Separating the effects of wall blocking and near-wall shear in the interaction between the wall and the free shear layer in a wall jet

Abstract

The statistical and structural characteristics of two plane jets, one developing along a real wall and the other along a frictionless wall (equivalent to a zero-shear, nondeformable free surface) are compared by way of highly resolved LES (large eddy simulation) solutions at computational conditions close to those of DNS (direct numerical simulation). The aim is to distinguish between two types of influence of the wall on the outer shear layer: one inviscid, arising from wall blocking, and the other, associated with the near-wall shear in the boundary layer. Results are presented for mean-flow properties, second moments and budgets thereof, structural characteristics, and the integral length scale. The comparisons demonstrate that the wall affects a significant proportion of the outer shear layer to a depth of approximately 3 times the thickness of the boundary layer, with or without wall shear. Outside the immediate near-wall layer, in the interaction region, the influence of the wall is affected by an interplay between turbulence diffusion toward the wall and inviscid processes associated with pressure fluctuations and their reflection from the wall. The addition of shear modifies substantially the statistical behavior and structure within the thin sheared region. The state of anisotropy and the energy-redistribution process among the normal-stress components change drastically, the structure is dominated by small scale, elongated eddies, and the near-wall layer is observed to ‘shield’ the wall from the penetration of large-scale vortices from the outer shear layer. The role of diffusion, in particular, renders the near-wall shear layer very different from a conventional equilibrium boundary layer, its integral scale being considerably enhanced by the influence of large-scale eddies originating in the outer shear layer and migrating toward the wall.