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Update on CE simulations
New Work
- I performed a new binary run with half the softening length but other parameters the same (run 116 with relaxation run 096) except that I also changed the initial positions and velocities of the particles so that the frame of reference is effectively rotated by 180 degrees.
Summary of New Results
- For the binary runs, the code slows down after about t = 10-15 days and the chombo files get much bigger, probably due to increasing refinement.
- The separation vs time graph is similar to the old run, and about equally similar to O+16a.
Detailed Results
Binary run 96/116 with half softening length of old run 062/088
Damp116) Extrapolated hydro BCs, Multipole expansion Poisson BCs, ambient dyne/cm, ambient density g/cc.
(Stampede 1 normal 1024 cores)
( cm, , 5 levels AMR)
2d density slice zoomed
2d density slice with mesh
- How does halving the softening length change the results? Let us compare the old run (062/088) with the new run (096/116) with softening length 2.4Rsun instead of 4.8Rsun (for both primary and secondary).
2d density comparison of 062/088 (left) and new run 096/116 (right)
Below are snapshots of density in the orbital plane from t = 10 days (left) and t = 20 days (right). Top row: old simulation 062/088. Next row: new simulation 096/116 with half the softening length. Bottom fig 1 from O+16a. In the O+16a figure, the '+' denotes the primary core particle, while the 'x' denotes the secondary point particle. The core and secondary are denoted as '0' and '1' respectively in my plots.
We see that the two runs are qualitatively very similar, as expected. How do they compare in detail?
On the bottom below I've plotted Fig 1 of Ohlmann+16a and the equivalent figure with these simulations for comparison. Note that the new run went up to 21 days while the old run went up to 20 days. In my plot of separation, I've shown the softening radius as a dotted horizontal line. Circles represent the softening radius, while the green dot shows the initial center of mass of the two stars. The very small green square in the upper right shows the smallest resolution element, which is the same for both runs (about 0.29 Rsun).
Discussion
- Up to a few days after the first minimum, the results look almost identical. Softening length begins to play a role thereafter.
- However it is not clear which run is more accurate because the softening length is not as well-resolved in the new run.
- The new run (096/116) takes about 6 days of wall time (including 2 days for the relaxation run).
- The code becomes even slower after t = 10 days of the binary run, probably because the AMR is refining to the max level over a larger and larger volume.
- Regardless, we need to get higher resolution, larger box size, longer relaxation runs, and longer binary simulation time.
- This will require that we force the resolution to be much lower away from the point particles.
Conclusions
- Halving the softening length produces obvious quantitative differences, but only starting toward the end of the first orbit.
- However, it is unclear whether the differences between the two runs with different softening lengths are due to differences in the softening length per se or to differences in the ratio of softening length to resolution.
Next Steps
- I managed to perform a low-ambient density run (rho = 1E-10 g/cm3 rather than 6.7E-9 g/cm3) on stampede 1 while on vacation…it will take a few more days to complete. This will tell us what difference the ambient density makes.
- We need to modify the code to make it able to refine to a high level only around the (moving) point particles.
- Then it will be feasible to increase the max resolution and isolate the effects of resolution and softening length.
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