Turbulent Drag Reduction by a Near Wall Surface Tension Active Interface

Abstract

In this work we study the turbulence modulation in a viscosity-stratified two-phase flow using Direct Numerical Simulation (DNS) of turbulence and the Phase FieldMethod (PFM) to simulate the interfacial phenomena. Specifically we consider the caseof two immiscible fluid layers driven in a closed rectangular channel by an imposedmean pressure gradient. The present problem, which may mimic the behaviour of anoil flowing under a thin layer of different oil, thickness ratioh2/h1=9, is describedby three main flow parameters: the shear Reynolds numberReτ(which quantifies theimportance of inertia compared to viscous effects), the Weber numberWe(which quan-tifies surface tension effects) and the viscosity ratioλ=ν1/ν2between the two fluids.For this first study, the density ratio of the two fluid layers is the same (ρ2=ρ1),we keepReτandWeconstant, but we consider three different values for the viscosityratio:λ=1,λ=0.875 andλ=0.75. Compared to a single phase flow at the sameshear Reynolds number (Reτ=100), in the two phase flow case we observe a decreaseof the wall-shear stress and a strong turbulence modulation in particular in the proxim-ity of the interface. Interestingly, we observe that the modulation of turbulence by theliquid-liquid interface extends up to the top wall (i.e. the closest to the interface) and pro-duces local shear stress inversions and flow recirculation regions. The observed resultsdepend primarily on the interface deformability and on the viscosity ratio between the twofluids (λ).