  Hydrodynamical interaction of spherical ejecta freely expanding at mildly
relativistic speeds into an ambient cold medium is studied in semi-analytical
and numerical ways to investigate how ejecta produced in energetic stellar
explosions dissipate their kinetic energy through the interaction with the
surrounding medium. We especially focus on the case in which the circumstellar
medium is well represented by a steady wind at a constant mass-loss rate having
been ejected from the stellar surface prior to the explosion. As a result of
the hydrodynamical interaction, the ejecta and circumstellar medium are swept
by the reverse and forward shocks, leading to the formation of a geometrically
thin shell. We present a semi-analytical model describing the dynamical
evolution of the shell and compare the results with numerical simulations. The
shell can give rise to bright emission as it gradually becomes transparent to
photons. while it is optically thick. We develop an emission model for the
expected emission from the optically thick shell, in which photons in the shell
gradually diffuse out to the interstellar space. Then, we investigate the
possibility that radiation powered by the hydrodynamical interaction is the
origin of an underluminous class of gamma-ray bursts.
