  With the objective of modeling both separate and disperse two-phase flows, we
use in this paper a methodology for deriving two-fluid models that do not
assume any flow topology. This methodology is based on a variational principle
and on entropy dissipation requirement. Some of the models that are such
derived and studied are already known in the contexts of the description of
separate-or disperse-phase flows. However, we here propose an arrangement of
these models into a hierarchy based on their links through relaxation
parameters. Moreover, the models are shown to be compatible with the
description of a monodisperse bubbly flow and, within this frame, the
relaxation parameters can be identified. This identification is finally
verified and discussed through comparisons with experimental measures of sound
dispersion and with dispersion relations of a reference model for bubbly media.
