2D MHD Colliding Flows and different Boundary Condition Effects

2D Mods to global.data

A 2D version of the colliding flows problem seems to be working. I just modified 3 points in the global.data file,

ndim = 2 !before was 3

…

mx = 64, 64, 1 !before was 64, 64, 64

…

xbounds = -25d0, -25d0, 0d0, 25d0, 25d0, 0.78125d0 !this has domain running from 0 to dx in z direction

Results

This reduces simulation time from ~hr. to ~min. While gravity in 2D might be wonky, this allows us to quickly debug the striations.

I checked that the striations at boundary still exist in 2D with extrapolating BCs. They do -

​http://www.pas.rochester.edu/~erica/2DCF_rho_extrapBCs.gif

I next tried to change the boundary on the left and right sides of the box (x1 and x2). I find that different boundary conditions produce wildly different results in the box,

  1. Using the BCs "reflecting, B-parallel" reduces or nearly completely removes the striations. These BCs force the magnetic field to remain normal at the wall.
  1. Using "reflecting, wall" seems to create near vacuum states in the colliding flows. The lower densities seem to be greatly reducing the time step, making these simulations run quite long (~>hr). I don't see a change in the max speed in the standard out however, which I am curious about. This is clearly not right. I am not sure why the boundary condition is leading to this behavior inside the flow.
  1. Using "periodic" boundary conditions leads to very strange behavior as well. It seems to make sense that you'd expect this boundary condition and reflecting to produce the same behavior, but they do not. Again, I am not sure why the boundary is effecting the flow in this way.

Here is a movie of the density for the different runs (the simulation is 2D in the x-y plane) - ​http://www.pas.rochester.edu/~erica/all4BCs_CFrho.gif

I will make note of these effects and when I can look more deeply at the code's prescription for boundary conditions will check my understanding. For now, I am running the 3D version of the code with the reflecting, B-parallel BCs to check that they look good still.

Directory Locations

Build directory is @: /grassdata/erica/CollidingFlows/scrambler_3.0

Runs are @: /grassdata/erica/CollidingFlows/CollidingFlows/2D/MHD

Comments

1. Jonathan -- 11 years ago

So reflecting wall is incompatible with the fields in the X-direction. It can't be done without producing divergence - so I am not surprised by the wonky behavior. The periodic run causes the colliding flow to be periodic as well… which causes issues at the boundary… If the colliding flow object was limited to only be the size of the box then this should give the same behavior as reflecting…. However it looks like reflecting B_parallel fixes the problem.

I would suggest redoing the 2D run with reflect_B_parallel at higher resolution and then trying it in 3D at 643 or 1283. But if those don't show striations - then we may be good to go.