  The linear stability of stratified two-phase flows in rectangular ducts is
studied numerically. The linear stability analysis takes into account all
possible infinitesimal three-dimensional disturbances and is carried out by
solution of the associated eigenproblem. The neutral stability boundary and the
corresponding critical wave number are obtained for liquid - liquid and air -
water systems. Depending on the problem parameters, the instability sets in
owing to short, intermediate, of long wave most unstable perturbations.
Patterns of the most unstable disturbances are reported and discussed. It is
shown that the instability arises due to shear, or interfacial mechanisms.
Effects of the surface tension and of width/height aspect ratio are also
studied. The results support the premise that the stability analysis of
stratified two-phase flow in the simpler geometry of two-infinite plates can
provide a reasonable estimation of the conditions for which this flow pattern
can be considered to be linearly stable.

Fluid Dynamics