Special Features of Near Wall Flow

In wall-bounded turbulent flows, the presence of solid walls is experimentally observed to have a strong damping effect on the transport of turbulence. In numerical calculation of flow, the velocity components and the turbulence energy may unambiguously be set to zero at wall through the so-called No-slip condition. But ambiguity exists about the often singular values of the turbulence dissipation parameter e or w at the solid boundary. Across a turbulent boundary layer, the flow has to undergo a transition from fully turbulent to completely laminar within the thin viscosity-dominated sublayer adjacent to the solid surface. In this laminar and transitional layer, the molecular viscosity has a direct damping effect on the turbulence. This phenomenon is termed as Low Reynolds number turbulence and the transition from high to low Reynolds number regions is determined by the local turbulence Reynolds number, R_t = ρk² /( με ) where k is the turbulence kinetic energy and ε is its dissipation. Two significant physical effects of the presence of a wall are:

1. Molecular viscosity diffuses vorticity and damps turbulence. In the Reynolds Stress transport equation near a solid wall, the viscous diffusion terms which are usually negligible compared to other terms in regions away from wall, become one of the largest terms to be balanced by the other terms.
   
2. Significant reduction of velocity fluctuation normal to the wall by the Pressure Reflection Mechanism which is perhaps controlled by non-viscous effects. But this effect is not understood quite well and isotropic eddy viscosity based models like k - ε or k - ω cannot separate this second effect from the viscosity effect.