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Mach stem exploration
Running Eddie's setup,
time = 39.7 tau; M = 30.02; d = distance between clumps centers;
Summary: Simulation Extrapolation/Interpolation
| d [R_clump]\gamma | 5/3 | 1.4 | 1.2 |
|---|---|---|---|
| 3 | TMS, SB | TMS, SB | TMS, SB |
| 3.25 | TMS, SB | TMS, SB | SMS |
| 3.5 | TMS, SB | TMS, SB | RR |
| 4 | TMS, SB | TMS, SB | RR |
| 4.5 | TMS, SB | TMS, SB | RR |
| 5 | TMS, SB | SMS | RR |
| 5.5 | TMS, SB | RR | RR |
| 6 | TMS, SB | RR | RR |
| 6.5 | TMS, SB | RR | RR |
| 7 | SMS | RR | RR |
| 7.5 | SMS | RR | RR |
| 8 | SMS | RR | RR |
| 8.5 | SMS | RR | RR |
| 9 | RR | RR | RR |
Snapshots:
gamma = 1.4; d = 6, 5, 4 clump radii:
Brainstorming for October shots - multiple wires?
The upcoming Astroshock campaign shot day on October 27 will feature driver tests; we also anticipate a few magnetized wire shots (possibly done with new drivers being tested on the same day).
Last year we tried to reproduce magnetospheric structures in an experiment at LLE by having a blast wave overrun a single wire of ~600 um thickness (conductor ~400 um thickness). Due to various real-world complications the magnetic influence on the flow was weak, at best.
With the upcoming shots, we would like to modify both the upstream (i.e. driver) and the downstream (wires) to make the magnetic effects much more obvious, one proposal involves using multiple wires.
Running with this, I was inspired by Eddie's work on Mach stems: Mach stems can be an obvious feature due to brightness, and they may be robust due to hysteresis:
Mach stem evolution can be controlled by adjusting the separation of the obstacles/clumps/wires.
The separation of the obstacles can be controlled by the state of the magnetic field.
Run of blast wave across magnetized wire up to late times
Attached - MPEG comparing density plots for runs with:
-Top, 10 T wire surface field
-Bottom, hydro
-The blast wave followed the behavior described in post aliao09082014; the density of the wire was 7.5 g/cc (80% Cu).
-Contours of the magnetic field strength are overlaid.
Key observations from the video:
-Before the rarefaction of the blast wave reaches the wire, i.e. before the peak ram pressure/density hits, the bow shock moves leeward monotonically. Once the ram pressure starts to decrease the bow shock relaxes windward.
-At early times, i.e. before the shock passes the center of the FOV, the contact discontinuity is visibly deflected by the magnetic field. At high latitudes with respect to the wire, this bowing out should be readily distinguishable from the hydro case regardless of possible projection or timing complications.
-At late times, the displacement of the nose of the bow shock in the magnetized run vis-a-vis the hydro run re-emerges after a long intermission where magnetic effects were suppressed due to the high sigma. At these late times, the ram pressure has dropped back down to low sigma regime.
-At late times, the dynamics of the near-wire region is dominated by the "ablation" of the most windward crescent of wire. The CD is visible in both runs to be between the "ablated" wire material and the wind. The dynamics of the runs are indistinguishable within this inner region.
-The wire temperature is set close to absolute zero, and the "ablation" at late times only spreads around cold, non-emitting matter in the inner region. Thus in both runs the inner region is not a significant source of emissions. I don't trust what the wire is doing at all here.
Image: emissivity of final frame of movie ~100 ns
Running magnetized wire problem with Peter Graham's blast wave
Previous runs of the magnetized wire problem used a uniform wind, Since early last month more realistic blast wave results have become available:
- Rise time of shock density-
10 ns for 1000 fold exponential increase with time from initial 0.01 mg/cc
- Decay time of rarefaction-
100 ns for 1000 fold exponential decay with time from the peak density of (1)
- Deceleration of the flow-
40 ns for 5 fold exponential decay with time from initial *150 km/s *This is already less than ½ of what Peter Graham starts with (>300 km/s)
The simulations w/; w/o B are running fairly slowly, much more so than what I've seen before. Before the rise phase of the blast wave passes completely by the wire we would be seeing times much later than what we've seen before.
Below: emissivity maps at 10 ns, separation between wire center and left boundary is 1.25 mm; the density peak of the blast wave has just emerged from the left boundary.
We should be able to see an effect on the flow from the magnetic field long before the bulk of the flow passes by the wire. If we can avoid going through the hump we can mitigate some of the effects of wire erosion and high sigma due to the large material flux.
Note: wire density in simulation is only 1% of copper, whatever erosion we see at later times are a gross overestimate.
Working around bubbles in high speed wind
Bubbles in wind were prevented from forming by keeping the magnetic field from extending to the grid boundary where the wind comes in.
Why bubbles form when a significant magnetic field is in contact with the edge from which a high speed wind comes into the grid remains unanswered.
Description of attached images:
Image 1 (left): Linear false-color plot of magnetic field strength at setup with wire edge marked by ring. Note the hard outer boundary of the magnetic field.
Image 2 (right): Optical band emissivity map of simulation with: wind density = 0.01 mg/cc; wind velocity = 150 km/s; B = 7.5 T; sigma = 10.08
Image 3: bow shock and CD altitudes of this run (empty and filled squares) plotted against data (empty and filled circles) from previous runs with much greater wind density (2 - 20 mg/cc) and slower speeds (< 70 km/s).
From the combined results as shown in the plot, sigma alone can determine the altitude of the magnetosphere structures; rho, v and B at wire surface are degenerate.
Bug? Bubbles in wind object
I tried to run a 2D simulation where a Wind passes by a Wire surrounded by a cylindrical magnetic field embedded inside an Ambient medium. Wire object extends Clump object to a cylindrical shape.
For this run,
the grid consists of a square of 2.5 mm; magnetic field at the wire surface is 7.5 T; Wind and Ambient temperature is 300 K; Wire temperature is 3e-3 K; Wind density is 1e-5 g/cc; Wire density is 7.5e-2 g/cc; Ambient density is 7.5e-7 g/cc; Wind velocity is 150 km/s.
sigma = 10 at surface of wire
Description of bug: bubbles inside the wind appear and grow to dominate the simulation.
MPEG attached.
Maximum time of movie does not exceed 0.6 ns. Left side: log density Right side: log energy
