Expansion into a vacuum: A one-dimensional model

The dynamics of a one-dimensional hard-core fluid is examined subsequent to a sudden increase in the size of its bounding container; attention is focused upon the standard hydrodynamical variables: density, current density, local temperature, and local pressure. The rather complicated equilibrating density pattern is found explicitly, and all other relevant quantities expressed in terms of this pattern. In particular, the local bulk diffusion constant retains an anomalous ideal-gas form, whereas the local pressure decomposes into a local-equilibrium part and a specific wall-reflection contribution.