Near wall dissipation revisited

The characteristics of the dissipation of the turbulent kinetic energy in wall-bounded flows are revisited through direct numerical simulations of turbulent channel flows realized in large computational domains up to the Reynolds number Reτ=hu¯τν=1100 where u¯τ is the friction velocity, ν and h stands for the kinematic viscosity and channel half width. It is shown that the local homogeneity assumption is acceptable in the whole layer, while the local isotropy is valid only in the far meso-layer and near the centerline. The mean dissipation in inner scale is Reynolds number dependent in the low-buffer and viscous sublayers. This is due to the local shear layers induced by the outer-layer irrotational eddies. A conceptual model is subsequently proposed to link the near wall dissipation to the large-scale structures. The dissipation statistics conditioned by fixed amplitudes of the velocity field are also analyzed in order to clarify whether the zero-crossings of the fluctuating velocity components contribute most to the energy dissipation or not. Incidentally, the occurrence of the Eulerian stagnation points is questioned. The largest contribution to the dissipation in the viscous sublayer comes from the level-crossings of the wall normal velocity v component in the spanwise direction, that peaks to 30% when v=0. The spanwise direction is the most active in terms of the level-crossing frequencies that are inversely proportional to the corresponding Taylor scales.