wiki:u/u/erica/EricasCFIntro

Version 20 (modified by Erica Kaminski, 11 years ago) ( diff )

2D MHD Colliding Flows with gravity and cooling

Diameter of colliding flows 25 pc
Mach 1.5
Number density 1 cm-3
Beta 1
Cells per Jeans Length 64
Amplitude of interface perturbations, scaled to largest flow diameter .015
Time to shut off flows 30 Myr
Cooling Modified II curve (10K at 1000/cc)
Final time 17 Myr
dt of framerate .12 Myr
resolution (642 x 1) + 3 levels
box size 50pc X 50pc X dx in x,y,z respectively
finest dx 0.09 pc
Boundary conditions reflecting, and keep B-normal in x, open in y and z
self-gravity? yes

(Note: The inputs in the data files are given above, however, visualization in Visit seems to show both of these values less by 0.5 — I have to check into this more)

Here are some stills of the initial conditions

First is the first frame (t=dt) showing the mesh,

Here is just the density at this first frame, the legend is in particles/cc:

(Note the 0th frame for the density just shows a uniform field = 1/cc)

And here is the x velocity for the 0th frame (t=0),

Here is a movie of the density, with field vectors overlaid

The field starts with a Beta=0.5 everywhere in the box. We see early on the NTSI develops and grows for the first 20 frames or so. By about 2 Myr, it seems the NTSI begins to collapse back toward the midplane. Without the troughs of the NTSI for the material to funnel toward, the material is instead splashed laterally in the y-direction. This creates large shear that excites KH modes especially at the tops and bottoms of the colliding streams. The first core (sink particle) forms late in the simulation, by 15 Myr, in the center of the slab corresponding to the global potential minima. Peak densities at this time are ~12,000 /cc << 1021 number densities Heitsch et al. regarded previously as the threshold when H2 can appreciably form in the clouds due to effective shielding.

movie

Here is a movie of the density, compared to hydro with and without cooling

Focusing on the left column of the plot, we see the field restricts the lateral dispersal of the collision region away from the colliding streams. This seems to have 3 important effects. 1) In the hydro run, the clumps fragment and become localized structures moving in the flow, whereas in the MHD run a long filamentary structure forms. 2) The 'clumping' of material in the MHD run is enhanced, producing higher densities than in the Hydro run. A sink particle forms in the MHD run by frame 134, but does not form (at least by the final frame 140) in the hydro run. 3) The collision region seems bound in the MHD run, but not in the hydro run.

The right column shows the following: without cooling, we see the collision region expands (quickly) — increasing density leads to increased pressure, which forces the evacuation of the gas. In the hydro case, we see little localized clumps of lower density material form within the collision region. These structures do not form in the MHD runs, but rather we see again longer filamentary structures form there.

The boundary conditions are reflecting on left and right, and extrapolating on top and bottom. The behavior in the hydro, non-cooling case makes sense with these BCs, but I am not entirely sure what is going on in the MHD non-cooling case where there is some weird flow coming back into the grid on the top and bottom..

movie

3D MHD Colliding Flows with gravity and cooling

Diameter of colliding flows 25 pc
Mach 1.5
Number density 1 cm-3
Beta 1
Cells per Jeans Length 64
Amplitude of interface perturbations, scaled to largest flow diameter .015
Time to shut off flows 30 Myr
Cooling Modified II curve (10K at 1000/cc)
Final time 17 Myr
dt of framerate .12 Myr
resolution (643) + 4 levels
box size 50pc x 50pc x 50 pc in x,y,z respectively
finest dx 0.05 pc
Boundary conditions reflecting, and keep B-normal in x, open in y and z
self-gravity? yes

(Note: The inputs in the data files are given above, however, visualization in Visit seems to show both of these values less by 0.5 — I have to check into this more)

The effective resolution of this run is 2x higher than the 2D runs. There appears to be some errors forming at the end of the simulation, and have not been able to finish this run at this resolution on the amount of cores I was running it on. Thus this run stops prematurely at t = 4.7 Myr.

Here is a movie of the density

Comparing the 3D higher resolution runs to the 2D lower resolution runs, I see the same global evolution of the NTSI on similar timescales, that is by about 2 Myr, the NTSI seems to collapse back in on itself, with its fingers becoming less prominent. I also see material being contained within the slab region in y-direction, and KH waves appearing at the top and bottom of the flow as before. The major difference between the 2D and 3D runs is that the density grows much more quickly in the 3D runs, leading to stronger cooler and stronger nonlinear density perturbations inside of the collision region. This is in contrast to the 2D run, which seems to instead form a single long filamentary structure. I am not sure if this is mainly due to the resolution or dimension.

movie

Attachments (6)

  • CFinitialvx.png (22.4 KB ) - added by Erica Kaminski 11 years ago.
  • CF.png (23.3 KB ) - added by Erica Kaminski 11 years ago.
  • meshCF.png (20.8 KB ) - added by Erica Kaminski 11 years ago.
  • fieldlinesrho2dCF.gif (9.1 MB ) - added by Erica Kaminski 11 years ago.
  • 2dCFcomparison.gif (24.0 MB ) - added by Erica Kaminski 11 years ago. Focusing on the left column of the plot, we see the field restricts the lateral dispersal of the collision region away from the colliding streams. This seems to have 3 important effects. 1) In the hydro run, the clumps fragment and become localized structures moving in the flow, whereas in the MHD run a long filamentary structure forms. 2) The 'clumping' of material in the MHD run is enhanced, producing higher densities than in the Hydro run. A sink particle forms in the MHD run by frame 134, but does not form (at least by the final frame 140) in the hydro run. 3) The collision region seems bound in the MHD run, but not in the hydro run. The right column shows the following: without cooling, we see the collision region expands (quickly) — increasing density leads to increased pressure, which forces the evacuation of the gas. In the hydro case, we see little localized clumps of lower density material form within the collision region. These structures do not form in the MHD runs, but rather we see again longer filamentary structures form there. The boundary conditions are reflecting on left and right, and extrapolating on top and bottom. The behavior in the hydro, non-cooling case makes sense with these BCs, but I am not entirely sure what is going on in the MHD non-cooling case where there is some weird flow coming back into the grid on the top and bottom..
  • rho3Dcfs.gif (1.9 MB ) - added by Erica Kaminski 11 years ago.
Note: See TracWiki for help on using the wiki.