Changes between Version 8 and Version 9 of u/erica/MHDshocksReorientation

Timestamp:

03/09/16 15:20:36 (9 years ago)

Author:

Erica Kaminski

Comment:

—

u/erica/MHDshocksReorientation

@@ -27 +27 @@
 When the scenario is finite, there is now pressure gradients between the collision refion and the ambient that allow for a radial expulsion of gas from the collision region. Over time in these plots, we see both the reorientation of the inner surface of the collision region, as well as the outer (fast) shock front. Here are 3 likely scenarios for the reorientation of the inner collision region. 
 
-'''More x-momentum due to kinking and shear flow and balance between pressure and radial expansion.'''[[br]]
-In this picture, the radial expansion of the flow drags the field lines out of the collision region. Depending on which side of the interface you are on, this is either enhanced by the shear, or partially cancelled out. Figure of close up. On the side where the field remains relatively straight, gas doesn't get deflected as strongly as on the other side, given the flow is tied to the field. (Here, could strengthen this argument by better understanding of the connection between field, velocity, in MHD shocks). This leads to more net x momentum on one side of the interface then the other, and thus, torque. A figure shows this to be the case. However, whether this picture for why the momentum gets amplified on one side of the interface is correct, is still unclear. 
+ '''More x-momentum due to kinking and shear flow and balance between pressure and radial expansion.'''[[br]]
+ In this picture, the radial expansion of the flow drags the field lines out of the collision region. Depending on which side of the interface you are on, this is either enhanced by the shear, or partially cancelled out. Figure of close up. On the side where the field remains relatively straight, gas doesn't get deflected as strongly as on the other side, given the flow is tied to the field. (Here, could strengthen this argument by better understanding of the connection between field, velocity, in MHD shocks). This leads to more net x momentum on one side of the interface then the other, and thus, torque. A figure shows this to be the case. However, whether this picture for why the momentum gets amplified on one side of the interface is correct, is still unclear. 
 
 
-'''The radial expansion of gas away from the collision region.'''[[br]]
-The infinite case teaches us that the velocity goes to zero in the center of the collision region in the MHD case. However, when there is a pressure gradient between the collision region and the surrounding ambient gas, we see a strong outward (relative to the center of the collision region) velocity field arise. That there is now an up/down (relative to the cylindrical axis of the flows) velocity field within the collision region, sends gas upward and downward, thus effectively straightening out the collision region. In the hydro case, this is not the case, as there the velocity field is the usual sheared flow field. (Which, by the way, uh-oh for our runs declaring to study a 'shear' effect).  Thus, we do not see a realignment of the interface in the hydro case. In visit, drawing a line along the original collision angle shows supports that the interface realligns in the MHD case, but not in the hydro case.
+ '''The radial expansion of gas away from the collision region.'''[[br]]
+ The infinite case teaches us that the velocity goes to zero in the center of the collision region in the MHD case. However, when there is a pressure gradient between the collision region and the surrounding ambient gas, we see a strong outward (relative to the center of the collision region) velocity field arise. That there is now an up/down (relative to the cylindrical axis of the flows) velocity field within the collision region, sends gas upward and downward, thus effectively straightening out the collision region. In the hydro case, this is not the case, as there the velocity field is the usual sheared flow field. (Which, by the way, uh-oh for our runs declaring to study a 'shear' effect).  Thus, we do not see a realignment of the interface in the hydro case. In visit, drawing a line along the original collision angle shows supports that the interface realligns in the MHD case, but not in the hydro case.
 
-'''In the 3rd scenario, it may be due to the tension in the field. (Rubber-band model).'''[[br]]
-Just a glance at the magnetic field threading the collision region shows suggests this would be a contributing factor.
+ '''In the 3rd scenario, it may be due to the tension in the field. (Rubber-band model).'''[[br]]
+ Just a glance at the magnetic field threading the collision region shows suggests this would be a contributing factor.
 
+
+For the outer wave front (fast shock), the apparent reorientation can be explained by a stalling of this wave front, as seen in the upper right/lower left corners of the collision interface. This is happening because there is a loss of magnetic pressure behind this wave front, and thus, the shock loses its support and stalls (see following figure of magnetic pressure map). The enhanced magnetic pressure, relatively speaking, in the opposite regions are due to the combined effect of 1. the deflection of material and 2. the enhanced pressure from radial expansion. In regions where the magnetic pressure is strongest, the wave front is supported and continues to move outward, thus appearing to straighten the outer wave front.   
 
 In the hydro case, we do not see this reorientation (at least in 2D). Instead, we find a staircase effect. This seems to be getting generated by vortices above and below the collision region. Material that is deflected away from the flows by the shear, falls back down onto the cylinder due to pressure gradients. This additional material then creates more x-momentum/ram pressure, which drives the stair-casing structure.