Scale interactions and spectral energy transfer in turbulent channel flow

Spectral turbulent energy transport is studied in the form of triadic wave interaction by focussing on the logarithmic layer. At the integral length scales, the main kinetic energy balance is formed between turbulence production and nonlinear turbulence transport, the latter of which plays the key role in the energy cascade. Visualisation of triadic wave interactions further revealed that there exist two new types of scale interaction processes highly active in the near-all and the lower logarithmic regions. First type of the scales interactions is associated with the energy cascade of relatively small energy-containing motions located in the region close to the wall, and considerable part of their energy transfer mechanisms from the integral to the adjacent small length scale in the energy cascade is found to be provided by the interactions between larger energy-containing motions. Second type of the scale interactions is the existence of a non-negligible amount of energy transfer from small to large integral scales. This scale interaction is predominant only for the streamwise and spanwise velocity components, and it plays a central role in the formation of the wall-reaching inactive part of large energy-containing motions which locally scales in inner units.