Water-lubricated channel flow.

We use direct numerical simulation (DNS) to study the problem of drag reduction in a lubricated channel, a flow instance in which two thin layers of a lubricating fluid (density ρ1, viscosity η1, thickness h1) are injected in the near-wall region of a plane channel, so to favor the transportation of a primary fluid (density ρ2, viscosity η2, thickness h2). All DNS are run within the constant power input (CPI) approach, which prescribes that the flow rate is adjusted according to the actual pressure gradient so to keep constant the power injected into the flow. The CPI approach has been purposely extended here for the first time to the case of multiphase flows. A phase-field method (PFM) is used to describe the dynamics of the liquid-liquid interface. We unambiguously show that a significant drag reduction (DR) can be achieved for all four configurations considered. Upon a detailed analysis of the turbulence activity in the two lubricating layers and of the interfacial wave dynamics, we are able to characterize the effects of surface tension forces, surfactant concentration, and viscosity contrast on the drag reduction performance.