ON THE DYNAMICS OF NEAR-WALL TURBULENCE

A model of the dynamic physical processes that occur in the near-wall region of a turbulent flow at high Reynolds numbers is described. The hairpin vortex is postulated to be the basic flow structure of the turbulent boundary layer. It is argued that the central features of the near-wall flow can be explained in terms of how asymmetric hairpin vortices interact with the background shear flow, with each other, and with the surface layer near the wall. The physical process that leads to the regeneration of new hairpin vortices near the surface is described, as well as the processes of evolution of such vortices to larger-scale motions farther from the surface. The model is supported by recent important developments in the theory of unsteady surface-layer separation and a number of 'kernel' experiments which serve to elucidate the basic fluid mechanics phenomena believed to be relevant to the turbulent boundary layer. Explanations for the kinematical behaviour observed in direct numerical simulations of low Reynolds number boundary-layer and channel flows are given. An important aspect of the model is that it has been formulated to be consistent with accepted rational mechanics concepts that are known to provide a proper mathematical description of high Reynolds number flow.