Others have just applied a certain degree of momentum coupling in model calculations carried out to study other aspects of the wind. We will give a brief overview of the most important of these studies, resulting in the conclusion that prior to our attempt, full two-fluid hydrodynamics has been presented only twice. First, single-fluid models are those in which only the momentum equation of the gas component is solved. All momentum due to radiation pressure on grains is transferred fully and instantaneously to the gas. The second class is that of the two-fluid models.
Grains can drift at non-equilibrium drift velocities.
Hence, grains and gas are neither forced to have equal velocity nor forced to have equal acceleration. The third category of models represents what we will call 1.5-fluid models. In these models, grains are assumed to drift at the equilibrium drift velocity with respect to the gas. No assumptions about position coupling are made. In other words, gas and grains are equally accelerated but do not necessarily have the same velocity.