A few observations are noteworthy for this simulation.
First, two entropy layers are predicted to evolve within the
granular material. The entropy layer generated at the piston
surface (and, thus, propagating at speed up = 100 m/s)
is a numerical relic resulting from artificial diffusion inherent
in the ENO method used in this work; such spurious
layers are a common feature of high-resolution shock capturing
methods. The entropy layer generated at the interface
between the initially precompacted and loose material propagates
at speed u_ = 166.3m/s and is a physical consequence
of the initial entropy difference that exists across this interface.