Hydrodynamics of Internal Shocks in Relativistic Outflows

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.