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Update on CE project
Recap of Last Post
- In the last post I presented the first runs that included a secondary point mass. I had forgotten to initialize the primary with an orbital velocity.
- I had also presented the first runs that attempted to translate the star across the grid. I had been implementing this incorrectly, as I had been adding the velocity every time step rather than just at the start of the run, and also not giving any velocity to the point mass.
New Work
This time I've tried the following:
- Translating the RG across the grid at 100 km/s. This is important because otherwise in the binary, there would be no way of knowing whether effects on the RG were caused by the secondary or by motion through the ambient medium. We want to make sure the RG is stable as it moves through the ambient medium.
- Evolving the RG from t=0 (now without translating) with a nested grid that minimizes resolution outside the RG. The point is to save computation time since there is no need to have high resolution outside the star.
- I've also written a script in IDL that calculates the required initial velocities for a given set of orbital parameters (masses, binary separation, eccentricity, orientation of the orbit on the x-y plane), and makes a simple animation of the orbit, showing the positions and velocities as a function of time in CM coordinates.
Summary of New Results
- I was able to translate the star across the grid. After about a dynamical time, it begins to become unstable on the trailing side.
- The simulation with low resolution in the ambient medium runs about 4-5 times faster than the previous sims which allowed for AMR in the ambient medium. However, the star becomes unstable earlier, and relatively large density contrasts develop in the ambient medium near the star.
Results
I) Translation across the grid at 100 km/s
Damp069) Extrapolated hydro BCs, Multipole expansion Poisson BCs, ambient dyne/cm
(bluehive standard 120 cores)
( cm, , 5 levels AMR)
(Restarted from run Damp062, at s, after Damping stopped, to s
2d density (continuous)
2d density (1 loop)
2d density and velocity (continuous)
2d density and velocity (1 loop)
For comparison, from a previous post, here is the same run without translating
Damp062) Extrapolated hydro BCs, Multipole expansion Poisson BCs, ambient dyne/cm
( cm, , 5 levels AMR)
(bluehive standard 120 cores up to frame 7 and then about 2 days on comet compute 864 cores, 2 cpus/task up to frame 150 )
2d density (continuous)
2d density (1 loop)
COMMENT: The star becomes unstable on the trailing side sooner than it takes to become unstable when it is not in motion.
II) Evolve RG with low resolution outside the star
Damp070) Periodic hydro BCs, Periodic Poisson BCs, ambient dyne/cm
(bluestreak standard 8192 cores, about 3-4 days computation time)
( cm, , 5 levels AMR)
(As Damp059 except that resolution reduced in ambient medium, with 2 buffer cells per level)
2d density (continuous)
2d density (1 loop)
For comparison, from a previous post, here is the same run without constraining refinement outside RG
Damp059) Periodic hydro BCs, Periodic Poisson BCs, ambient dyne/cm
(bluestreak standard 8192 cores, about 15 days computation time up to s)
( cm, , 5 levels AMR, run up to s)
2d density (continuous)
2d density (1 loop)
Damp078) Extrapolated hydro BCs, Multipole Expansion Poisson BCs, ambient dyne/cm
(bluehive standard 120 cores, about 34 hours computation time)
( cm, , 5 levels AMR)
(As Damp070 except that different BCs, and now 8 buffer cells per level)
2d density (continuous)
2d density (1 loop)
2d density with mesh (continuous)
2d density with mesh (1 loop)
COMMENT: The star becomes unstable sooner than it takes to become unstable when low resolution is not imposed on the ambient medium. But there is a tradeoff as the computation time is reduced by a factor of 4-5.
III) Circular binary orbit with 1 solar mass secondary (as Ohlmann+16a)
(comet compute 1728 cores, 2 cores per task to increase memory per task, a little over 1 day computation time)
( cm, , 5 levels AMR)
2d density (continuous)
2d density (1 loop)
2d density (2x zoom, continuous)
2d density (2x zoom, 1 loop)
2d density (4x zoom, continuous)
2d density (4x zoom, 1 loop)
2d density (Edge-on, continuous)
2d density (Edge-on, 1 loop)
2d density (Edge-on, 2x zoom, continuous)
2d density (Edge-on, 2x zoom, 1 loop)
2d density (Edge-on, 4x zoom, continuous)
2d density (Edge-on, 4x zoom, 1 loop)
Discussion
The RG is still not quite stable enough after damping is turned off and it is allowed to evolve for a few dynamical times. The situation worsens somewhat when the star is translated across the grid. It is worth comparing with Ohlmann+ to see what differences may be important between their setup and ours. Here is a table comparing the two setups:
The most obvious difference is the density of the ambient medium being much larger in our setup. I am currenlty trying to reduce this ambient density to see if it will help to improve the stability of the RG.
The other curious thing is that they used a very small box in their relaxation run, and they used periodic BCs. Does this explain the oscillations they were getting?
Next steps
- Experiment with lower ambient densities
- Longer binary runs
Comments
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