Reorienting MHD Colliding Flows

MHD, w/cooling, mach=1.5, beta=1, beta_ram=3.8

30-degree angle

(Movie ​here)

Here we see that the outer shock stalls on the top right and bottom left of the collision interface. This has the effect of 'reorienting' the shock front over time, so that it becomes normal to the magnetic field. Internally, it takes much longer for the internal collision surface to reorient, and then, it only becomes normal to the flows near the flow/ambient boundary. That happens to coincide with the field becoming parallel to this inner shock layer. Late in the sim, we also see what appears to be magnetic field waves, leaving the collision region and heading toward the flow boundaries. The entire interface then becomes unstable, and begins to break down.

Note also that while the 1D solution (without cooling), as well as the cooling case of this run, show velocity vectors going to zero across the slow shock, the present case with cooling shows strong velocity vectors 'falling onto the main filament'. This must be do to the cooling/compression of the gas. This motion in the cooling case leads to a more kinked field.

60-degree angle

Wow, we get much stronger re-orientation in this case, and in fact, it seems from a slightly different mechanism — one in which the deformity of the field plays a big role. Notice in the below movie of rho, that instead of 'blow-out' regions from the central shock zone, we start to see the flow turn around and re-enter the collision region! The flow is following the strongly deformed field. The flow vectors seem to be pushing on the central interface, causing the entire thing to reorient.

​movie

The field structure is also fascinating to watch. Over time, we see a central 'line' of black appear… this is the strongest field strength in the plot.

​movie

It is interesting that despite the cooling, the shock fronts are so nicely supported. Clearly, they are magnetically supported.

MHD, w/cooling, mach=10, beta=1, beta_ram=38

movie​

The strong shock creates strong post-shock pressures that cause a fast ejection of material from the collision region. Because of the strong shock, we also get strong compressions and thus, strong cooling. This reduces the thermal pressure in the collision zone, which triggers thermal instability, and subsequent NTSI, KH, RT modes. The fast shock is not longer supported by either thermal or magnetic pressure, so it collapses down on the thin boundary. If the field were stronger, we would likely see more magnetic support and thus a stronger fast shock front.

Despite the ripples of the collision interface, we do see reorientation occur. While the instabilities are growing in the center, you can see the field 'ballooning'. Oppositely directed field comes together, and looks to dissipate in strength (go from dark to light in color).

MHD, w/cooling, mach=1.5, beta=10, beta_ram=38

These are the same parameters as the 3D MHD colliding flows paper. In that case, the 30 degree shock didn't seem to reorient, but the 60-degree shock did.

30-degree angle

Weakening the field leads to less reorientation. Both the outer shocks and the inner, do not reorient as much as the beta=1 case.

We also see that the ejected material pushes the field out to a further radius, which is expected. This is the same as saying the flow can escape the collision region easier in this case, and this is why we start to see a straightening of the upper and lower 'knobs' of ejected material. They are the only parts of the flow that reorient, and become normal to the flows. The internal collision layer doesn't reorient

movie of rho​ movie of field​​

60-degree angle

​movie of rho ​movie of field

MHD, w/cooling, mach=1.5, beta=.1, beta_ram=.38

movie​

Keeping the mach constant, but increasing the beta, we see that the field is strong enough to resist significant ejection from the collision region. Without this ejection, the outer shocks do not stall, and therefore, we do not see reorientation occur.

MHD, w/cooling, mach=10, beta=.1, beta_ram=3.8

movie​​

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