Turbulent drag reduction by compliant surface tension active wall layer

In this work we use Direct Numerical Simulation (DNS) together with a Phase Field technique to study the turbulent Poiseuille flow of two immiscible liquid layers inside a channel. A thin liquid layer (fluid 1) flows on top of a thick liquid layer (fluid 2), such that their thickness ratio is h1/h2 = 0.075. The two liquid layers have the same density but different viscosities η, specifically we consider the case η1 < η2. The problem is described by the shear Reynolds number (Reτ), by the Weber number (We, which quantifies surface tension effects) and by the viscosity ratio λ (ratio between the viscosity of the two fluids). Compared to a single phase flow at the same shear Reynolds number (Reτ = 300), in the stratified case we observe an increase of the flow rate of fluid 2 and a strong modification of the turbulence structures near the liquid-liquid interface. Altogether, these observations support the presence of a significant Drag Reduction (DR), whose efficiency depends strongly on the viscosity ratio (λ).