Turbulence and Interface Waves in Stratified Oil–Water Channel Flow at Large Viscosity Ratio

Abstract We investigate the dynamics of turbulence and interfacial waves in an oil–water channel flow. We consider a stratified configuration, in which a thin layer of oil flows on top of a thick layer of water. The oil–water interface that separates the two layers mutually interacts with the surrounding flow field, and is characterized by the formation and propagation of interfacial waves. We perform direct numerical simulation of the Navier-Stokes equations coupled with a phase field method to describe the interface dynamics. For a given shear Reynolds number, $$Re_\tau =300$$ R e τ = 300 , and Weber number, $$We=0.5$$ W e = 0.5 , we consider three different types of oils, characterized by different viscosities, and thus different oil-to-water viscosity ratios $$\mu _r=\mu _o/\mu _w$$ μ r = μ o / μ w (being $$\mu _o$$ μ o and $$\mu _w$$ μ w oil and water viscosities). Starting from a matched viscosity case, $$\mu _r=1$$ μ r = 1 , we increase the oil-to-water viscosity ratio up to $$\mu _r=100$$ μ r = 100 . By increasing $$\mu _r$$ μ r , we observe significant changes both in turbulence and in the dynamics of the oil–water interface. In particular, the large viscosity of oil controls the flow regime in the thin oil layer, as well as the turbulence activity in the thick water layer, with direct consequences on the overall channel flow rate, which decreases when the oil viscosity is increased. Correspondingly, we observe remarkable changes in the dynamics of waves that propagate at the oil–water interface. In particular, increasing the viscosity ratio from $$\mu _r=1$$ μ r = 1 to $$\mu _r=100$$ μ r = 100 , waves change from a two-dimensional, nearly-isotropic pattern, to an almost monochromatic one.