SHEAR-FREE TURBULENT BOUNDARY-LAYERS .1. PHYSICAL INSIGHTS INTO NEAR-WALL TURBULENCE

Direct numerical simulation is used to examine the interaction of turb ulence with a wall in the absence of mean shear. The influence of a so lid wall on turbulence is analysed by first considering two 'simpler' types of boundaries. The first boundary is an idealized permeable wall . This boundary isolates and elucidates the viscous effects created by the wall. The second boundary is an idealized free surface. This boun dary complements the first by allowing one to isolate and investigate the kinematic effects that occur near boundaries. The knowledge gained from these two simpler flows is then used to understand how turbulenc e is influenced by solid walls where both viscous and kinematic effect s occur in combination. Examination of the instantaneous flow fields c onfirms the presence of previously hypothesized structures (splats), a nd reveals an additional class of structures (antisplats). Statistical analysis of the Reynolds stresses and Reynolds stress transport equat ions indicates the relative importance of dissipation, intercomponent energy transfer, and energy transport. It is found that it is not the structures themselves, but the imbalance between structures which lead s to intercomponent energy transfer. Remarkably, this imbalance (and h ence near-wall intercomponent energy transfer) is controlled by viscou s processes such as dissipation and diffusion. The analysis presented herein is a departure from past notions of how boundaries influence tu rbulence. The efficacy of these qualitative physical concepts is demon strated in Part 2 where improved near-wall turbulence models are deriv ed based on these ideas.