  Study of the propagation of sound in a single non-ideal fluid originates with
Stokes in 1845 and Kirchhoff in 1868. The situation is much more complex in the
case of two-fluid flow, both from the physical point of view, as the
configuration of the flow matters greatly, and from the analytical point of
view. The principle two-fluid models currently in use for CFD are the focus of
this article. It is shown that analytical expressions for the speed of sound
depend heavily on the chosen model. These sound speed expressions are compared
with experimental values. The consequences for CFD models are discussed in the
final section of this paper. It is found that numerical models with inaccurate
wave speeds lead to incorrect numerical solutions, despite the accuracy of the
numerical scheme.
