Skinfriction in turbulent channel flow

Abstract

The main focus of this thesis is turbulent flow in a lubricated plane channel. This flow is mainly studied by direct numerical simulations (DNS). Both the lubricating film and the bulk-fluid velocity and pressure fields are computed within the DNS approach. 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 channel walls, and is instead influenced by the dynamic interface between the lubricating film and the bulk-fluid. 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 coherent structures in the near interface region, which are responsible for a major part of the turbulence production, resemble those close to solid walls.

The thin lubricating film shows a high degree of anisotropy, and the only major component is the streamwise velocity fluctuations. The interface reduces the interaction between the bulk-flow and the solid wall by preventing inrush of high momentum fluid towards the wall. This leads to a reduction of Reynolds shear stress in this region, and inside the film the Reynolds shear stress is vanishing. This observation suggests that a thin lubricating film can be used for drag reduction purposes. Simulations, using turbulence modelling, show that drag reduction is possible to achieve in high Reynolds number gas-pipelines if the film remains stable.

The thin lubricating film can also be replaced by a set of simplified slip- flow boundary conditions applied at the film bulk-fluid interface. By replacing the lubricating film with slip-flow conditions, both turbulence statistics as well as the coherent near-interface structures are surprisingly well reproduced. In this approach the need to resolve the flow within the lubricating film is eliminated and the computational efforts are therefore reduced.

A direct numerical simulation of a conventional channel flow is also performed. A generic model for the skin-friction pattern associated with the near wall coherent structures is deduced. The generic model of the footprint of the coherent structures shows that high-skin friction occurs just aside of the tip of the vortex tail due to the downwash of high-speed fluid induced by the vortex.

Solid spherical particles injected into a turbulent channel flow are studied by DNS. The focus is on the interaction between the particles and the near wall coherent structures. The accumulation of particles in the very near wall region is thought to be an effect of an imbalance between the efficiency of sweeps and ejections in transporting particles.