to try one change at a time, so I worked on the density issue (using bluehive).
  • The lower ambient density runs are very slow. I present the results of tests below.
  • In parallel, Baowei, is trying to optimize the code on Comet.

Detailed Results

  • Run with half of previous softening length (2.4Rsun instead of 4.8Rsun) but same resolution 0.29 Rsun: central density decreases by 35% from to s when damping is turned off. This can be compared with 11% for the original run with 4.8Rsun softening length. Should resolution be doubled? For comparison Ohlmann+16a used softening radius of 2.8Rsun and resolution 0.14Rsun for relaxation run.

Relaxation run with single RG star
Damp104) Extrapolated hydro BCs, Multipole expansion Poisson BCs, ambient dyne/cm, ambient density g/cc.
(bluehive standard 120 cores)
( cm, , 5 levels AMR)
lTrackDensityProtections=T, lTrackPressureProtections=T, MinDensity=1d-14
​2d density + velocity
​2d pressure
​2d temperature

Damp105) Extrapolated hydro BCs, Multipole expansion Poisson BCs, ambient dyne/cm, ambient density g/cc.
(bluehive standard 120 cores)
( cm, , 5 levels AMR)
​2d density + velocity
​2d pressure
​2d temperature

Discussion

  • The run with lower density (Damp104) is very slow (estimated wall time 6 months at frame 6.7 of 1500) and eventually crashed (timeout error)
  • The more pronounced artefacts in Damp104 compared to Damp105 could be the reason, but it is not clear
  • The larger temperature in Damp104 could mean that the computational unit TempScale~ K is too small. However, I've now played around a lot with the computational units and this does not make a significant difference.
  • It is possible that AMR is causing the problem, so I'm now doing smaller box runs with and without AMR to see if it makes any difference.

Next Steps

  • First try to fix this problem with low ambient densities
  • We should use half the smoothing length (now done) but should we double the resolution? Should we increase the time for the relaxation run?

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