Methodologies have been developed for accurate numerical simulations of unsteady
two layer cocurrent and countercurrent turbulent flows driven by gravity and pressure.
The basis of these methodologies is projection method within the bulk of each
phase, with rigorously derived boundary conditions being imposed at the gas/liquid
interface. A judiciously chosen high – order spatial discretisation allows for direct numerical
simulations (DNS) of the turbulence in each phase. A method for conducting
simulations with smallscale eddies modelled and largescale eddies resolved (wall – resolved
LES), was also implemented. The gas/liquid interface is forced to remain flat so
as to allow the a priori constructed structured mesh to adequately resolve the turbulent
boundary layer around the interface for the duration of the simulation. The method is
numerically efficient when applied to the flows where time scales (andj therefore kinematic
viscosities) in the two phases are of the same order of magnitude, but no restriction
is placed on the density or viscosity ratio. For example, flow of air over viscous oil
can be efficiently simulated. Single phase versions of both the DNS and the LES codes
are validated against the literature. Results for the two layer system provide insight
into the structure and the energy budget of turbulence near a liquid film on a wall. The
turbulence statistics in the less viscous phase are compared to those derived from those
collected from singlephase flows over a moving wall. The results are used to validate
(and possibly replace) semiempirical profiles used previously in stability analyses and
to provide further information that is otherwise unknown.