In this work we use Direct Numerical Simulation (DNS) to study the turbulent Poiseuille flow of two immiscible liquid layers inside a rectangular channel. A thin liquid layer (fluid 1) flows on top of a thick liquid layer (fluid 2), such that their thickness ratio is . The two liquid layers have the same density but different viscosities (viscosity-stratified fluids). In particular, we consider three different values of the viscosity ratio : and . Numerical Simulations are based on a Phase Field method to describe the interaction between the two liquid layers. Although a small viscosity ratio is assumed, this physical setup aims at mimicking the situation where water (less viscous fluid) is used to favour the transport of oil (large viscous fluid) inside pipelines. Compared with the case of a single phase flow, the presence of a liquid-liquid interface produces a remarkable turbulence modulation inside the channel, since a significant proportion of the kinetic energy is subtracted from the mean flow and converted into work to deform the interface. This induces a strong turbulence reduction in the proximity of the interface and causes a substantial increase of the volume-flowrate. These effects become more pronounced with decreasing λ.
