  We study the hydrodynamical effects of two colliding shells, adopted to model
internal shocks in various relativistic outflows such as gamma-ray bursts and
blazars. We find that the density profiles are significantly affected by the
propagation of rarefaction waves. A split-feature appears at the contact
discontinuity of the two shells. The shell spreading with a few ten percent of
the speed of light is also shown to be a notable aspect. The conversion
efficiency of the bulk kinetic energy to internal one shows deviations from the
widely-used inelastic two-point-mass-collision model. Observational
implications are also shortly discussed.
