Drag reduction in a lubricated turbulent channel

Abstract

Direct numerical simulation (DNS) was utilized to study the drag reduction in a lubricated channel, characterized by the injection of a lubricating fluid layer in the near wall region of a planar channel to favor the transportation of the core fluid. In the present work the fluids have equal density but different viscosity, allowing for the definition of a viscosity ratio λ = η1/η2. In the scope of this work both a lubricated channel with λ = 0.01 and a single phase reference case were considered to quantify the drag reduction enabled by the lubricating layer and observe flow dynamics at large viscosity ratios. All DNS were run with a constant power input (CPI) approach. This allows for the power injected into the flow to be constant across both cases considered by adjusting the pressure gradient according to the flow rate of the channel. This was accomplished here by extending the CPI approach to lubricated channel flow. The dynamics of the liquid-liquid interface is described by a phase-field method (PFM). The simulation results indicate a significant drag reduction for the lubricated case. The flow rate increase has been quantified to be ≃ 63%. An analysis of the mean and turbulent kinetic energy budgets gives insight into the drag reduction mechanisms of the lubricated channel. For the multi-phase case, turbulence is sustained in the lubricating layer due to the local increase in Reynolds number. The core flow experiences a laminarization due to the hindering of transfer of turbulent kinetic energy from the boundary layer to the core.