CONTINUUM HYDRODYNAMICAL EQUATIONS FOR MULTIPHASE ASTROPHYSICAL FLOWS

We present a formalism for averaging mutually interacting multiphase hydrodynamical flows. Only averaging over space is discussed. The outcome of this formulation is a set of nonlinear first-order integrodifferential equations. We then apply the formalism to the problem of H I regions immersed in H II regions and obtain approximated equations. The H I regions are treated as a dilute suspension of fluid spheres immersed in H II continuum. The equations include terms and coefficients that have to be evaluated from a ''mesoscopic'' hydrodynamical theory. The hydrodynamics of the flow past a single cloud, which is defined here as the ''mesoscale'' of a ''fluid particle'' for the system under consideration should provide the input physics to the formalism presented here.