  We perform simulations in a simple model that aims to mimic the hydrodynamic
evolution of a relativistic fluid during a cosmological first-order phase
transitions. The observable we are concerned with is hereby the spectrum of
gravitational radiation produced by colliding fluid shells. We present simple
parameterizations of our results as functions of the wall velocity, the
duration of the phase transition and the latent heat. We also improve on
previous results in the envelope approximation and compare with hydrodynamic
simulations.
