Multiphase flow modelling is still a major challenge in fluid dynamics and, although
many different models have been derived, there is no clear evidence of their relevance
to certain flow situations. That is particularly valid for bubbly flows, because most
of the studies have considered the case of fluidized beds. In the present study
we give a general formulation to five existing models and study their relevance
to bubbly flows. The results of the linear analysis of those models clearly show
that only two of them are applicable to that case. They both show a very similar
qualitative linear stability behaviour. In the subsequent asymptotic analysis we derive
an equation hierarchy which describes the weakly nonlinear stability of the models.
Their qualitative behaviour up to first order with respect to the small parameter is
again identical. A permanent-wave solution of the first two equations of the hierarchy
is found. It is shown, however, that the permanent-wave (soliton) solution is very
unlikely to occur for the most common case of gas bubbles in water. The reason is
that the weakly nonlinear equations are unstable due to the low magnitude of the
bulk modulus of elasticity. Physically relevant stabilization can eventually be achieved
using some available experimental data. Finally, a necessary condition for existence
of a fully nonlinear soliton is derived.