The stability of two superposed viscous fluids in a channel

The Couette/Poiseuille flow of two superposed viscous fluids in a channel is studied. There are two mechanisms for instability of this flow: one mechanism is associated with the presence of the interface, the other with the presence of the boundary walls. Analytical results are presented based on the short wave perturbation scheme of Hooper and Boyd [J. Fluid Mech. 179, 201 (1987)] and the long wave perturbation scheme of Yih [J. Fluid Mech. 27, 337 (1967)], together with numerical results. Similarities between the stability of plane Poiseuille flow and one fluid and two‐layer Couette/Poiseuille flow are discussed.