Series of Sims

Spherical Sims‏

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Spherical flows should be the simplest since the geometry is (or should be) one-dimensional provided that the background is as well.

The lower row of two panels shows the initial conditions used for the sims above them. A spherical AGB wind (rho /propto 1/r^2) is used for the sims in the right panel and the sum of an AGB wind and a torus is on the left. The flows launched into the background are relatively dense (top row) and sparse (third row) and in between (second row).

Right panels: Aside from some obvious wiggles that develop near the x and y axes the winds blow round bubbles. A hot, low-density bubble forms in the case of the low-density ("sparse"). The thermal pressure of the bubble pushes against the swept-up shell (called a dense rim) ahead of it. The leading edge of the bubble will wrinkle (that is, form ripples) under certain conditions — especially when the pressure gradient on opposite sides of the rim differ significantly. In the upper panel no hot bubble develops. The spherical winds push almost directly on the rim causing it to flutter into the "thin-shell instability".

Left panels: Note how the presence of the torus influences the growth of the leading rim. In the lower model the rim barely advances so the influence of the external torus is slow to develop. In the panel above it the hot bubble punctures the rim near the y axis and quickly flows forwards at about its sound speed. Once again, in the upper panel no hot bubble develops. The fast spherical winds push almost directly on the rim causing thin-shell instabilities.

In the right column you will see odd wiggles in the advancing rim near the x and y axes. These are artifacts of the code. Accordingly, the knots or jets that develop in them cannot be taken seriously. Below is the result of a test run in which a fine grid was imposed on the gas to the left of the y axis. Comparing this region to that near the x axis (where no special grid was used) shows that spartial resolution matters. We have also discovered that the wiggles go away if we initially and gradually accelerate the low-density winds rather than launch them at their full speed at t=0. We are investigating this problem at the moment since we don't fully understand why this procedure works.