  We study the steady state motion of bubble walls in cosmological phase
transitions. Taking into account the boundary and continuity conditions for the
fluid variables, we calculate numerically the wall velocity as a function of
the nucleation temperature, the latent heat, and a friction parameter. We
determine regions in the space of these parameters in which detonations and/or
deflagrations are allowed. In order to apply the results to a physical case, we
calculate these quantities in a specific model, which consists of an extension
of the Standard Model with singlet scalar fields. We also obtain analytic
approximations for the wall velocity, both in the case of deflagrations and of
detonations.
