Rayleigh Taylor
---------------

This is a common test for hydrodynamics (see Abel 2011, Saitoh and Makino 2013, Hopkins 2013).
It consists in a low density layer of fluid at the bottom and a high density layer at the top.
Due to a constant vertical gravitational acceleration, the two fluids mix togerther through the
Rayleigh Taylor instability (e.g. nuclear mushroom cloud).

In this example, we follow the implementation of Saitoh and Makino 2013, but with a longer box in order
to avoid an interaction with the wall (see last image in Figure 10).

The code needs to be compiled with the following options: `--with-hydro-dimension=2`,
`--with-ext-potential=constant`, `--enable-boundary-particles` and `--with-adiabatic-index=7/5`.
I also recommend to use `--with-kernel=wendland-C2`.

The option `--enable-boundary-particles` requires the ID of the last boundary particle.
This value is provided by the script makeIC.py and depends on the resolution.