Direct numerical simulation of lubricated channel flow

Low Reynolds number turbulent flow in a lubricated plane channel is studied by means of direct numerical simulations. The very thin lubricating film is more viscous than the bulk-fluid and is present at both channel walls. The bulk-fluid is no longer exposed to the no-slip condition at the solid surfaces. Despite the rather large interfacial motions the bulk-flow does in many aspects perceive the interface as a solid wall, except in the immediate vicinity of the lubricating film where the interfacial motions heavily influence the bulk-flow. The lubricating film shows a high degree of anisotropy, and the only major component is the streamwise velocity fluctuations. Also the bulk-flow adjacent to the interface is closer to one-component turbulence than what is commonly observed close to solid surfaces. The interface is populated by very organized low-speed streaks with a mean spacing close to 100 wall units. The coherent structures in the near-interface region do resemble those close to solid walls, but they are situated somewhat closer to the interface since the coherent velocity field is slightly penetrating the lubricating film.