Hydro Colliding Flows with shear - Tests with fixed grid

Checking staircase effect

Running the 2D shear flow, fixed grid, shows the stair casing effect is not a function of the AMR grid placement. The effect seemed most prominent in the 30-degree shear angle case from my previous post, so am using that for comparison here. Fixed grid, 5122 is on bottom. Movie attached.

movie​

I also checked whether the AMR speed-up factor predicted by astrobear.log for the non-shear and 45-degree shear cases were valid. These were run on bamboo, 8 processors, machine not busy.

Checking AMR speed-up factor

Non-shear case

To make the table, I took the data from astrobear.log at the time-step right before the last frame of the simulation.

Simulation Filling fractions Info allocations Efficiency Speed-up Actual Speed-up
642 + 3 0.135 0.614 0.762 18.3 mb 3.6 mb 54% 3.3 ~5
5122 na 41.4 mb 5.2 mb 82% 5 na

movie​

45-shear case

Simulation Filling fractions Info allocations Efficiency Speed-up Actual Speed-up
642 + 3 0.207 0.643 0.789 25 mb 5.4 mb 44% 1.6 ~4
5122 na 41.4 mb 5.2 mb 82% 5.3 na

movie​

  • Between the simulations of varying shear, I notice an increase in the filling fraction on level 0. This is because of the increase in the collision region. So, shear runs take longer.
  • In both cases here, the amr speed-up factor as reported in the .log file for the AMR cases is an UNDER-estimate. AMR speeds up the simulation by ~5 times, even though in the high shear run it estimated the speed-up would be closer to 1.5.
  • Current refinement criteria has the collision region nicely refined, but the splashing effects are not captured at high resolution. We see some features in these areas being lost by the refinement when compared to fixed grid. Is this something we would like to improve?

Resolution

Here is the fixed grid, 5122 45-degree resolution, and the mesh for the AMR (64+3) for comparison,

A possible concern here might be some loss of detail in the shear flows being splashed away from collision region with current refinement criteria (arrows in left plot). This might be a region that would otherwise form sink particles, but since the highest level of mesh isn't there, no sinks can be generated in these regions.

To get an idea of how large the flow region is, and the velocity field, here is a movie of vx:

movie​

  • We see entrenched gas with high, positive speed in the x-direction in the splashing region. This likely would encounter similar gas from the other direction, which would collide and form dense regions of possible core-formation. This seems to indicate we might want to increase resolution into these regions. Other than that, should I try to reduce refinement in the collision region and check for loss of features in an attempt to speed of computation time?
  • There is an apparent asymmetry in the high density forming regions, at least at early times, in the lower half of the collision region. This is curious.
  • There appears to be some improper nesting occurring due to the tilted cross-section. This might be problematic should there be sharp gradients across this boundary, possibly resulting in nans.

Cone-shape

Looking at these simulations again today, I am curious about this cone-shape feature that appears at the splashing region of the collision area. I am interested in why this shape is so smooth, and remains so as it expands over time? By first impression, it looks too smooth. Would expect some turbulent features to appear along the boundary as it is pushed into the surrounding medium. Am left thinking it must have to do with the thermodynamics/EOS, which in this case is ideal with gamma = 5/3 + cooling.

movie​

  • The clump-crushing time came to mind in thinking about this; the crushing time might be > the time of this sim. Further, the density of the ambient is lower than the material in the cone. It just doesn't seem to provide enough drag on the incoming material to disrupt the flow..

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